JPH0779516B2 - Parametric speaker - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a loudspeaker (parametric loudspeaker) that uses nonlinearity of air with respect to ultrasonic waves and has a sharp directivity.

2. Description of the Related Art Conventionally, in a loudspeaker, the directivity of sound is made sharp like a spotlight, and the request to hear only certain people around without being affected by ambient noise is required at individual exhibitions. There was a strong thing when I wanted to give a separate explanation for each booth, or as a guide broadcast at a station platform.

Conventionally, a horn speaker has been mainly used for such an application, but in order to obtain sharp directivity up to a low frequency such as voice using the horn speaker, both length and diameter are extremely large. was there.

By the way, a speaker (parametric speaker) utilizing the non-linearity of air with respect to ultrasonic waves has recently been attracting attention because it can provide a much sharper directivity than conventional speakers.

First, a conventional parametric speaker will be described (for example, JP-A-58-119293). FIG. 6 shows the structure of a conventional parametric speaker. 1 is an ultrasonic transducer having a sound pressure peak at 4 kHz, and 2 is an ultrasonic wave generator (speaker) arranged in a honeycomb shape. . Reference numeral 3 is an audio signal source, and the audio signal from this is input to the modulator 4. On the other hand, a carrier wave of 40 kHz is input from the carrier wave oscillator 5 to the modulator and amplitude-modulated with a voice signal. The amplitude-modulated signal is input to the speaker 2 via the power amplifier 6 and radiated into the air as an ultrasonic wave having a finite amplitude level. As a result, a strong ultrasonic field (parametric array) is formed in the air, and the original audio signal is generated by the nonlinear interaction. Reference numeral 7 is an acoustic filter for protecting the listener 8 from strong ultrasonic waves. Here, the amplitude-modulated ultrasonic wave radiated from the sound source is called a primary wave, and the audio signal generated as a result of the nonlinear interaction of the primary wave is called a secondary wave.

By the way, in the parametric speaker, the conversion efficiency from the primary wave to the secondary wave is extremely low (1% or less). Therefore, extremely high primary wave sound pressure is required to generate a practical level of audible sound. For example, to obtain a secondary wave sound pressure of 90 dB
A primary wave sound pressure of 140 dB or higher is required.

Problems to be Solved by the Invention However, in order to increase the conversion efficiency from the primary wave to the secondary wave with the above configuration, (1) increase the primary wave sound pressure (2) decrease the primary wave frequency (3 ) The only way I could say was to increase the degree of modulation. Among these, it is necessary to improve the performance of the ultrasonic transducer in order to increase the primary wave sound pressure, but there is a limit to this, and if the primary wave sound pressure is increased too much, rapid non-linear attenuation occurs and As shown in FIG. 3, there is a problem that the secondary wave is rather small. Further, even if the primary frequency is lowered, the limit is about 25 kHz, and there is a problem that the distortion component increases in proportion to the square of the modulation degree when the modulation degree is increased.

In view of the above problems, the present invention provides a parametric speaker with dramatically improved conversion efficiency.

Means for Solving the Problems In order to solve the above problems, a parametric speaker according to the present invention includes an audio signal source, a carrier oscillator, a distorted waveform generating circuit to which an output of the carrier oscillator is input, and the audio. An audio signal from a signal source and an output of the distorted waveform generation circuit are respectively input, and a modulator for modulating the output of the distorted waveform generation circuit as an audio signal with an audio signal, and an ultrasonic wave generation driven by the output of the modulator Waveform generator circuit for distorting the initial waveform of the particle velocity u of the carrier wave of the modulated ultrasonic wave radiated into the air from the ultrasonic wave generator in a direction opposite to the distortion caused by the nonlinearity of the medium. Is configured.

Action The present invention has the above-described configuration to extend the shock wave front formation distance, increase the conversion efficiency, and raise the sound pressure level.

EXAMPLES Examples of the present invention will be described below with reference to the drawings. First
The structure of one embodiment of the present invention is shown in the drawing.

In FIG. 1, the speaker 2 is composed of ultrasonic transducers having a center frequency of 40 kHz arranged in a 1000-honeycomb shape and all connected in parallel. Reference numeral 3 is an audio signal source and 4 is a modulator.
Reference numeral 5 is a carrier wave oscillator, and 5a is a distorted waveform generation circuit for distorting the output waveform of the carrier wave oscillator 5. The distorted waveform generating circuit 5a used in this embodiment can generate an arbitrary waveform, and the distorted waveform generating circuit 5a and the carrier wave oscillator 5 constitute an arbitrary waveform generator 5 '. In the distorted waveform generation circuit 5a, the initial waveform of the particle velocity of the primary wave is Such a carrier wave is generated, this is modulated with an audio signal, and the signal is input to the speaker 2 through the power amplifier 6.

Hereinafter, the formula A will be described in more detail. First 1
A process in which a secondary wave is subjected to nonlinear distortion and a secondary wave is generated will be briefly described.

The sound velocity C of the finite amplitude sound wave depends on the particle velocity u and is represented by C = C ₀ + βu (1). Where C _{0 is} the sound velocity at infinite small amplitude, β is the nonlinear parameter of the medium (β = 1.2 for air). Therefore, when a finite-amplitude sine wave travels a distance x, a large waveform distortion occurs at the place where the amplitude is large, and the waveform is Is represented. Where u ₀ ; maximum amplitude of particle velocity, ω; angular frequency, t; time.

This is shown in FIG. In FIG. 4, A shows an initial waveform and B shows a waveform after the propagation of the distance x. The shock wave front formation distance is given by the distance at which the wave front at point 0 becomes infinite, and is represented by = (βu ₀ ω / C ₀ ² ) ^-1 (4). C indicates that a shock wave front is formed. Therefore, the lower the frequency of the primary wave, the longer the shock wave front formation distance.

When the primary wave is two ultrasonic waves having slightly different frequencies, the energy of the secondary wave is given by the distortion of the envelope function due to the distortion of the primary wave. An example of the result obtained by computer simulation is shown in FIG. A in FIG.
When using 45kHz and 50kHz as the primary wave, B is 15kHz
And 20 kHz, where the horizontal axis represents the propagation distance and the vertical axis represents the ratio of the secondary wave energy and the primary wave energy of 5 kHz. From this, it is understood that when the shock wave front formation distance is exceeded, the primary wave abruptly attenuates due to the non-linear attenuation, and the secondary wave does not increase much further.

This simulation was performed by ignoring the linear damping. In reality, due to the linear damping, the shock wave front formation distance becomes far, and the secondary wave sound pressure also increases (conversion efficiency increases). However, if a shock wave is formed, the secondary wave is hardly generated.

In the present embodiment, the waveform of the primary wave is preliminarily distorted in the opposite manner to the non-linear distortion by the above-described configuration, so that the shock wave front formation distance is extended and the conversion efficiency is increased. First, in the arbitrary waveform generator, the initial waveform of the particle velocity of the primary wave is Such a carrier wave is created, modulated with a signal wave, and then input to the speaker. The initial waveform of the primary wave is shown in FIG. 2B. In this case, the shock wave front forming distance 'becomes' = 2 as compared with the case where the conventional sine wave is used as the primary wave, ignoring the linear attenuation, and the conversion efficiency is doubled. The result of this simulation is shown in FIG. 5C. Carrier frequency is
45kHz and 50kHz were used. It can be seen that the shock wave front forming distance is doubled and the sound pressure level is improved by 6 dB as compared with FIG. 5A. The equation (5) is obtained with the sound velocity C = C ₀ −βu, but with C = C ₀ + βu, the waveform obtained when it is assumed that the sound wave travels only in the negative direction. Since they have the same shape as, I will write.

As described above, according to this embodiment, when the input voltage to the speaker is changed and the carrier wave is a conventional sine wave and the "distorted wave" of this embodiment is compared, the secondary wave sound pressure levels are compared. When the input voltage was small, that is, when the initial sound pressure was small, there was no large change, but as the input voltage was increased, the value of this example using the “distorted wave” was larger, and the initial sound pressure was
Above 0 dB, it improved by over 6 dB. When a conventional sine wave is used, when the initial sound pressure exceeds 150 dB, the shock wave front formation distance becomes short, so the secondary wave sound pressure level decreases rather as shown in Fig. 3. In the case of the embodiment, the higher the initial sound pressure, the more remarkable the result.

Although the arbitrary waveform generator is used as the carrier wave oscillator in the present embodiment, in actuality, if the optimum waveform is determined, it can be configured by an electronic circuit and incorporated in a modulator or a power amplifier, resulting in cost increase. Is almost never. Also, the method of distorting the carrier wave may be any other method as long as it is the distortion opposite to the nonlinear distortion due to the medium, and the degree of distortion is necessary when this method is used. There is no difference.

EFFECTS OF THE INVENTION The present invention can drastically improve the conversion efficiency from the primary wave to the secondary wave by using the waveform distorted as the carrier wave of the parametric speaker, which is opposite to the distortion caused by the nonlinearity of the medium. In particular, there has been a development in which the sound pressure level of the secondary wave is rather lowered when the initial sound pressure level of the primary wave is extremely high. However, according to the present invention, when the sound pressure level of the primary wave is high, Also has the effect of increasing the secondary wave sound pressure level significantly. This increases the shock wave front formation distance,
That is, the so-called array length becomes long. As a result, the directivity becomes wider as the initial sound pressure is increased, and the advantage of the parametric speaker tends to be impaired, but this is greatly improved, and even if the initial sound pressure is increased, a sufficiently sharp directivity is ensured. Create a new effect. Further, since the carrier wave generating circuit of the present invention can be realized by a relatively simple electronic circuit, it has an effect of not increasing the cost.

Further, the generation of the secondary wave is almost not performed after the shock wave front is formed, while the harmonic component of the secondary wave continues to increase even after the shock wave front is formed. There is a drawback in that distortion is remarkably increased when an attempt is made to improve the conversion efficiency by increasing the level, but in the present invention, since the conversion efficiency is improved by extending the array length, the secondary wave sound pressure level is increased. There is a great effect that there is no increase in distortion even if is raised.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a parametric speaker in one embodiment of the present invention, FIG. 2 is an initial waveform diagram of a primary wave of this embodiment, and FIG. 3 is a primary wave sound pressure level and a secondary wave sound pressure. FIG. 4 is a characteristic diagram showing a level relationship, FIG. 4 is a waveform diagram showing a waveform distortion due to nonlinearity of a medium, FIG. 5 is a characteristic diagram showing a relationship between sound wave propagation distance and secondary wave energy, and FIG. It is a block diagram which shows the structure of the conventional parametric speaker. 1 ... Ultrasonic transducer, 2 ... Ultrasonic generator (speaker), 3 ... Voice signal source, 4 ... Modulator, 5 ... Carrier oscillator, 5a ... Distorted waveform generation circuit, 5 '... Arbitrary waveform generator, 6 ... Power amplifier, 7 ... Acoustic filter, 8 ...
Listener.

Claims (2)

[Claims] 1. An audio signal source, a carrier wave oscillator, a distorted waveform generating circuit to which an output of the carrier wave oscillator is input, and an audio signal from the audio signal source and an output of the distorted waveform generating circuit are input and distorted, respectively. A modulator for modulating the output of the waveform generation circuit with a sound signal as a carrier wave, and an ultrasonic wave generator driven by the output of the modulator, and the modulated ultrasonic wave radiated into the air from the ultrasonic wave generator. A parametric speaker characterized in that the distorted waveform generating circuit is configured so as to distort an initial waveform of a particle velocity u of a carrier wave of a sound wave in a direction opposite to a distortion caused by nonlinearity of a medium. 2. The initial waveform of the particle velocity u of the carrier wave of the modulated ultrasonic wave radiated in the air from the ultrasonic wave generator is 0 ≤ α ≤ (βu ₀ ω / C ₀ ² ) ^-1 where u ₀ ; maximum amplitude of particle velocity ω; angular frequency of carrier C ₀ ; sound velocity at infinite small amplitude β; nonlinear parameter of medium τ = t- The first aspect of the present invention is characterized in that α / C ₀ (t; time) is set.
Parametric speaker according to the item.