JPS62296698A - Parametric speaker - Google Patents

Abstract

PURPOSE:To reproduce a wide-band signal and to improve the efficiency of low-frequency conversion by driving a speaker with two kinds of carriers and modulating one carrier with a signal wave passed through an LPF. CONSTITUTION:Two piezoelectric ceramic plates 9 are stucked on each other having the mutually opposite polarizing direction to constitute a bimorph plate, and a resonator 10 and a coupling shaft 12 are adhered and fixed in the center of the bimorph plate; and 100 ultrasonic vibrators to which a terminal plate 11 is fitted are arranged in a honeycomb shape and all connected in parallel, thus forming a speaker. The oscillation frequencies f1 and f2 of carrier oscillators 5a and 5b are set to 22 and 45 kHz and the cutoff frequency of an LPF 13 is set to 1.5 kHz. Amplitude modulation is imposed on a signal lower than 1.5 kHz by both carriers of 22 and 35 kHz or on a signal higher than 1.5 kHz by only the carrier of 45 kHz and the resulting amplitudemodulated signal is inputted to the speaker 12.

Description

DETAILED DESCRIPTION OF THE INVENTION 3. Detailed Description of the Invention Field of Industrial Application The present invention relates to a speaker that uses the nonlinearity of air with respect to ultrasonic waves, a so-called parametric speaker.

Conventional Technology Traditionally, there has been a demand for loudspeaker equipment to sharpen the directionality of sound like a spotlight so that only a certain range of people can hear it without being affected by ambient noise, such as at exhibitions. When you want to provide separate explanations for each individual item,
It was useful for applications such as announcements on station platforms.

Conventionally, horn speakers have been mainly used for such applications, but the directivity of a horn speaker strongly depends on its length and diameter. The drawback was that both diameters were extremely large.

On the other hand, in recent years, a so-called parametric speaker, which utilizes the nonlinearity 1'4 of air with respect to ultrasonic waves, has been attracting attention because it can obtain much sharper directivity than conventional speakers (for example, Japanese Patent Laid-Open No. 119293/1982).

First, a conventional parametric link speaker will be explained.

FIG. 6 shows the configuration of a conventional parametric speaker. In FIG. 6, 1 is an ultrasonic transducer, and 2 is an ultrasonic generator (hereinafter referred to as a speaker) constructed by arranging the ultrasonic transducers in a honeycomb shape. 3 is an audio signal source, the output of which is input to a modulator 4; Here, the audio signal (modulated wave) is amplitude-modulated using a carrier wave of 40 kHz, and is input to the speaker 2 via the power amplifier 6. 7 is an acoustic filter for absorbing ultrasonic waves, and is installed between the speaker 2 and the listener 8.

In the above configuration, ultrasonic waves having a spectrum as shown on the right side of FIG. 7 are radiated into the air from the speaker. In the J-wave, where the amplitude of the ultrasonic wave is large and is considered to be at a finite amplitude level, the carrier wave and the upper and lower sideband waves cause nonlinear interaction in the air, and a modulated wave with sharp directivity is generated. Here, the modulated ultrasonic wave emitted from the speaker 2 is called a primary wave, and the original audio signal (modulated wave) generated as a result of the nonlinear interaction of the primary waves is called a secondary wave.

By the way, in a parametric speaker, since the conversion efficiency from a primary wave to a secondary wave is low, a strong opening θU is required in order to generate a practical level of secondary waves. In particular, the sound pressure level of the secondary wave in the low range is proportional to the square of the secondary wave frequency, so the sound pressure level in the low range is insufficient, causing a problem in sound quality1.On the other hand, conversion efficiency From this point of view, it is known that it is better to lower the carrier frequency. , primary wave and 2
Figure 8 shows an example of calculating the relationship between the sound pressure level of the next wave using the carrier frequency as a parameter (Journal of the Acoustical Society of Japan 41
, 6 (1985), 378). It is meaningless for the purpose of a speaker to lower the frequency of the carrier wave to an audio frequency, but according to Figure 8, to obtain a secondary wave sound pressure level of 110 dB, for example, the carrier wave frequency must be At 40 kHz, a primary wave sound pressure level of approximately 145 dB is required, whereas at 20 kHz, it is 135 dB.
It turns out that you can get away with it. Recently, however, attempts have been made to improve the conversion efficiency by lowering the frequency of the carrier wave (Acoustical Society of Japan, Spring 1986, 1-4-18 P, 31ts).

The problem that the invention aims to solve is that when the frequency of the carrier wave is slightly lowered, as shown in Figure 9, when the lower side band of the primary wave reaches the audio frequency, the sound of the primary wave becomes loud, and the important secondary wave becomes louder. Since the sound of the waves becomes difficult to hear, the band of the signal wave must be narrowed so that it does not reach the audio frequency range, and the problem is that the frequency of the carrier wave can only be lowered to about 25 kHz at most. It had a point.

In view of the above problems, the present invention can reproduce broadband signals,
The present invention also provides a parametric speaker with improved low frequency conversion efficiency.

Means for Solving the Problems In order to solve the above problems, the parametric speaker of the present invention includes a low-pass filter and a first carrier wave for modulating a first carrier wave with a signal wave passed through the low-pass filter. a second carrier for modulating a second carrier higher than the first carrier with the original audio signal wave from the low-pass filtered audio signal source; The ultrasonic generator is equipped with a modulator and an ultrasonic generator driven by the summed output of the two modulators.

Operation According to the above-mentioned configuration, the present invention divides the audio signal wave into two waves, one of which is passed through a low-pass filter, and the other is passed through a low-pass filter. input to the second modulator. The signals modulated by each modulator drive a speaker through an adder.

Since the signal wave input to the first modulator is only in the low frequency range, the sideband wave does not reach the audio frequency and the primary wave is not heard. Also, the two carrier frequencies are 20 klh
Because of the distance above, secondary waves generated by nonlinear interaction between carrier waves cannot be heard. Since the low-frequency signal with low conversion efficiency is modulated by two carrier waves, and one uses the low-frequency wave with high conversion efficiency as the carrier wave,
The secondary wave sound pressure in the low range is greatly improved.

EXAMPLE An example of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a parametric speaker in one embodiment of the present invention.

The speaker of this embodiment uses ultrasonic transducers having the structure shown in FIG. 2 arranged in a honeycomb pattern and all connected in parallel. In FIG. 2, 9 is a piezoelectric ceramic plate, and a bimorph is constructed by bonding two piezoelectric ceramic plates with their polarization directions opposite to each other. 1o is a resonator adhesively fixed to the center of the bimorph, 11 is a terminal plate, and 12 is a coupling shaft.

The characteristics of this ultrasonic transducer include the shape of the bimorph and resonator,
You can change it to "Material". Figure 3 shows the sound pressure frequency characteristics of a commercially available 26kl (+) ultrasonic transducer.As the diameter of this ultrasonic transducer is increased,
When the material of the resonator was changed from aluminum to mica-reinforced epoxy resin, the first peak decreased from 25 kHz to 22 kHz, and the second peak decreased from 25 kHz to 22 kHz.
The peak was from 36 kll+ to 46 kHz. So the first thing. The oscillation frequency f1 of the second carrier wave oscillator 5a, 5b. f2, 22kHz, 4e, respectively
s klb and toshida. Further, the cutoff frequency of the low-pass filter 13 was set to 1.5 kflz. Note that 2 is an ultrasonic generator that constitutes a speaker.

3 is an audio signal source, and 6 is a power amplifier, which have the same configuration as the conventional example.

With the above-described configuration, this embodiment uses both 22 kHz and 46 kHz as carrier waves for the low frequency range below 1.5 kHz, and uses 45 kt (for the high frequency range above 1.5 kl) as the carrier wave.
Only z is used as a carrier wave, amplitude modulated, and input to the speaker.

As a result, a primary wave having a spectrum as shown in FIG. 4 is radiated into the air from the speaker 2. Then, nonlinear interaction occurs in the air, and the signal wave is regenerated. The frequency characteristics of the secondary wave are as shown in Figure 5a. Figure 6, c
are the first. This is the characteristic when modulation is applied only by the second modulator. As is clear from this, according to this embodiment, the sound pressure level in the low range can be reduced by 1 od compared to when carrier frequencies around 40 kHz were conventionally used (corresponding to characteristic C).
B and above can also be improved.

Note that in this example, an ultrasonic transducer made of piezoelectric ceramic and exhibiting bimodal frequency characteristics was used as the speaker, but the ultrasonic transducer used is not limited to this; Needless to say, an electrodynamic speaker or the like may be used.

In that case, it is also possible to modulate at more frequencies than two frequencies. Furthermore, as a modulation method for the first carrier wave (lower one), double side band amplitude modulation (DS
B) If instead of using single sideband modulation (SSB) and using only the upper sideband, the carrier frequency would be lower than the audible limit of 20
It can be lowered to kHz.

Effects of the Invention As described above, according to the IJ sock peaker of the present invention, it is driven by two types of carrier waves, fl and f2 (fl<f2), and fl is modulated by a signal wave passed through a low-pass filter. As a result, (1) low-frequency conversion efficiency is greatly improved and broadband reproduction is possible, and (2) sideband waves of the primary wave do not extend to the audio frequency.
Noise-free playback is possible, and other excellent effects can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of a parametric speaker in an embodiment of the present invention, Fig. 2 is a structural diagram of an ultrasonic transducer, and Fig. 3 shows the sound of the conventional ultrasonic transducer and the ultrasonic transducer used in this embodiment. The pressure level frequency characteristic diagram, FIG. 4, is 1 in the present invention.
Characteristic diagram showing the frequency spectrum of the secondary wave and the secondary wave, 5th
The figure is a sound pressure level frequency characteristic diagram of the secondary wave in an embodiment of the present invention, Figure 6 is a block diagram showing the configuration of a conventional parametric speaker, and Figure 7 is a diagram of the primary wave and secondary wave of a conventional parametric link speaker. A characteristic diagram showing the frequency spectrum of the secondary wave. Figure 8 is a characteristic diagram showing the relationship between the primary wave and secondary wave sound pressure levels. Figure 9 is a characteristic diagram showing the frequency spectrum of the primary wave when the carrier frequency is low. It is. 1... Ultrasonic transducer, 2... Ultrasonic generator (speaker), 3... Audio signal source, 4.4
a, 4b...Modulator, 5, 5a, esb
... Carrier wave oscillator, 6 ... Power amplifier, 7 ... Acoustic filter, 8 ... Listener, 9 ... Piezoelectric ceramic, 10 ...Resonator, 11...Terminal plate, 12...Coupling shaft, 13...Low pass filter, 14...
・Adder. Name of agent: Patent attorney Toshio Nakao and one other person
Tamen → Hani (9p) 4 (＞'1 arc rumor

Claims (3)

[Claims] (1) a low-pass filter that receives an audio signal from an audio signal source; a first carrier wave oscillator that oscillates a first carrier wave; a second modulator for modulating a second carrier output from a second carrier oscillator with an audio signal from the audio signal source; and an adder to which the outputs of the second modulator and the second modulator are inputted, and an ultrasonic generator to which the output of the adder is inputted, A parametric speaker characterized by generating an original audio signal in the air by nonlinear interaction of modulated ultrasonic waves. (2) The parametric speaker according to claim 1, wherein the frequency of the second carrier wave is higher than the frequency of the first carrier wave by 20 kHz or more. (3) The parametric speaker according to claim 1 or 2, wherein the first carrier wave has a frequency of 20 to 25 kHz.