JPH0754996B2 - Electro-acoustic transducer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroacoustic transducer for radiating an electric signal in the audible frequency band into the air as an acoustic signal.

2. Description of the Related Art Currently, electrodynamic direct-radiation speakers and horn-loaded speakers are the mainstream electro-acoustic transducers, but in either method, a vibration wave in the air vibrates a diaphragm to create a compressional wave of air, which causes mechanical vibration. It converts energy into acoustic energy.

The present invention utilizes a completely different means from the acoustic transducer such as the conventional speaker, that is, the parametric action of the finite amplitude sound wave due to the non-linearity of air, but it is self-demodulated in the air by the parametric action. Sound wave (2
The next wave) is characterized by having a directivity pattern equivalent to that of a carrier sound wave in the ultrasonic range.

Then, an ultrasonic wave whose amplitude is modulated by a signal in the audible frequency band is radiated into a medium such as air or water at a finite amplitude level, and a self-demodulating action based on the nonlinear effect of air causes the medium to propagate in the medium. Various methods have already been reported as a parametric speaker for the method of using the demodulated sound wave generated in 1. Parametric loudspeakers that utilize this nonlinear phenomenon of sound waves have one characteristic in the sharpness of their directivity.However, in general, when the frequency of ultrasonic waves increases, the sound waves emitted from the transducer go straight in a beam shape. To do.

If it is assumed that ultrasonic waves that have been amplitude-modulated from a transducer array of radius α are radiated in the form of beams, then x
The sound pressure P at the point of distance can be expressed by the following equation.

(However, C _O is the speed of sound, α is the attenuation coefficient of the sound wave of angular frequency ω _O , P _O is the initial sound pressure, m is the degree of modulation, and g (t) is the modulated wave.) The sound pressure of a secondary wave generated by demodulating an ultrasonic wave of a finite amplitude level in air by a nonlinear parametric action is represented by the following non-homogeneous wave equation.

In the equation (2), P _S is the sound pressure of the secondary wave, ρ _O is the density of air, q is the virtual sound source density of the secondary wave generated in the primary wave beam, and this q can be expressed by the following equation.

Therefore, if the virtual sound source density at the point of distance x (on the axis) from the array is calculated from the equations (1) and (3), the following equation is obtained. The first term on the right side of the equation (4) represents the virtual sound source density based on the signal component, and the second term represents the virtual sound source density of the distortion component.

Furthermore, as a modulation method to reduce the distortion component of the secondary wave, There is.

this Adds a direct current component to the modulated signal Then, the modulated signal is multiplied by the carrier signal. In this case, the modulated signal can be expressed by the following equation.

Therefore, the sound pressure of the primary wave (modulated ultrasonic wave) at the distance x from the transducer array is Becomes The virtual sound source density of the secondary wave in this case is calculated by using the equation (3), Becomes Therefore, when this modulation method is used, the distortion component as shown in the second term on the right side of the equation (4) is reduced, and the quality of reproduced sound is significantly improved.

However, when the electroacoustic transducer using the parametric effect as described above is used as a speaker, a listener receives a high-power primary wave (ultrasonic wave), which is a safety problem.

The object of the present invention is to solve the problems that occur when the electroacoustic transducer utilizing the parametric effect as described above is used as a speaker, and more specifically, high power harmful to the human body. The purpose of the present invention is to provide an electroacoustic transducer that cuts ultrasonic waves and has a small secondary wave loss.

Configuration In order to achieve the above object, the present invention modulates a carrier signal in an ultrasonic frequency band with a signal from a signal source in an audible frequency band, amplifies the power, and then guides the ultrasonic wave to the ultrasonic transducer to generate the modulated wave. In the electroacoustic transducer that converts the sound wave into a sound wave of a finite amplitude level and radiates it into the air, and reproduces the original audible sound by the non-linear effect of air, in the direction of ultrasonic wave radiation from the front surface of the ultrasonic transducer. , An acoustic filter for blocking the primary wave of the ultrasonic wave is provided, and a distance L between the ultrasonic transducer and the acoustic filter is a Rayleigh length R ₀ , and an attenuation coefficient of the ultrasonic wave per Rayleigh length in air. Is characterized by having a relationship of L = R ₀ / 2α.

The configuration of the present invention will be described below based on examples.

A speaker using the parametric effect is affected by the nonlinear characteristics of air and propagates in the air as finite amplitude ultrasonic waves whose amplitude is modulated by audible sound is emitted from the ultrasonic transducer array and As a result of demodulation, 1
Since the virtual sound source of the secondary wave (modulation wave = audible sound) is formed as a vertical array in the beam of the secondary wave, the secondary wave sound field having a sharp directivity is provided. Therefore, when listening to the reproduced sound from the parametric speaker, the listener needs to listen at a sufficient distance from the ultrasonic transducer array surface. If the distance is not sufficient, the virtual sound source array is not sufficiently formed and a reproduced sound having a satisfactory sound pressure level cannot be obtained. When listening at an appropriate distance from the ultrasonic transducer, the listener hears the secondary sound and is also exposed to the primary wave. In this case, since the sound pressure of the primary wave is considerably high, it is feared that the human body will be affected if exposed for a long time. Therefore, we considered that an acoustic filter with a diameter sufficiently larger than the beam diameter should be inserted into the beam after the virtual sound source array was formed to cut the primary wave and at the same time pass the secondary wave with as little attenuation as possible. In that case, the following two problems arise.

(1), insertion position of acoustic filter (distance from ultrasonic transducer).

(2), configuration and characteristics of acoustic filter.

With regard to (1), if the insertion position is too close to the vibrator, the primary wave is blocked before the virtual sound source array is formed, so sufficient secondary sound pressure cannot be obtained. On the other hand, if the insertion position is too far from the vibrator, there is no concern about cutting the virtual sound source array, but handling is inconvenient. Therefore, it is necessary to determine the optimum insertion position.

Fig. 1 shows an example of computer simulation results for the relationship between the normalized distance L / R _O (actual distance divided by the Rayleigh length) from the oscillator and the secondary sound pressure. As a result, it is considered that the virtual sound source array is almost completed in the vicinity of the normalized distance L / R _O = 1 / 2α. Therefore, the insertion position of the acoustic filter is appropriate near L = R _O / 2α from the viewpoint of the secondary sound pressure.

Next, regarding the item (2), the desired characteristics of the acoustic filter are that the absorption of the primary wave is large and the transmittance of the secondary wave is as high as possible. Various materials can be considered as such a material, but in the case of a primary wave frequency of 40 kHz, urethane foam, air pad, etc. are effective. The Rayleigh length and the absorption coefficient (attenuation coefficient) will be described below.

Rayleigh length: The amplitude of a sound wave radiated from a sound source having a finite size exhibits a complicated property due to the fundamental property of the wave, that is, interference. This is particularly noticeable near the sound source, and the amplitude repeats unevenness. On the other hand, when the distance from the sound source is increased to some extent, this interference weakens, and the amplitude gradually decreases with spherical diffusion along with the distance from the sound source. The Rayleigh length is one standard distance that gives a boundary between a region where the wave amplitude changes in a complicated manner and a region where the wave amplitude changes gently, and is determined by the size of the sound source and the wavelength of the wave.

Absorption coefficient: Elastic media that transmit sound have more or less viscosity and thermal conductivity. This property corresponds to frictional resistance in mechanics, and the elastic energy of the wave is called the absorption coefficient, which is the rate of attenuation with propagation due to this resistance. This absorption coefficient strongly depends not only on the medium constant but also on environmental conditions such as temperature and humidity, and further on the used frequency. The higher the frequency, the larger the absorption coefficient, and the wave is significantly attenuated.

Fig. 2 shows the experimental results of the relationship between the insertion position and the insertion loss when an air pad is used as the acoustic filter. From the figure, the insertion position of the air pad (acoustic filter) is secondary from 7 to 10m. It shows that the wave loss is large. In Fig. 2, ○ is 1kHz and □ is 3kH.
z, △ show 5kHz, ● shows 40kHz (primary wave).

FIG. 3 is a diagram showing an example of a parametric speaker system in which an acoustic filter of a primary wave cut, which is the gist of the present invention, is inserted. In the figure, 1 is a signal source (audible sound) and 2 is an ultrasonic frequency. The area oscillator, 3 is an AM modulator, 4 is a power amplifier, 5 is an ultrasonic transducer array, 6 is an acoustic filter (air pattern curtain), and 7 is a listening area. An acoustic filter 6 is provided at a distance of approximately L = R _O / 2α from the front surface of the acoustic wave transducer array 5. As described above, according to the present invention, in order to block the ultrasonic wave of the primary wave, the acoustic filter is installed perpendicularly to the ultrasonic beam at the position of R ₀ / 2α from the speaker (ultrasonic transducer), which is harmful to the human body. It is an acoustic radiation system that gradually reduces high-power ultrasonic waves to a safe level. Therefore, if only the distance between the ultrasonic transducer and the acoustic filter is known, the intended purpose can be sufficiently achieved. That is, the reproduced audible sound increases in size as it moves away from the speaker,
A peak appears at the position of R ₀ / 2α, and thereafter, it is gradually attenuated by spherical diffusion. This means that R ₀ / 2α (α
Is the attenuation coefficient of the ultrasonic wave per Rayleigh length in the air), the strong primary ultrasonic wave does not contribute to the reproduction of the audible sound, and it is harmful to the human body to hear in the vicinity. Therefore, it is necessary to reduce the ultrasonic wave by some method. The directivity, which is a feature of this speaker, depends on the length of the vertical array, and since the array is already completed before R ₀ / 2α, it does not change even if the ultrasonic waves are gradually reduced.

Effect As is clear from the above description, according to the present invention, the distance L between the ultrasonic transducer and the acoustic filter is L = R ₀ / 2α
(R ₀ : Rayleigh length, α: attenuation coefficient of ultrasonic wave per Rayleigh length in air) is installed so that high-power ultrasonic waves harmful to the human body are cut, and 2 It is possible to provide an electroacoustic transducer with less secondary wave loss.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the normalized distance from the vibrator and the secondary sound pressure, FIG. 2 is a diagram showing the relationship between the insertion position of the air pad and the insertion loss, and FIG. 3 is the present invention. It is a block diagram for explaining one Example. 1 ... Signal source, 2 ... Ultrasonic frequency domain oscillator, 3 ... AM modulator, 4 ... Power amplifier, 5 ... Ultrasonic transducer array, 6 ... Acoustic filter, 7 ... Listening area.

Claims (1)

[Claims] 1. A carrier signal in an ultrasonic frequency band is modulated by a signal from a signal source in an audible frequency band, power-amplified, and then guided to an ultrasonic transducer to convert the modulated wave into a sound wave having a finite amplitude level. Then, in the electroacoustic transducer that is radiated into the air and reproduces the original audible sound by the non-linear effect of the air, in the ultrasonic radiation direction from the front face of the ultrasonic transducer, the primary wave of the ultrasonic wave is emitted. When an acoustic filter for shutting off is provided and the distance L between the ultrasonic transducer and the acoustic filter is R ₀ , and the attenuation coefficient of the ultrasonic wave per Rayleigh length in air is α, L = An electroacoustic transducer having a relationship of R ₀ / 2α.