AU2008200358B2 - Electrical and electronic musical instruments - Google Patents

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD DIVISIONAL PATENT Applicant: Shelley Katz 130A Shelford Road Cambridge CB2 9NF United Kingdom Actual Inventors: Shelley Katz Address for Service: HODGKINSON McINNES PATENTS Patent & Trade Mark Attorneys Levels 21, 201 Elizabeth Street Sydney NSW 2000 HMcIP Ref: P20342AU01 Invention Title: Electrical and electronic musical instruments Divisional Parent: No. 2003202084 Filed 22 January 2003 The following statement is a full description of this invention, including the best method of performing it known to us: P1 IOAHAU ELECTRICAL AND ELECTRONIC MUSICAL INSTRUMENTS This invention relates to electrical and electronic musical instruments and in particular provides such an 5 instrument having a novel loudspeaker arrangement. The invention is especially, but not exclusively, applicable to digital pianos. For decades, the production and reproduction of sound 10 electrically has been the subject of constant research and development. One aspect of the research concerns the performance of loudspeakers with a view to improving the fidelity and quality of the sound they produce, for example with a view to producing a loudspeaker whose .15 moving elements will faithfully follow the variations in the electrical signal driving them. Another aspect of the research and development concerns techniques for encoding, recording, broadcasting and decoding electrical signals representing sound with a view to 20 improving the realism of the sounds as reproduced by loudspeakers driven by the decoded signals. 25 2 Fidelity and Loudspeaker Design As a result of the search for improvement in loudspeakers, there are currently available a number of 5 different types of loudspeaker operating on respective different principles. For example, there is the conventional cone loudspeaker in which a compliantly mounted cone of relatively rigid 10 material is driven electromagnetically by means of a moving coil or a moving magnet. This type of loudspeaker has probably achieved widest popularity and is used in a wide variety of different applications, each having its own specific design requirements. Examples of 15 applications are in home entertainment systems for reproducing music, speech or audio-visual material, public address systems, audio outputs of computers, cinema and theatre sound systems, in-car entertainment, public address systems and electrical or electronic 20 musical instruments, such as digital pianos. No cone loudspeaker has yet been devised which can generate sound with good fidelity across the whole range of audio frequencies, specifically from 50 Hz or less to 25 about 20 kHz. Hence good-quality sound, for example for 3 faithful music reproduction, can only be achieved with cone loudspeakers when two or more units, each designed for a particular frequency band, are used in combination. The driving signal to these units is accordingly passed 5 through circuits, known as crossover circuits, which direct the different frequency bands of the driving signal to the appropriate cone loudspeaker. Many good quality speaker systems typically employ at least three cone loudspeakers, one for the high frequencies, this 10 being known as a tweeter, one for the mid-range and the third for the low frequencies, known as a woofer or a sub-woofer. Modern cone loudspeaker systems can reproduce sound with a relatively flat frequency response over substantially the whole of the frequency range 15 audible to human beings with relatively high fidelity, and are currently by far the most widely used type of loudspeaker for the reproduction of music. Another type of loudspeaker which is in use is the 20 electrostatic loudspeaker in which a light stretched plastic membrane is caused to vibrate by an alternating electrostatic field produced from the driving signal by a pair of electrodes between which the membrane is positioned. Because the membrane is particularly light 25 and therefore has low mechanical inertia, the motion of 4 the membrane can reproduce the applied signal even at high frequencies with relatively high fidelity. A particular characteristic of electrostatic loudspeakers is the high degree of clarity of sound which they produce 5 compared to other forms of loudspeaker. However, electrostatic loudspeakers as currently available commercially cannot produce the lowest frequencies which arise in music. Despite this, and despite the fact that electrostatic loudspeakers produce lower sound pressure 10 levels than can be produced by cone loudspeakers, many consider that electrostatic loudspeakers do provide particularly high fidelity over the frequency range at which they can operate. 15 The above described cone loudspeakers and electrostatic loudspeakers generate sound by pistonic motion of the cone or membrane. A third type of loudspeaker, which does not rely upon 20 pistonic motion, has come into use in recent years. This is known as a distributed mode loudspeaker. These are described in numerous publications, for example: (a) PCT application WO 97/09842 25 5 (b) US patent 6,399,870 (c) An article entitled "NXT Up Against the Wall" by Henry Azima which appeared in the September 1998 5 edition of the journal "Audio" published by Hachette Filipacchi Magazines Inc. (d) A paper entitled The Distributed Mode Loudspeaker (DML) as a Broad-Band Acoustic Radiator by Neil 10 Harris and Malcolm Omar Hawksford presented at the 103rd Audio Engineering Society Convention 1997 September 26-29 New York. (e) A paper entitled Boundary Interaction of Diffuse 15 Field Distributed Mode Radiators by Henry Azima and Neil Harris presented at the 103rd Audio Engineering Society Convention 1997 September 26-29 New York. 20 (f) A paper entitled Distributed Mode Loudspeaker Simulation Model by Joerg W. Panzer and Neil Harris presented at the 104th Audio Engineering Society Convention 1998 May 16-19 Amsterdam. 25 (g) A paper entitled Distributed Mode Loudspeaker 6 Radiation Simulation by Joerg Panzer and Neil Harris presented at the 105th Audio Engineering Society Convention 1998 September 26-29 San Francisco, California. 5 The contents of these publications are incorporated herein by reference. The structure and operation of such distributed mode loudspeakers has also been described in numerous other published papers, some of which are 10 referenced in the above referred to articles and papers. As is described in the above publications, a distributed mode loudspeaker comprises a panel and one or more transducers which are attached to the panel and, when 15 activated by an electrical audio signal, generate resonant bending waves in the panel, which waves are distributed in a complex pattern over the surface, or the required part of the surface, of the panel. The excitation of the panel into these distributed resonant 20 modes by the transducer requires that the panel be constructed so that it is capable of being excited into these resonant modes and that the transducer or transducers be carefully positioned having regard to the characteristics of the panel so that the required 25 resonant modes are produced in the panel. Those skilled 7 in the art of distributed mode loudspeakers are able to design such loudspeakers in a variety of sizes and using a variety of different materials and different forms of transducer. 5 The successful design of a distributed mode loudspeaker is a complex operation since the manner in which the panel vibrates and the frequency response is dependent upon a large number of different parameters including the 10 panel width, height, thickness, material, the density of the material, the Poisson ratio, the bending rigidity, the damping factor, the shear ratio, the shear modulus, the nature and positioning of the transducers and the number of transducers employed. In practice, it is 15 necessary to compute the frequency response from mathematical equations, in which connection, reference is made to the above identified publications relating to distributed mode loudspeakers. A computer program for performing these calculations and thereby facilitating 20 the successful design of distributed mode loudspeakers is commercially available from New Transducers Limited, Signet House, Kingfisher Way, Hinchingbrook Business Park, Huntingdon postcode PE29 6FW. This computer program allows the designer to enter or select the 25 various relevant parameters of the proposed loudspeaker 8 and the computer program computes the resulting frequency response and vibration characteristics of the proposed loudspeaker to allow the designer to make appropriate design decisions. 5 Apart from the above computer program, distributed mode loudspeakers are commercially available from a number of different sources, for example, Amina Technologies Ltd, Cirrus House, Glebe Road, Huntingdon, Cambridgeshire PE29 10 7DX, England; Tannoy Limited, Coatbridge ML5 4TF, Scotland; Mission (UK) Ltd, Stonehill, Huntingdon, Cambridgeshire PE29 6EY, England; or Armstrong World Industries, 2500 Columbia Avenue, Lancaster, PA 17603, USA. 15 Various proposals have been made for the deployment of distributed mode loudspeakers in combination with other forms of loudspeaker. For example, because currently available distributed mode loudspeakers are not capable 20 of reproducing faithfully frequencies below about 100 Hz, it has been proposed to use them with a sub-woofer which is separate from the distributed mode loudspeaker, the two loudspeakers being driven via appropriate filtering (crossover) circuits. Also, loudspeaker assemblies are 25 commercially available in which there is provided in 9 combination, in a common casing, a distributed mode loudspeaker used as a tweeter, one or more conventional cone loudspeakers acting as a woofer and/or mid-range loudspeakers, and conventional crossover circuits so that 5 the distributed mode loudspeaker is driven exclusively by frequencies in the band appropriate to tweeters and the cone loudspeaker or cone loudspeakers is or are driven exclusively by frequencies in the bands appropriate to woofers and mid-range loudspeakers. 10 Another proposal for a full frequency range loudspeaker system utilising a distributed mode loudspeaker is described in US patent 6,351,542 (Azima et al). In this patent, a distributed mode loudspeaker forms one wall of 15 a closed chamber which is connected through a pipe to an enclosure containing a low frequency loudspeaker (woofer) so that the air pressure variations generated in the enclosure containing the woofer are transmitted through the pipe to the closed chamber. The distributed 20 mode loudspeaker is supported at its edges by compliant material so that it may move pistonically in response to the air pressure variations in the closed chamber and thereby produce low frequency sound, effectively by pistonically vibrating in sympathy with the woofer. 25 10 Published PCT application WO 97/09842, already referred to above, itself discloses a large number of potential applications and arrangements for distributed mode loudspeakers. One arrangement disclosed in this PCT 5 application proposes utilising curved panels in distributed mode loudspeakers with a view to focussing the sound in a particular direction. Another arrangement (figure 65 of the PCT application) proposes forming at least one wall of the casing of a conventional 10 loudspeaker as a passive panel which is capable of resonant mode vibration but is caused to vibrate not through an electromagnetic transducer but by sympathetic vibrations induced by the air pressure variations arising within the loudspeaker enclosure when the conventional 15 cone loudspeakers, which include a woofer, a mid-range and a tweeter, are driven. This arrangement is said to enable desirable colouration to be achieved. An important advantage of distributed mode loudspeakers 20 is that they can be made with a relatively flat profile, enabling them to be used in situations where a cone loudspeaker would be inconvenient or visually intrusive. However, distributed mode loudspeakers have not yet achieved wide use in the field of high fidelity music 25 reproduction, possibly because, although they can be made * 11 to- produce sounds over a large part of the frequency range audible to human beings, their frequency response is not yet as flat as can be achieved with well designed cone loudspeakers. 5 Realism and Sound Encoding Techniques The research into improvement in the realism of reproduced sound resulted in the development of 10 stereophonic systems in which a recording or broadcast includes distinct signals, provided in separate channels, for driving loudspeakers positioned in front of the listener to his left and to his right. These systems came into wide commercial use in the 1950s and 15 1960s and continue in use. They make it possible to create an impression, for the listener, of sounds being produced at different locations in front of him and of sounds moving across the sound "stage". They also provide an impression of the spaciousness and full sound produced 20 by, for example, a symphony orchestra or musical instrument such as a piano, which is superior to that given by reproducing sound using a single signal channel. With a view to further improving the impression of 25 spaciousness, and providing for the possibility of 12 special effects such as the apparent movement of the sound source through a three dimensional space in which a listener is located, four channel systems, known as quadraphonic systems, were developed in the 1960s. 5 Although some recordings and some broadcasts were made at the time, quadrophonic systems did not then come into wide use. One of the problems with the systems is that they required special recording and broadcast techniques in which different signals were encoded in different 10 channels and four loudspeakers positioned, in essence, at the four corners of the listening room, with the listener located in the central area of the space between the four loudspeakers. Although these systems did not at the time come into wide use, they were able 15 to produce an improvement, relative to stereophonic systems, both in the impression of spaciousness i.e. the impression that the listener is listening to the sound in a room substantially larger than the room he is actually listening in, and in "envelopment" i.e. 20 generating a feeling in the listener that he is enveloped by the sound. Together, these impressions give the listener a psycho-acoustic experience which more closely resembles the experience which he would have in listening to music in a concert hall whose acoustics are such as 25 to provide appropriate levels of reflection of sounds 13 from the walls and appropriate reverberation times. In recent years so called "surround sound" systems have come into use, particularly in the cinema and in so 5 called "home cinema" entertainment systems. One of the main purposes of the surround sound system is the production of special acoustic effects, such as the simulation of the sound of a vehicle passing through the space containing the surround sound system and the 10 listeners. Typically, surround sound systems comprise five channels, respectively for driving left and right loudspeakers in front of the listener, left and right loudspeakers at the sides of the listener and a front centre loudspeaker. The systems require the signal to 15 be reproduced to be encoded individually in each of five different channels so that each separate loudspeaker can be individually driven by its own dedicated signal, as encoded in the recording or broadcast signal. With appropriate signal encoding, the surround systems can 20 provide an improvement in the spaciousness and envelopment of the sound compared to stereophonic systems. The Invention 25 14 Despite the extensive improvements which have been achieved over past decades in loudspeaker design and signal encoding as discussed, there remains a need for a 5 system which can produce enhanced sounds from electrical signals at reasonable cost, and particularly a system in which the spaciousness of the sound is improved, and which can be employed in electrical and electronic musical instruments. 10 The present invention provides an electrical or electronic musical instrument comprising playing means operable by a player for selecting musical notes to be sounded; electronic signal generator means responsive to 15 operation of the playing means for generating electrical signals corresponding to the selected musical notes; and a loudspeaker arrangement which is responsive to said electrical signals for producing the selected musical notes; wherein said loudspeaker arrangement: 20 (a) comprises distributed mode and pistonic loudspeaker means each of which is arranged for producing at the same time the same musical notes when the playing means is operated; and (b) is such that each said loudspeaker means is 25 operable to generate sound, when the playing means is 15 operated, with a frequency response which, measured under anechoic conditions, comprises an operating range and roll-off ranges respectively above and below the upper and lower ends of the operating range, said 5 operating ranges of said distributed mode and said pistonic loudspeaker means overlapping over an overlapping range which comprises a plurality of octaves at least one of which is in the frequency range made up of the 1kHz to 4 kHz octave bands. 10 It has been surprisingly found that, by reproducing sound with such an arrangement, enhanced spaciousness and/or envelopment can be achieved, even in single channel (mono) systems. Hence, the invention is 15 applicable to single channel, stereophonic and multi channel systems. The enhancements in spaciousness, and also enhancements in envelopment and warmth of the sound will be further 20 discussed below by reference to comparative experiments which have been conducted, and the results of which are represented in figures 1 to 7 of the accompanying drawings. Practical embodiments of the invention in digital pianos will be described with reference to 25 figures 8 to 13 of the accompanying drawings.

16 In the accompanying drawings: Figure 1 is a bar chart showing the Lateral Early Energy 5 Fraction (LEF) measured in a first experiment using a stereophonic sound reproduction system, comparing the LEFs obtained using conventional cone loudspeakers alone, distributed mode loudspeakers alone and a combination of both in accordance with an embodiment of 10 the invention; Figure 2 is a bar chart showing Inter-Aural Cross Correlation Coefficient (IACC) measured in the experiment referred to in connection with figure 1; Figures 3 and 4 are bar charts similar to figures 1 and 2 but showing the LEFs and IACCs obtained in a second experiment in which a mono sound reproduction system is used; 20 Figures 5 and 6 show respectively the frequency responses of a conventional cone loudspeaker and a distributed mode loudspeaker used in a third experiment in relation to the invention; 25 17 Figure 7 shows the frequency responses obtained when the loudspeakers to which figures 5 and 6 relate are driven simultaneously, with the respective different curves of figure 7 showing the results obtained when the relative 5 sound pressure levels of the two loudspeakers are varied; Figure 8 is a perspective view of a digital grand piano in accordance with an embodiment of the present 10 invention; Figure 9 is a schematic block diagram of the digital piano of figure 8; .15 Figure 10 is a rear perspective view of a digital upright piano according to an embodiment of the invention; Figure 11 is a block diagram of the digital piano of 20 figure 10; Figure 12 is a block diagram of further embodiment of a digital piano in accordance with the invention; and 25 Figure 13 is a block diagram of an add-on unit, S 18 according to a further embodiment of the invention, shown connected to a conventional digital piano so that the add-on unit and the conventional digital piano in combination implement the invention. S EXPERIMENTS General Introduction to Experiments 10 In order to investigate the surprising increase in the impression of spaciousness achieved by combining distributed mode and pistonic loudspeakers in accordance with the invention, a number of experiments have been conducted in a number of different environments and 15 utilising, in the different environments, different loudspeakers and other equipment. All of the experiments involved the use of top quality studio monitor cone loudspeaker systems and top quality 20 distributed mode loudspeakers. Each experiment included subjective listening tests performed on expert listeners, different listeners present in the different experiments. 25 One experiment (the first to be described in detail below) employed a stereophonic system located in a professional listening room designed to emulate acoustically typical listening conditions encountered 5 in a living room in a home. The listening room was roughly equivalent to the size of a large living room with randomly distributed reflective and non-reflective surfaces around the walls and with carpet on the floor and with some items of highly absorbent furniture. The 10 acoustic was relatively "dead" as in a living room containing soft furnishings and curtains. The second experiment was conducted using mono equipment 15 in a different professional listening room half the size of that used in the first mentioned experiment above and having walls which absorb frequencies above 10,000Hz. Like the first room, the acoustic was relatively "dead". Mono equipment was used. 20 Mono equipment was also used in the third experiment to be described. The loudspeakers were positioned in a particularly unfavourable acoustic at the intersection of two corridors each about 2 metres wide and extending 25 at right angles to each other. Listening tests were conducted with the subjects in one of the corridors about 3 metres from the loudspeakers with the loudspeakers oriented to face into the corridor in which the listeners were located. 5 In all experiments, the subjective listening tests disclosed a perceived improvement in spaciousness when the distributed mode and cone loudspeakers were driven simultaneously as compared to when the cone loudspeakers 10 were driven alone or the distributed mode loudspeakers were driven alone. In order to determine whether or not the perceived increase in spaciousness was merely subjective or whether it had a scientific basis, the Lateral Early Energy Fraction (LEF) and the Inter Aural 15 Cross Correlation Coefficient (IACC) were measured in accordance with conventional measurement techniques in the first and second experiments. The nature of the LEF and IACC is explained more fully below. 20 The subjective quality of the sound produced by the different equipment used in the experiments was also considered in the listening tests. In the first expe-riment (stereophonic) the sound pressure levels of the distributed mode and cone loudspeakers were set to 25 be substantially the same value at the measuring microphone and it was subjectively considered that the quality of the sound from the combined loudspeakers was better than the quality of sound produced by the cone loudspeakers alone or by the distributed mode 5 loudspeakers alone. In the second experiment, the sound pressure level of the distributed mode loudspeaker was set to be 4.2 decibels less than the sound pressure level produced by the cone 10 loudspeaker (as measured at the microphone). This figure was selected after trying a number of different relative sound pressure levels. The quality of the sound produced by the combined loudspeakers was also considered to be better than that produced by the cone loudspeakers alone 15 or the distributed mode loudspeaker alone. In the third experiment, the effect of varying the relative sound pressure levels of the distributed mode and cone loudspeakers on the subjective quality of the 20 sound was tested and it was found that driving the distributed mode loudspeakers at 5 decibels +/-3 decibels less than the cone loudspeakers gave optimum perceived quality and this quality was found to be better than that perceived when listening to the distributed mode 25 loudspeaker or to the cone loudspeaker alone.

The third experiment also included measurements to determine the frequency responses of the two loudspeakers alone and the frequency response of the combined loudspeaker. Frequency responses of the combined 5 loudspeaker were determined for a number of different relative levels at which the loudspeakers were driven. It was found from these experiments and calculations that frequency response curves for the combined loudspeakers could be obtained which were smoother than the frequency 10 response of the distributed mode loudspeaker alone. Each of the three experiments will now be described in more detail. 15 Details of First Experiment (Stereo) The first experiment was conducted under the following conditions and with the following equipment: 20 (a) The room was a typical dry room with extra absorption panels randomly placed on the sides and back wall. The room size was 6.87m long x 4.6m wide x 2.79m high = volume of 88.17 cubic meters. 25 (b) The loudspeakers and Microphone were located at apices of an equilateral triangle. The distance between loudspeakers and microphone was 2 Meters (each side of the triangle = 2 meters). The loudspeakers were positioned spaced from the walls 5 of the room. (c) The loudspeakers consisted of two conventional cone loudspeakers and two distributed mode loudspeakers. 10 (d) The conventional cone loudspeakers were the Genelec model 1029A 40W monitors with integrated amplifier. Technical data: Bass5" drive unit; Treble 3/4" metal dome drive unit; Crossover frequency is 3.3kHz; frequency response 70 - 18,000 Hz. 15 (e) The distributed mode loudspeakers were manufactured by Amina Technologies Limited (referred to above) and comprised 610mm x 492mm aluminium core, polyester skin, 4 exciter (10w/exciter). Each was 20 a 40w open back panel. The frequency response was 80Hz to 20 KHz. (f) 'For the LEF measurements, a CALREC Soundfield microphone model ST250 was used. For the IACC 25 measurements used a B+K Head and Torso Simulater * 24 (HATS) microphone model 4100 was used. (g) For these measurements, a Maximum Length Sequence signal was used, the Impulse Response was 5 extracted, and spatial measurements performed on the Impulse Response. The software used was the P.C. version of Cool Edit Pro with the Aurora Plugins. 10 (h) The distributed mode loudspeaker and the cone loudspeaker were driven to provide substantially equally sound pressure levels at the microphone. A listener positioned in a listening space, such as a 15 concert hall receives sound energy both directly from the source (for example an orchestra) and after reflection at the boundaries of the listening space. The proportion of the total energy received by the listener which is received after reflection from the boundaries of the 20 listening space within a period of approximately 50 milliseconds after receipt of the direct sound is known as the Lateral Early Energy Fraction (LEF). Since the higher frequencies are absorbed more than the lower frequencies and also generally contain less energy than 25 the lower frequencies, most of the reflected energy is in the lower frequency ranges. LEF is conventionally measured in a number of frequency bands, particularly the following bands: 5 88 to 176Hz (known as the 125 octave band) 176 to 353Hz (known as the 250 octave band) 353 to 707Hz (known as the 500 octave band) 707 to 1,414Hz (known as the 1,000 octave band) 10 It is widely considered that, within limits, the higher the LEF in these frequency bands the greater the impression of spaciousness, in the environment of a concert hall. Since the object of the invention is to create an improved impression of spaciousness, 15 measurements of the LEF of the combined sound field created by driving the distributed mode loudspeakers and cone loudspeakers simultaneously were made and compared to measurements of the LEF when the distributed mode and cone loudspeakers were driven separately. 20 Figure 1 is a bar chart showing the results. As can be seen in that figure, the vertically hatched bars 500a, 500b', 500c and 500d respectively show the LEF for the cone loudspeakers alone in each of the above defined 25 octave bands. Horizontally hatched bars 502a to 502d show the LEF for the distributed mode loudspeakers alone in the same octave bands. Cross hatched bars 504a to 504d show the LEF with the combined distributed mode and cone loudspeakers in accordance with the present 5 invention. As can be seen in figure 1, the LEF for the sound field produced by the combined distributed mode and cone loudspeakers in accordance with the invention is markedly 10 higher in each of the octave bands than the LEF in the sound field produced from the cone loudspeaker alone. Thus, figure 1 shows that the subjectively perceived marked increase in the impression of spaciousness achieved with the combined loudspeakers in accordance 15 with the invention compared to the cone loudspeakers is consistent with the increase in LEF in the octave bands referred to which, in turn, is consistent with the widely held opinion that increasing the LEF in these frequency bands provides an increase in the impression of 20 spaciousness in the concert hall environment. In other words, the sound field generated by the combined loudspeakers in accordance with the invention in the first experiment differs from the sound field generated by the cone loudspeakers alone in that then is an 25 increase in the value of an important parameter (LEF) wh-ich is considered to be associated with improved impressions of spaciousness. It is considered that this is confirmation that the increase in spaciousness perceived in the subjective listening tests referred to 5 above arises from the creation of a physically different sound field when the combination in accordance with the invention is used as compared to that with cone loudspeakers alone. 10 It can also be observed that in the 125 and 250 octave frequency bands the magnitude of the difference in LEF between the combined loudspeakers and the cone loudspeakers is substantially greater than the difference in LEF in the 500 and 1,000 octave frequency bands. This 15 characteristic is associated with increased warmth in the sound. Expressed differently, if the magnitude of the difference in LEF in each of the four octave frequency bands shown in figure 1 were the same, this would be indicative of an increase in spaciousness but not an 20 increase in the warmth of the sound. It will also be noted from figure 1 that the LEFs of the distributed mode loudspeakers alone are greater than those of the cone loudspeakers alone in all four 25 frequency bands. In the 125 and 250 octave frequency bands the LEFs of the distributed mode loudspeakers alone are less than those of the combined loudspeakers. In the 500 and 1,000 octave bands the LEFs of the distributed mode loudspeakers alone and the combined loudspeakers are 5 of a similar magnitude, the differences shown in these octave bands being almost certainly, of themselves, imperceptible. These results suggest, as confirmed by the subjective listening tests, that the increase in spaciousness using the combined loudspeakers in 10 accordance with the invention compared to the distributed mode loudspeakers is not so great as compared to the cone loudspeakers alone and that the distributed mode loudspeakers provide greater spaciousness than the cone loudspeakers. However, the importance of the invention 15 is in achieving a substantial increase in spaciousness over that obtained with cone loudspeakers alone whilst maintaining fidelity. It will be recalled that distributed mode loudspeakers are not yet in wide use in applications requiring the highest levels of fidelity. 20 The Inter Aural Cross Correlation Coefficient (IACC) is a measure of the degree of correlation between the sound pressure signals received in the two ears of a listener. IACC is consequently conventionally measured using a 25 dummy head having apertures in it in the positions of the human aural passages and a small microphone in each aperture. It is widely considered that low values of this correlation coefficient in one or more of the 1,000, 2,000 and 4,000 octave frequency bands is indicative of 5 increased spaciousness, the values of these bands being as follows: 1,000 band: 707 to 1,414Hz (as already indicated above) 2,000 band: 1,414 to 2,825Hz 10 4,000 band: 2,825 to 5,650Hz Figure 2 is a bar chart showing the results of IACC measurements performed in the first experiment. In figure 2, the vertically hatched bars 506a to 506d, the 15 horizontally hatched bars 508a to 508d and the cross hatched bars 510a to 510d show respectively the IACCs of the cone loudspeakers alone, distributed mode loudspeakers alone and the combination of both in accordance with the invention, in each of four octave 20 bands, namely the 500 octave band (described above with reference to LEF) and the 1;000, 2,000 and 4,000 octave bands. As can be readily seen from the drawing, the value of the IACC for the combined loudspeakers in accordance with the invention is markedly smaller than 25 that of the cone loudspeakers alone in the 1,000, 2,000 * 30 and 4,000 octave bands. This, as with the LEF values, again is indicative of an increase in spaciousness in the sound because as already explained, the lower the value of this coefficient in these frequency bands the greater 5 the impression of spaciousness. Although figure 2 includes IACC measurements in the 500 octave band, these are in fact not relevant because, as will be appreciated, the wave length of the sound energy 10 in this frequency band is such that a high correlation factor is to be expected in the energy received by the two ears of a listener. Thus, it can be seen that both LEF and IACC measurements 15 on the stereophonic equipment used in the first experiment are consistent with the subjective impression of spaciousness observed in the listening tests. Details of Second Experiment (Mono) 20 The second experiment was conducted under the following conditions and with the following equipment: (a) The room was acoustically treated to absorb sound 25 above 10,000Hz - average Reverberation Time 0.3 Sec.

3' The room size was 5.6m long x 3.2m wide x 2.4m high = volume of 43.28 cubic meters. (b) The microphone in these measurements was located 1.5 5 meters on axis from the loudspeakers. The front surface of the distributed mode loudspeaker was approximately in vertical alignment with the back surface of the casing of the conventional cone loudspeaker. 10 (c) The cone loudspeaker was a Tannoy Near Field Dual Concentric Monitor Model 6NFM Mark II having a frequency response of 44Hz to 20 KHz. The advantage of this unit is that the HF and LF are at the same 15 axis point for measurements. The distributed mode loudspeaker panel used was manufactured by Amina Technologies Limited and comprises a 500 x 700mm resin dipped paper honeycomb core and skin with 4 exciters (10w/exciter). It was a 40w open back 20 panel with a frequency response of 80Hz to 20KHz. (d) For the LEF measurements an AKG C34 Variable Polar - Microphone was used. For the IACC measurements, a Neuman Binaural Head was used. 25 (e) Again the Impulse Response and the spatial data was extracted from a Maximum Length Signal. The software used was the MLSSA P.C. software. 5 (f) Using a separate amplifier, the DML panel was set 4.2dB lower than the Tannoy. Figures 3 and 4 are bar charts showing the LEFs and IACCs measured in the second experiment. The hatching of the 10 bars has the same significance as in figures 1 and 2. Thus, bars 512a to 512d in figure 3 and bars 518a to 518d in figure 4 represent respectively the LEFs and IACCs for the cone loudspeakers alone; bars 514a to 514d in figure 3 and bars 520a to 520d in figure 4 represent the LEFs 15 and IACCs for the distributed mode loudspeaker alone; and bars 516a to 516d and 522a to 522d in figures 3 and 4 respectively represent the LEFs and IACCs for the combined digital mode and cone loudspeakers in accordance with the invention. As in figures 1 and 2, each bar in 20 figures 3 and 4 represents the relevant value in a specific octave frequency band, as indicated in the drawings. Examination of figure 3 will show that the LEF values for 25 the combined loudspeakers in accordance with the S 33 invention are higher in each octave frequency band than the LEFs for the conventional cone loudspeaker alone and for the distributed mode loudspeaker alone. This is consistent, as previously explained, with the perceived 5 increase in spaciousness observed in the subjective tests. The fact that the magnitude of the difference between bars 516b and 512b is greater than the magnitude of the difference between bars 516c and 512c (LEF values in the 250 and 500 octave bands) is indicative of a 10 marked improvement in the warmth of the sound. Figure 4 shows that in the 1,000 and 4,000 octave bands, the IACC of the combined loudspeakers in accordance with the invention is significantly lower than that of the 15 cone loudspeakers alone. This is consistent with the observed increase in spaciousness, However, in the 2,000 octave band the IACC values 518c and 522c are almost the same. As described with reference to figure 2, the IACC values shown in figure 4 of the 500 octave band are not 20 relevant. Figures 3 and 4 together are consistent with the surprising and marked increase in the impression of spaciousness perceived in the subjective tests with the 25 mono system used in the second experiment.

Details of Third Experiment (Mono) This experiment was performed in an environment, and with equipment, different from the environment and equipment 5 of the first and second experiments described above. The experiment consisted of subjective tests using a single channel (mono) with the loudspeaker in an unfavourable acoustic (as already described above), and frequency response measurements performed in an anechoic chamber. 10 Measurements of LEF and IACC were not carried out in this experiment. The conventional cone loudspeaker used throughout these measurements was a JBL LSR32 passive studio monitor comprising three drive-units covering a frequency range 15 of approximately 30Hz to 20kHz. The DML was manufactured by Amina Technologies Ltd. The DML panel comprised a resin dipped paper honeycomb core with a resin impregnated fibreglass skin and measured 60cm x 60cm. There were four electromagnetic exciters and the 20 frequency response was 80Hz to 20KHz. The DML panel was placed on top of the conventional loudspeaker and attached with strong double-sided adhesive tape. Pink noise was used as the test signal for all measurements ensuring adequate signal-to-noise ratios 25 at all frequencies of interest (pink noise is a broadband *3 random signal which contains equal signal energy per octave of bandwidth). The output signal from the signal generator was connected to the inputs of a two-channel power amplifier via a pair of universal filter sets to 5 allow tailoring of the frequency range of the signals fed to each loudspeaker. The relative output levels of the two loudspeaker were adjusted using a switchable attenuator in line with the drive to the amplifier powering the conventional loudspeaker. 10 On-Axis Frequency Response Figure 5 shows the magnitude of the on-axis frequency response of the conventional cone loudspeaker, and Figure 15 6, that for the DML panel. The low-frequency response of the DML panel was cut below 100Hz in accordance with the manufacturer's recommendations. An in-line attenuator was adjusted to give approximately the same on-axis level from the two loudspeakers in the range of frequencies 20 from 500Hz to 5kHz. The frequency responses shown in Figures 5 and 6 were summed on a computer both with and without regard for phase. A comparison between the resultant frequency responses and a measurement of the frequency response 25 obtained when the loudspeakers were combined clearly showed that phase addition, and hence interference, occurs between the two loudspeakers. It follows that, surprisingly, the frequency response of the combined output of the two loudspeakers can be accurately 5 determined by summing the individually-measured responses with regard for phase. This being the case, it is possible to determine the response of the combination with arbitrary relative output levels. Figure 7 shows the frequency response of the combination, determined by 10 summing, on a computer as just described, with the level of the DML varied from -12dB to +12dB in 3dB steps relative to that of the conventional loudspeaker. Subjective Appraisal 15 Subjective tests were carried out on the two loudspeakers using the same equipment as used in the measurements, but in a semi-reverberant space (intercepting corridors as described above) instead of the anechoic chamber. 20 All listening was carried out in single-channel mono using a variety of commercially available music recordings played from compact discs. First a marked change in spaciousness was perceived as the DML panel was 25 switched in and out. The subjects agreed that the 3~S addition of the panel improved the feeling of spaciousness compared to the conventional cone loudspeaker alone. Second, the relative levels of the two loudspeakers were varied until the subjects agreed on an 5 optimum level for the perceived improvement. This was carried out using the 'balance' control on the amplifier to maintain a constant overall level. The subjects agreed on an optimum level setting of -5dB output from the DML panel relative to the level used in the above 10 measurements (equal level from 500Hz to 5kHz). The subjects agreed on this value within ±3dB. A further test involved establishing thresholds of relative level, beyond which no change in sound could be 15 detected as one loudspeaker was switched in and out while the other remained constant. The levels were quickly determined as about -35dB detection threshold for the DML panel and -20dB for the conventional loudspeaker. 20 Discussion The frequency response measurements clearly show that the DML panel has a less smooth frequency response than the 25 conventional cone loudspeaker. This is not entirely S surprising when the different radiation mechanisms are considered. The DML panel also suffers a pronounced dip in response at around 7kHz. What is surprising, however, is the degree to which the DML panel and conventional 5 loudspeaker interfere. DML panels radiate sound the way they do because the vibration field over the surface of the panel is approximately diffuse in nature. Therefore one may quite reasonably expect there to be no particular phase associated with the radiated sound field; however, 10 the results of these measurements show that, at least at a single point in space and over a narrow frequency band (but for all audio frequencies), the panel has a measurable and repeatable phase response which gives rise to constructive interference with the sound radiation 15 from another loudspeaker. Figure 7 shows the frequency response of the combined output of the two loudspeakers when the level of the DML panel is varied relative to that of the conventional cone 20 loudspeaker. As could be expected, low levels of DML output have little effect on the response of the conventional cone loudspeaker and high relative levels show- the response dominated by that of the panel. The most interesting point to note about this figure however, 25 is the relative level at which the otherwise smooth response of the conventional cone loudspeaker is upset by interference from the output of the DML. The f igure shows that for relative levels above -3dB, the response is, in this experiment, adversely affected by the panel, 5 but for lower levels, this is not the case. This is in accordance with the subjective observation that a relative level of -5dB is about optimum for preferred sound quality; it is possible that for higher relative levels of DML output the improvement in spaciousness is 10 a trade-off against a poorer frequency response. Summary of Conclusions from this Experiment As with the first and second experiments, the subjective 15 tests indicated a marked increase in the impression of spaciousness, this being in a monaural system with the loudspeaker in an unfavourable acoustic. The frequency response measurements and calculations indicated that the combined loudspeakers in accordance with the invention 20 may achieve, with appropriate levels of driving signal, a frequency response significantly better than the distributed mode loudspeaker alone. PRACTICAL EMBODIMENTS 25 S +o As will be appreciated from the above discussion of the experiments which have been conducted, both the subjective tests and the scientific measurements, the invention provides a practical and simple solution to 5 the problem of enhancing the spaciousness of sound produced by loudspeakers, in particular making it possible to utilise a high quality wide frequency range pistonic loudspeaker and enhance the spaciousness of the sound produced, without losing the high fidelity of the 10 sound, by the addition of distributed mode loudspeakers and without the need for additional signal channels or complex signal encoding. In putting the invention into practice, the distributed 15 mode and pistonic loudspeakers must operate over a common part of the audible frequency band as specified * in claim 1. Preferably, both loudspeakers operate over substantially the whole of the audible frequency band, for example the pistonic loudspeaker may operate from 20 20Hz to 20KHz and the distributed mode loudspeaker may operate from about 100Hz to 20KHz or, when the art of distributed mode loudspeakers is further developed, both loudspeakers might operate over the whole of the audible frequency range i.e. 20Hz to 20KHz. It is within the 25 scope of the invention for narrower frequency bands to be used dependent upon circumstances, such as cost and intended use. For example, the distributed mode loudspeaker could be restricted to, for example, a plurality of octaves within the range up to 4,000Hz or up to 6,000Hz. By way of a further specific example, 5 the frequency band of the distributed mode loudspeaker or loudspeakers might be from 100 to 6,000Hz and the frequency band of the pistonic loudspeakers might be from 800 to 8,000KHz. 10 As a further alternative, provided the distributed mode and pistonic loudspeakers operate over a common part of the frequency range as specified in the claims, the highest frequency of the pistonic loudspeakers might be substantially lower than the highest frequency of the 15 distributed mode loudspeakers so that in the higher frequencies the tweeter is constituted by the * distributed mode loudspeaker. In general, therefore, the frequency range of the 20 distributed mode loudspeaker can be wider than or narrower than that of the pistonic loudspeaker; the lower extremity of the frequency range of the distributed mode loudspeaker may be lower than or higher than the lower extremity of the frequency range of the 25 pistonic loudspeaker; and the upper extremity of the frequency range of the distributed mode loudspeaker may be lower than or higher than that of the pistonic loudspeaker. The best frequency ranges for the pistonic and distributed mode loudspeakers may be determined by experiment dependent upon applications and costs and 5 particular requirements for different uses or different markets. A number of practical embodiments of the invention will be described below. In these embodiments specific 10 frequencies will not be given. These should be taken to be selected from the ranges foreshadowed in the above discussion dependent upon the circumstances of the particular product to be designed in accordance with the description of the embodiments. 15 It should also be understood that tests (not referred to in the above description of the experiments) have indicated that the introduction of a delay between the output of the pistonic and distributed mode loudspeakers 20 may be employed and in particular the signals applied to the distributed mode loudspeakers may be delayed relative to those applied to the pistonic loudspeakers. Where a delay is applied, it should in general never exceed 80msecs and preferably it should not exceed 25 35msecs. The introduction of such delays has been found in informal tests to enhance the "imaging" or "localisation" of the sounds in stereophonic applications or surround sound applications. Such improvements in imaging and localisation improve the musical 5 intelligibility, when music is listened to. It is considered that this improvement arises because, as a result of the delay, the transients in the sounds from the pistonic loudspeakers are not masked by the 10 transients from the distributed mode loudspeakers but the transients from the distributed mode loudspeakers are masked by the transients from the pistonic loudspeakers. 15 Although in the above discussion in relation to LEF and IACC and in relation to the effect of delays theoretical explanations underlying the effects have been given, it should be understood that the applicant is not bound by these explanations. The explanations given are based 20 upon current knowledge and measurement techniques in the audio field but it is well recognised that the psychoacoustic effects in human beings of the sounds to which they are subjected is an extremely complex subject and accurate explanations are difficult (or in some 25 situations impossible) to give.

Embodiments of the Invention in Digital Pianos It is well known that even the best quality digital pianos which are currently available produce a sound 5 which is relatively unmusical. Embodying the teachings of the present invention in digital pianos makes it possible to provide a much more realistic musical sound from a digital pianos. Specifically, digital pianos according to the invention may produce highly enriched 10 sounds with random components which, provided the piano is constructed to an adequate quality, give to the listener the impression that the sounds produced are close in quality to the sounds of a high-quality acoustic grand piano. This may be achieved at much *15 lower cost than that of a top-quality acoustic piano. Grand Piano Figure 8 shows in perspective view a digital grand piano according to an embodiment of the invention. The piano 20 comprises a casing 200 supported on legs 201 and having a conventional hinged lid 202 shown supported in the open position by a conventional stay 203. A keyboard 204 of the kind conventionally used in digital pianos and a set of pedals 205 performing the normal functions 25 are supported in the usual positions by the casing 200.

The piano preferably includes (as is already known in the art of digital pianos) a conventional high-quality grand piano action (not shown) connected to the keyboard for the purpose of providing the player with the feel of 5 a good-quality concert grand. The casing 4 supports two loudspeaker assemblies 210 and 212, each comprising a tweeter 216, a mid-range unit 214 and a sub-woofer 218, arranged respectively in alignment 10 with the treble, mid-range and base portions of the keyboard. Each of the loudspeakers 216, 214 and 218 is a conventional electro-magnetically driven cone loudspeaker arranged so that the cones face vertically upwardly and the sounds produced thereby are reflected -15 by the lid of the piano. Although this orientation is particularly preferred for the tweeter and mid-range loudspeakers, the orientation of the sub-woofers is not particularly significant. 20 The casing 4 also supports two distributed mode loudspeakers 220 and 222 which are positioned horizontally so as to radiate sound upwardly and downwardly. By way of example, the panels of the distributed mode loudspeakers 220, 222 might measure 700 25 x 500 mm each.

W L5384010.02 It should be understood that the positioning of the loudspeakers shown in Fig. 8, with the conventional loudspeakers grouped near to the keyboard and the 5 distributed mode loudspeakers remote from the keyboard, is merely exemplary. The conventional and distributed mode loudspeakers may alternatively be interleaved with each other or the positions reversed relative to that shown in the drawing. Also, the tweeters, mid-range 10 units and sub-woofers of the conventional loudspeaker assemblies may be separated and interleaved with the distributed mode loudspeakers. As shown in Fig. 9, signals from foot pedal switches 206 15 and a midi signal generated in response to signals from infrared pickups 207 (not shown in Fig. 8) arranged beneath the keyboard 204 of the piano are passed to a first audio signal generator 230 and a second audio signal generator 231 both arranged to respond to the 20 MIDI signals and the foot pedal switches 107 for generating audio signals replicating the sound of a grand piano. The first and second generation 230, 231 may be implemented by means of a conventional computer containing known software for this purpose. 2rD 5384010.02 The first audio signal generator 230 in this embodiment comprises a first high quality sample library 232 and a first sound module 234. The second audio signal generator comprises a second high quality sample library 5 237, a second sound module 238 and a physical modelling unit 239. The respective libraries 232 and 237 comprise digital samples recorded from different high quality grand pianos preferably concert grands. Thus for example the first sound sample library 232 might 10 comprise a 1.6 Gb Steinway sample library and the second sample library .15 20 25 237 might comprise a Yamaha 30 Mb sample sound library. The sound modules 234, 238 comprise software programs for selecting sound samples from the sound libraries 232, 237 in response to received MIDI signals and signals from the 5 foot pedal switches 107. As the second sample library 237 is (in this embodiment) significantly smaller than the first sample library 232, the second audio signal generator 231 also includes a physical modelling unit 239 arranged to modify, in a conventional manner, the sound 10 samples selected by the second sound module 238. The audio signal from the generator 230 is amplified by an amplifier 240 and the amplified signal drives the two to the two distributed mode loudspeaker 220, 222. 15 The audio signal from the generator 231 is amplified by an amplifier 245 and supplied to crossover units 247 to drive the two conventional speakers 210, 212. The crossover units 247, in a conventional manner, pass high 20 frequency, mid range and low frequency signals to the tweeter 216, woofer 214 and base 218 of the conventional speakers 210, 212 respectively. The cone loudspeakers reproduce sound over substantially 25 the whole of the audio frequency range, say from 20 Hz to 20 kHz or from 45Hz to 20KHz. The distributed mode loudspeakers produce sound over as much of that range as practical, say 80Hz to 20KHz or 100 Hz to 20 kHz. The sound pressure levels provided by the distributed mode 5 loudspeakers may be adjusted to be less than those produced by the conventional cone loudspeakers, again as previously described or dependent upon the acoustics of the auditorium or room in which the piano is played, or the output of the distributed mode loudspeakers may have 10 substantially the same sound pressure level or even higher sound pressure level than that of the conventional cone loudspeakers. To enable the sound pressure levels of the distributed mode loudspeakers to be varied independently of the sound pressure levels of the 15 conventional cone loudspeakers, independent volume controls for the distributed mode and cone loudspeakers are preferably provided although these are not shown in the drawings. 20 When the piano is played, the distributed mode loudspeakers and the conventional loudspeakers are driven simultaneously. As a result, the different air disturbance patterns which are propagated respectively by the distributed mode loudspeakers and conventional cone 25 loudspeakers combine to produce air disturbance patterns D having a complexity and richness, arising from randomly varying interactions between the different patterns, to provide a substantially richer sound than could be produced by either type of loudspeaker individually. 5 This richness is further enhanced in that the signals used for driving the distributed mode loudspeakers differ from those used for driving the conventional loudspeakers. 10 Furthermore, as explained with reference to the above described experiments, improvements in spaciousness are achieved by the combination of distributed mode and conventional cone loudspeakers. 15 Although in this embodiment samples from two different models of grand piano are employed, further richness may be achieved by utilising samples from three or more different grand pianos in which case different ones of the loudspeakers might be driven bu signals derived from 20 respective different sets of samples. Further, samples other than Steinway and Yamaha samples may be used and the sound libraries which are employed may be, and for the highest quality instruments preferably are, such that they both contain the maximum number of samples available 25 having regard to the current state-of-the-art. More specifically, as computer memory increases in capacity and reduces in cost, it is possible to provide sound libraries containing more and more samples and therefore possibilities for better and better quality of sound. S Digital Upright Piano With reference to Fig. 10, digital upright piano, which may produce a sound of lower quality than that produced by the piano of figures 26 and 27 but which may be of 10 lower cost, comprises a casing 251 having back panel 252 supporting a pair of distributed mode loudspeakers 253 and also a pair of conventional cone loudspeakers 254. In this embodiment in contrast to the previous embodiment only two cone loudspeakers are provided, in 15 order to reduce cost. The distributed mode loudspeakers 253 are provided oriented parallel to the plane of the back panel 252 of the casing 251. The cones of the two conventional speakers 254 are oriented with the axis of the cone perpendicular to the plane of the back panel 20 252. Figure 11 is a schematic block diagram of the piano of Figure 10. In contrast to the previous embodiment, only a single audio signal generator 230, comprising a sound 25 library 262 and a sound generation module 260, is provided to save costs. The generator 230 generates an audio signal using the sound library 262 on the basis of the received signals from the infrared pickups 207 and foot pedal switches 207. 5 As in the previous embodiment the distributed mode loudspeakers 253 and conventional cone loudspeakers 254 are arranged so as to be driven simultaneously through amplifiers 245, 247 so that the electronic piano is 10 caused to create an air disturbance pattern which is the combination of sound output by the distributed mode loudspeaker 253 and the conventional loudspeakers 254, thereby more closely emulating the propagation of sound generated by an acoustic instrument as explained above. 15 Further, although, as is clear from figure 11, no crossover circuits are included since it is assumed that the two cone loudspeakers 254 are identical and thus have a relatively restricted frequency range, the frequency range of the distributed mode loudspeakers 20 should overlap the frequency range of the conventional cone loudspeakers as far as possible to enhance spaciousness of the sound as discussed above in connection with the experiments. However, it will not be possible for the quality of the sound produced by the 25 embodiment of figures 10 and 11 to be as good as that produced the in the embodiment of figures 8 and 9 although, with appropriate quality of components, even the embodiment of figures 10 and 11 should be capable of achieving greater overall quality than many currently 5 available digital pianos. With a view to providing some further improvement, a modification to the circuit of figure 11 is shown in figure 12. In this, the audio signal generated by the 10 generator 230 260 is passed to a signal modification unit 265 such as a digital signal processing unit, which generates a modified signal that is passed to the amplifier 247 which drives the cone loudspeakers 254. The signal modification unit 265 may be arranged to 15 alter the timing and timbre of the audio signal output by the sound generation unit 260. This signal modification unit 265 includes a conventional user interface (not shown) which enables a user to select the manner in which signals output by the sound generation 20 unit 260 are modified. In this way, the richness of the sound may be enhanced to some degree because the qualities of the signal which drive the cone loudspeakers differ slightly from the qualities of the signal which drive the distributed mode loudspeakers. If 25 desired, a further signal modification unit could be interposed between the sound generation module 260 and the amplifier 245 which drives the distributed mode loudspeakers to introduce further richness. 5 Although in the previous three embodiments, sound has been described as being output through pairs of distributed mode loudspeakers and pairs of conventional loudspeakers, it will be appreciated that a similar random mixing of air disturbance patterns could be 10 achieved by outputting sound simultaneously corresponding to the same notes through a single distributed mode loudspeaker and a single cone loudspeaker. 15 Although in previous embodiments an infrared motion detection system for detecting the motion of keys has been described utilizing infrared motion detection, other means may be used to detect the depression keys for example an electromechanical motion detection system 20 could be used to detect the position, pressure and velocity of key activation. Auxiliary Unit for Digital Piano Many people already own digital pianos. Figure 13 25 illustrates a conventional digital piano of 300 connected to an auxiliary unit 302 to form a piano which embodies the present invention. As seen in Figure 13, the auxiliary unit 302 comprises a 5 distributed mode loudspeaker 304 driven by an amplifier 306 which receives signals from an audio signal generator (which is as previously described) connected via a cable 310 and a plug 312 to the MIDI signal output 314 conventionally provided on currently available 10 digital pianos. The conventional digital piano 300 includes a three-way conventional cone loudspeaker system 316 comprising a woofer 318, a mid-range unit 320 and a tweeter 322 15 driven through conventional crossover circuits (not shown) and thereby operable to produce substantially the full audio frequency range of from, say, 20 Hz to 20 kHz. The distributed mode loudspeaker 304 is operable to produce frequencies over a substantial part of the 20 frequency range produced by the loudspeaker system 316, for example 100 Hz to 20 kHz. Although not shown in Figure 13, the conventional digital piano 300 operates using a sound library of 25 samples recorded from, typically, a good-quality concert grand. The audio signal generator 230 also contains a sound library preferably containing samples recorded from a different model of good-quality concert grand, for the reasons explained in relation to the embodiment 5 of figures 8 and 9. The auxiliary unit 302 may be made and sold separately from digital pianos so that it may be connected to an existing digital piano owned by the purchaser. By 10 simply connecting the auxiliary unit 302 to the existing MIDI output of the digital piano and ensuring that the volume control of the digital piano is set at a level so .* that sound is produced by the conventional speaker system 316 in addition to sound being produced by the 1.5 distributed mode loudspeaker 304, the benefits of the invention can be achieved. VARIATIONS AND MODIFICATIONS In addition to digital pianos, the invention may be 20 employed for reproducing sound from other electrical or electronic musical instruments, such as electric guitars. Although in each of the above described and illustrated 25 embodiments, conventional cone loudspeakers have been employed it is possible instead to use, at least in certain circumstances, alternative forms of pistonic loudspeaker, such as an electrostatic loudspeaker or a piezo electric loudspeaker comprising a flat panel or 5 membrane mounted for vibratory motion and driven by a piezo electric transducer. However, in most circumstances electromagnetically driven cone loudspeakers will be preferred. 10 The introduction of a delay in the signal applied to the distributed mode loudspeakers relative to the signal applied to the pistonic loudspeakers has been discussed above. This may be provided in any of the embodiments described with reference to the drawings and in any 15 other embodiments. Preferably, where a delay is provided, this will be adjustable by the user to suit the circumstances in which the invention is to be deployed. 20 Further, additional similar processing of the signals applied to the distributed mode and/or pistonic loudspeakers could be provided, for example to provide reverberation, equalisation, or other effects. 25 Various preferred frequency ranges and relative output levels have been given in the above description. It should be understood that these are examples only and that many variations are possible. It is, however, important that the frequency range of the distributed 5 mode loudspeaker or loudspeakers should overlap the frequency range of the pistonic loudspeaker or loudspeakers as defined in claim 1. Further, although in many instances, by arranging for the sound pressure level of the distributed mode loudspeaker to be slightly 10 less than that of the cone loudspeakers, e.g. just a few decibels difference, improvements are achieved where the distributed mode loudspeaker produces a substantially lower sound pressure level than the cone loudspeakers provided, of course, that the pressure level of the 15 distributed mode loudspeakers is not so low that the sound produced thereby is imperceptible. In the experiments, distributed mode loudspeaker sound pressure levels of as low as -35 decibels relative to the cone loudspeakers have still achieved perceptible, but at 20 this level only marginal, improvement. In certain situations it may be desirable for the sound pressure level produced by the distributed mode loudspeakers to be greater than that produced by the 25 pistonic loudspeakers.

Claims (33)

1. An electrical or electronic musical instrument comprising playing means operable by a player for selecting musical notes to be sounded; electronic 5 signal generator means responsive to operation of the playing means for generating electrical signals corresponding to the selected musical notes; and a loudspeaker arrangement which is responsive to said electrical signals for producing the selected musical 10 notes; wherein said loudspeaker arrangement: (a) comprises distributed mode and pistonic loudspeaker means each of which is arranged for - producing at the same time the same musical notes when the playing means is operated; and 15 (b) is such that each said loudspeaker means is operable to generate sound, when the playing means is operated, with a frequency response which, measured under anechoic conditions, comprises an operating range and roll-off ranges respectively above and below 20 the upper and lower ends of the operating range, said operating ranges of said distributed mode and said pistonic loudspeaker means overlapping over an overlapping range which comprises a plurality of octaves at least one of which is in the frequency 25 range made up of the 1 kHz to 4 kHz octave bands.

2. An instrument according to claim one, which is a keyboard instrument, said playing means comprising a keyboard. 5

3. An instrument according to claim 2, which is a digital piano.

4. An instrument according to claim 3, in which said .10 electronic signal generator means comprises a digital library of piano sounds from which said electrical -, ; signals are derived.

5. An instrument according to claim 3, in which said electronic signal generator means comprises first and second digital libraries of piano sounds from which said electrical signals are derived, the piano sounds of said first and second digital libraries having audible qualities different from each other. 20

6. An instrument according to claim 3, in which said electronic signal generator means comprises first and second digital libraries of piano sounds, the first library comprising digital samples recorded from a 25 first acoustic piano and the second library comprising digital samples recorded from a second acoustic piano different from said first acoustic piano.

7. An instrument according to claim 5 or 6, in which, 5 when said playing means is operated, the sounds generated by said distributed mode and pistonic loudspeaker means are derived respectively from said first and second libraries. .10

8. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 500 Hz and . 1 kHz octave bands.

, 9. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 250 Hz, 500 Hz and 1 kHz octave bands.

10. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 125 Hz, 20 250 Hz, 500 Hz and 1 kHz octave bands.

11. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 1 kHz and 2 kHz octave bands. 25

12. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 1 kHz, 2 kHz and 4 kHz octave bands. 5

13. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 2 kHz and 4 kHz octave bands.

14. An instrument according to any of claims 1 to 7, ',0 wherein said overlapping range includes the 500 Hz, 1 kHz and 2 kHz octave bands.

15. An instrument according to any of claims 1 to 7, . wherein said overlapping range includes the 250 Hz, 500 Hz, 1 kHz, and 2 kHz octave bands.

16. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 125 Hz, 250 Hz, 500 Hz, 1 kHz and 2 kHz octave bands. 20

17. An instrument according to any of claims 1 to 7, wherein said overlapping range includes the 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, and 4 kHz octave bands. 25

18. An instrument according to any of claims 1 to 17, wherein said operating ranges of said distributed mode and pistonic loudspeaker means extend over substantially the same frequency range. 5

19. An instrument according to any of claims 1 to 17, wherein said operating range of said distributed mode loudspeaker means extends over a narrower frequency range than said operating range of said pistonic loudspeaker means. '410 ..0

.' 20. An instrument according to any of claims 1 to 17, -. : wherein said operating range of said distributed mode loudspeaker means is wholly within said operating - range of said pistonic loudspeaker means.

21. An instrument according to any of claims 1 to 20, wherein said loudspeaker means is operable to produce sound in which, at a position spaced from said loudspeaker means by a distance suitable for 20 listening, the sound pressure level produced by the distributed mode loudspeaker means is no greater than the sound pressure level produced by said pistonic loudspeaker means. 25

22. An instrument according to any of claims 1 to 20, wherein the loudspeaker means is operable to produce sound in which, at a position spaced from said loudspeaker means by a distance suitable for listening, the sound pressure level produced by the 5 distributed mode loudspeaker means is less than the sound pressure level produced by said pistonic loudspeaker means.

23. An instrument according to claim 22, wherein the 0 loudspeaker means is operable to produce sound in which, at a position spaced from said instrument by a distance suitable for listening, the sound pressure level produced by the distributed mode loudspeaker -. means is 5 +/- 3 decibels less than the sound pressure 15,; level produced by said pistonic loudspeaker means.

24. An instrument according to any of claims 1 to 20, including electrical signal attenuator means for attenuating the signals applied to the distributed 20 mode loudspeaker means relative to the signals applied to the pistonic loudspeaker means.

25. An instrument according to any of claims 1 to 20, including means for adjusting the relative sound 25 pressure levels of the distributed mode loudspeaker means and the pistonic loudspeaker means.

26. An instrument according to any of claims 1 to 25, including delay means for introducing a delay between 5 the output of the pistonic and distributed loudspeaker means.

27. An instrument according to claim 26, wherein said delay means is operable to introduce a delay to the '.10 sound produced by the distributed mode loudspeaker means relative to that produced by the pistonic loudspeaker means.

28. An instrument according to claim 27, wherein said delay means is operable such that said delay is not more than 80msecs.

29. An instrument according to claim 27, wherein said delay means is operable such that said delay is not 20 more than 35msecs.

30. An instrument according to any of claims I to 29, wherein said electronic signal generator means includes signal conditioning means for modifying 25 signals applied to said distributed mode loudspeaker relative to those applied to said pistonic loudspeaker means.

31. An instrument according to claim 30, wherein said 5 signal conditioning means includes equalising means.

32. An instrument according to claim 30 or 31 as dependent directly or indirectly upon claim 25, wherein said signal conditioning means includes said .10 relative sound pressure level adjusting means.

33. A digital piano substantially as hearing .* . described with reference to any of the accompanying , drawings. tooe