JPH0793754B2 - Parametric speaker - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a parametric speaker using the nonlinearity of air with respect to ultrasonic waves.

2. Description of the Related Art Conventionally, in a loudspeaker, the directivity of sound was made sharp like a spotlight, and it was requested that only a specific range of people could be heard without being affected by ambient noise. There were strong applications such as when you wanted to give a separate explanation for each, or for information broadcasting at station platforms.

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, recently, a speaker utilizing the non-linearity of air with respect to ultrasonic waves (hereinafter referred to as a parametric speaker) has attracted attention because it can obtain a much sharper directivity than the conventional one.

First, a conventional parametric speaker will be described (for example, JP-A-58-119293). Fig. 4 shows the structure of a conventional parametric speaker, where 1 is 40kHz.
2 is an ultrasonic wave oscillator having a sound pressure peak, and 2 is an ultrasonic wave generator (hereinafter referred to as a 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 sound field (parametric array) is formed in the air, and the original speech 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 referred to as a secondary wave, and the audio signal generated as a result of the nonlinear interaction of the primary wave and the primary 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 sound pressure of the primary wave (2) decrease the frequency of the primary wave (3 ) The only way I could say was to increase the degree of modulation. In order to raise the sound pressure of the first-order wave, it is necessary to improve the performance of the ultrasonic transducer, but there is a limit to this, and if the sound pressure of the first-order wave is raised too much, rapid non-linear deceleration will occur, and the secondary There is a problem that the waves become smaller. 25kHz even if the primary wave frequency is lowered
However, when the modulation degree is increased, the distortion component becomes 2
There was a problem that it increased in proportion to the power.

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 of the present invention includes a carrier oscillator, a modulator for modulating a carrier with an audio signal, and ultrasonic wave generation driven by the output of the modulator. And the output of the carrier wave oscillator is composed of a combination of intermittent waves or a plurality of waveforms having different amplitudes.

It is known that the secondary wave sound pressure level of a parametric speaker is proportional to the square of the primary wave sound pressure level by solving the Westervelt equation based on the quasi-linear theory. On the other hand, the primary wave sound pressure level is proportional to the input power to the ultrasonic generator. Therefore, if the same primary wave sound pressure is obtained, driving with an input waveform that requires less input power than in the case of sinusoidal driving improves the conversion efficiency from the primary wave to the secondary wave.

The present invention is made on the basis of the above concept, and improves the conversion efficiency by using a waveform of a carrier wave having an intermittent wave or a combination of a plurality of waves having different amplitudes.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a parametric speaker according to an embodiment of the present invention. In FIG. 1, a speaker 2 has 1000 ultrasonic transducers having a center frequency of 40 kHz, arranged in a honeycomb shape, and all of them are connected in parallel. 3 is an audio signal source,
4 is a modulator. 5a is an arbitrary waveform generator used as a carrier wave oscillator.

First, a carrier wave as shown in FIG. 2B is created by the arbitrary waveform generator 5a, modulated with a voice signal, and input to the speaker 2 through the power amplifier 6.

This embodiment will be described below in more detail with reference to FIG.

First, FIG. 2A shows a conventional sine wave, which is represented by V = V _O sin ωt. FIG. 2b shows the intermittent wave used in this embodiment. It is represented by.

Therefore, the average input power of each waveform shown in FIGS.

By the way, when sound waves are emitted in FIG. 2b,
The sound pressure level of the primary wave is 3 dB higher than that of the continuous wave because the voltage is ▲ √ ▼ times that of the continuous wave.
Since the secondary wave sound pressure is proportional to the square of the primary wave sound pressure, the secondary wave sound pressure level becomes 6 dB higher. On the other hand, when there is no sound wave, no secondary wave naturally occurs, but on the average, it is 3 dB higher than when it is a continuous wave. That is, the electroacoustic conversion efficiency is 2 by driving with the waveform shown in FIG. 2b.
Double.

Next, a second embodiment will be described. Since the intermittent wave is used as the carrier wave in the first embodiment, the fundamental frequency is 1/2 that of the continuous wave. Therefore, when the frequency of the continuous wave is 40kHz or more, it does not cause a problem so much, but when it is less than 40kHz, the subharmonic is in the audible band and becomes noise. Therefore, in this embodiment, instead of the intermittent wave, a waveform obtained by combining a plurality of waves having different amplitudes is used. Specifically, for example, as shown in FIG. 3, it is assumed that waves having large amplitude and waves having small amplitude are alternately combined. By doing so, it is possible to increase the conversion efficiency from the primary wave to the secondary wave and suppress the occurrence of subharmonics. There is no particular limitation on how to combine these plural waves, and it is sufficient that the peak voltage is higher than that of continuous waves of the same amplitude when the average power is constant. For example, a combination of one wave having a large amplitude and a wave having three small amplitudes or a combination of three waves having a large amplitude and one wave having a small amplitude may be used. Further, the amplitude may be continuously changed as shown in FIG.

In the first embodiment, a waveform that is repeatedly turned on and off for each wave is used. However, when an oscillator having a high resonance Q is used, any of the waveforms shown in FIGS. Even if driven by, there is no great change in the output waveform as a sound wave and the effect is small. In that case, you may use the waveform which turns on and off for every several waves. In addition, although the fundamental wave is a sine wave in this embodiment, it is a matter of course that a distorted wave (for example, a sawtooth wave) other than the sine wave may be used.

EFFECTS OF THE INVENTION The present invention can improve the conversion efficiency from a primary wave to a secondary wave by using an intermittent wave or a wave in which a plurality of waves having different amplitudes are combined as a carrier wave of a parametric speaker. Further, the carrier wave generation circuit of the present invention can be realized by a simple electronic circuit, so that it has an effect of not increasing the cost.

Also, the conversion efficiency is improved, so the amplifier can be small.

[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. 2a is a waveform diagram showing a conventional carrier wave, and FIG. 2b is a waveform diagram showing a carrier wave in this embodiment. FIG. 4 is a waveform diagram showing a carrier wave of another embodiment of the present invention, FIG. 4 is a block diagram showing a configuration of a conventional parametric speaker, and FIG. 5 is a waveform diagram showing a carrier wave of another embodiment of the present invention. . 1 ... Ultrasonic transducer, 2 ... Ultrasonic generator (speaker), 3 ... Voice signal source, 4 ... Modulator, 5 ... Carrier generator, 5a ... Arbitrary waveform generator, 6 ... Power Amplifier,
7 ... acoustic filter, 8 ... listener.

Claims (4)

[Claims] 1. An audio signal source, a carrier wave oscillator, a modulator for modulating a carrier wave with an audio signal, and an ultrasonic wave generator driven by the output of the modulator, wherein the waveform of the carrier wave is intermittent. Parametric speaker characterized by being waves. 2. The parametric speaker according to claim 1, wherein the fundamental frequency of the carrier wave is 20 kHz or higher. 3. An audio signal source, a carrier wave oscillator, a modulator for modulating a carrier wave with an audio signal, and an ultrasonic wave generator driven by the output of the modulator, wherein the waveform of the carrier wave is an amplitude. When the average power is constant for multiple waves of different
A parametric speaker characterized by being combined so that the peak voltage is higher than that of a continuous wave of the same amplitude. 4. The parametric speaker according to claim 3, wherein the fundamental frequency of the carrier wave is 20 kHz or higher.