JP3668180B2 - Ultrasonic reproduction method / Ultrasonic reproduction device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic reproduction method and apparatus for limiting reproduction sound to be heard only in a desired region, and more particularly to an ultrasonic reproduction method and apparatus for suppressing emitted ultrasonic energy to a low level. .
[0002]
[Prior art]
In recent years, a video conference system using a personal computer (hereinafter referred to as a desktop conference system; referred to as a Desk Top Conference (DTC) system) is becoming widespread. Conventionally, a normal speaker is used as means for reproducing the voice of the other party in these conference systems.
[0003]
[Problems to be solved by the invention]
However, ordinary speakers generally have a wide directivity, and not only the person using the TV conference system but also the other party's voice (reproduced sound) can be heard in the vicinity, and it can be heard as unnecessary noise around. There was a problem. As one method for solving this, a method using a parametric loudspeaker configured to amplitude-modulate an ultrasonic signal with an audible sound signal and radiate the amplitude-modulated ultrasonic signal from the ultrasonic element is conceivable.
The principle of the parametric speaker and its superdirectivity will be briefly described below. A parametric speaker emits strong ultrasonic waves (about 100 dB or more), and generates audible sound using the nonlinearity of air. When the intensity of the sound wave at the propagation destination is expressed as a function of the intensity at the drive source at a normal sound level (about 70 dB), the second and higher order components of the intensity of the drive source are negligibly small. However, when considering the propagation of sound waves of 100 dB or more, the second and higher order components cannot be ignored. The sound pressure of the second and higher components is about 60 to 70 dB when the ultrasonic sound pressure is 100 dB. When this sound pressure level is reached, it can be heard as an audible sound. It is a parametric speaker that generates audible sound using the nonlinear term of air.
[0004]
Among these audible sound generation methods, a relatively efficient method is ultrasonic amplitude modulation. An ultrasonic element that emits ultrasonic waves has a resonance frequency. The ultrasonic signal having a frequency near the resonance frequency is used as a carrier to modulate the amplitude with an audible sound signal. Then, the ultrasonic wave emitted from the ultrasonic element naturally generates audible sound while propagating through the air due to the self-demodulation action based on the nonlinearity of the air (Tomoo Kamakura “Basics of Nonlinear Acoustics” (Ai Satoshi Publishing) 1996 pp. 107-110). However, as can be seen from the frequency analysis of the amplitude modulation shown in FIG. 7, the upper sideband SU and the lower sideband SD generated by the amplitude modulation are about 50 dB of the ultrasonic signal SU (carrier; 40 KHz in the figure), and 100 dB Even if the above ultrasonic waves are output, only an audible sound (sound, etc.) of about 70 dB can be generated, so that there is a problem that the energy efficiency is about 1/1000.
[0005]
An object of the present invention is to provide an ultrasonic reproduction method and apparatus capable of improving the radiant energy efficiency of audible sound.
[0006]
[Means for Solving the Problems]
According to the present invention, in an ultrasonic reproduction method in which an ultrasonic signal is amplitude-modulated with an audible sound signal, an electroacoustic transducer is driven by the amplitude-modulated ultrasonic signal, and an ultrasonic wave is emitted from the electroacoustic transducer, The conversion unit has a plurality of resonance frequencies, and the frequency of the ultrasonic wave to be amplitude-modulated is sandwiched between the plurality of resonance frequencies, so that the amplitude output characteristic for the ultrasonic signal driving the electroacoustic conversion unit is minimized. We propose an ultrasonic reproduction method.
The present invention further includes an ultrasonic signal oscillator, an amplitude modulator that amplitude-modulates an ultrasonic signal oscillated by the ultrasonic signal oscillator with an audible sound signal, and an electroacoustic converter that radiates the amplitude-modulated ultrasonic signal. In the ultrasonic reproducing apparatus, the electroacoustic conversion unit has a plurality of resonance frequencies, and the frequency of the ultrasonic wave to be amplitude-modulated is sandwiched between the plurality of resonance frequencies, and the amplitude with respect to the ultrasonic signal that drives the electroacoustic conversion unit We propose an ultrasonic reproducing device with a frequency that minimizes the output characteristics.
[0007]
The present invention further proposes an ultrasonic reproducing apparatus in which, in the ultrasonic reproducing apparatus described above, a plurality of piezoelectric elements having resonance frequencies different from each other are mounted on a common substrate.
Action According to the ultrasonic reproduction method and apparatus according to the present invention, the drive frequency of the electroacoustic transducer is selected as a frequency at which the radiation characteristic of the ultrasonic wave sandwiched between the resonance frequencies of the electroacoustic transducer is minimized. Therefore, even if it is driven with an energy lower than the driving energy of the conventional ultrasonic reproduction method in which an ultrasonic signal having a resonance frequency is applied to an ultrasonic element having a single resonance frequency characteristic, an audible sound of the same level as that of the conventional technique is reproduced. be able to. The reason will be clarified by the examples described below.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of an audible sound generating apparatus according to the present invention. In the figure, 1 is an ultrasonic signal source, 2 is an audible sound signal source, 3 is an amplitude modulator, 4 is an amplifier, and 5 is an electroacoustic converter.
The ultrasonic signal source 1 oscillates an ultrasonic signal of fc = 40 KHz, for example. The audible sound signal source 2 inputs an audible sound signal having a frequency in the audible frequency band to the amplitude modulator 3, and the amplitude modulator 3 amplitude-modulates the ultrasonic signal output from the ultrasonic signal source 1. The amplitude-modulated ultrasonic signal is output to the electroacoustic conversion unit 5 amplified by the amplifier 4 as necessary. The electroacoustic transducer 5 can also be configured by arranging a plurality of ultrasonic elements EL in a matrix or concentric circles on one surface.
[0009]
The configuration so far is the same as that of the conventional audible sound generating apparatus. However, in the present invention, the ultrasonic element EL used for the electroacoustic transducer 5 has a plurality of resonance frequencies f1 and f2 as shown in FIG. It is characterized by the point of having a configuration.
In the example shown in FIG. 2, an example is shown in which resonance frequencies are provided at two frequencies f1 and f2. An ultrasonic element EL having a plurality of resonance frequencies f1 and f2 can be obtained by the structure shown in FIG. In FIG. 3, piezoelectric elements 7A and 7B having resonance frequencies f1 and f2 are mounted on one surface of a base 6A that also serves as an electrode, and a ring-shaped electrode 6B is attached to the upper surfaces of the piezoelectric elements 7A and 7B. By connecting the ring-shaped electrode 6B in common, an ultrasonic element EL having a plurality of resonance frequencies can be obtained.
[0010]
As shown in FIG. 2, there is a frequency at which the amplitude output characteristic with respect to the ultrasonic signal for driving the electroacoustic transducer 5 is minimized in the portion sandwiched between the resonance frequencies f1 and f2. The oscillation frequency fc of the signal source 1 is matched, and the electroacoustic transducer 5 is driven around the frequency fc.
Here, when the ultrasonic signal is amplitude-modulated with an audible sound signal having an upper limit frequency of 20 kHz by a double-sideband type amplitude modulator, as shown in FIG. 4, the upper side and the lower side of the frequency fc of the ultrasonic signal as a carrier are shown. A double sideband signal having an upper sideband SU and a lower sideband SD on the side is generated.
[0011]
Assuming that the resonance frequencies f1 and f2 of the electroacoustic transducer 5 shown in FIG. 2 coincide with the frequencies of the substantially center of the upper side band SU and the lower side band SD as shown by broken lines in FIG. Is suppressed in the valley portion of the resonance characteristics, and the radiation level is suppressed compared to the case where the resonance frequency of the conventional ultrasonic transducer and the frequency of the ultrasonic signal coincide with each other. On the other hand, the upper sideband SU and the lower sideband SD are emphasized by the resonance frequencies f1 and f2. Therefore, it is possible to reproduce the audible sound equivalent to the conventional one while keeping the radiation level of the carrier component low.
The reason will be described with reference to FIG. FIG. 5A shows ultrasonic radiation characteristics when the conventional ultrasonic reproducing method shown in FIG. 7 is adopted. Here, the radiation levels of the components of the ultrasonic signal US and the frequencies f1 and f2 are shown. According to the conventional ultrasonic reproducing method, since the frequency of the ultrasonic signal US is matched with the resonance frequency of the ultrasonic element, the ultrasonic signal US is emphasized by the resonance characteristics from the components of the frequencies f1 and f2. The radiation level is assumed to be “3” here, and the radiation levels of the components of the frequencies f1 and f2 are assumed to be “1” here.
[0012]
On the other hand, the ultrasonic radiation level by the ultrasonic reproducing method according to the present invention is shown in FIG. 5B. In the ultrasonic reproducing method according to the present invention, since the frequency of the ultrasonic signal US is set to a frequency at which the ultrasonic radiation characteristic becomes a minimum value, the radiation level of the ultrasonic signal US is suppressed. For example, the radiation level is “2”. On the other hand, since the components of the frequencies f1 and f2 use the frequencies f1 and f2 as resonance frequencies in the present invention, the radiation level is emphasized by the resonance characteristics, for example, the radiation level is emphasized to “1.5”. .
When calculating the radiation energy and sound pressure power of ultrasonic waves from these assumptions,
The ultrasonic radiation energy W1 in the conventional case is W1 = 1 ² +3 ² +1 ² = 11.
The sound pressure power SW1 in the conventional case is SW1 = 3 × 1 + 3 × 1 = 6.
It becomes.
[0013]
The ultrasonic radiation energy W2 in the case of this invention is W2 = 1.5 ² +2 ² +1.5 ² = 8.5.
In the present invention, the sound pressure power SW2 is SW2 = 2 × 1.5 + 2 × 1.5 = 6.
It becomes.
As is clear from these calculation examples, according to the present invention, the ultrasonic radiation energy is suppressed from “11” to “8.5” in order to obtain the same sound pressure power as in the prior art, and is equivalent to the conventional one with a small energy. Sound pressure power can be obtained.
[0014]
In the above description, only the sound pressure power of the components of the frequencies f1 and f2 is calculated for the ultrasonic signal US, but the upper side band SU and the lower side band that are originally generated above and below the ultrasonic signal US. Since the SD component is integrated and emphasized, the sound pressure power SW2 is further emphasized. Therefore, if the same sound pressure power SW2 is obtained, the ultrasonic radiation energy W2 can be further suppressed.
[0015]
【The invention's effect】
As described above, according to the present invention, it is possible to radiate an ultrasonic wave having a sound pressure equivalent to that of the prior art with energy smaller than that of the prior art. Therefore, there is an advantage that the ultrasonic waves can be generated efficiently.
[Brief description of the drawings]
FIG. 1 is a block diagram for explaining an embodiment of the present invention.
FIG. 2 is a characteristic curve diagram for explaining frequency characteristics of an ultrasonic element used in the present invention.
3 is a cross-sectional view for explaining an example of a structure of an ultrasonic element exhibiting frequency characteristics shown in FIG. 2;
4 is a graph for explaining the relationship between the frequency characteristics of the ultrasonic element shown in FIG. 2 and the signal components of an amplitude-modulated ultrasonic signal. FIG.
FIG. 5 is a graph for explaining the effect of the present invention.
FIG. 6 is a curve diagram for explaining the ultrasonic radiation characteristics obtained by the apparatus according to the present invention.
FIG. 7 is a curve diagram for explaining ultrasonic radiation characteristics in a conventional apparatus.
[Explanation of symbols]
1 Ultrasonic Signal Source EL Ultrasonic Element 2 Audible Sound Signal Source 6A Base 3 Amplitude Modulator 6B Electrode 4 Amplifier 7A, 7B Piezoelectric Element 5 Electroacoustic Converter

Claims (3)

In the ultrasonic reproduction method of amplitude-modulating an ultrasonic signal with an audible sound signal, driving the electroacoustic conversion unit with the amplitude-modulated ultrasonic signal and emitting ultrasonic waves from the electroacoustic conversion unit,
The electroacoustic transducer has a plurality of resonance frequencies, and the amplitude-modulated ultrasonic frequency is sandwiched between the plurality of resonance frequencies, and the amplitude output characteristic for the ultrasonic signal that drives the electroacoustic transducer is minimal. An ultrasonic reproduction method characterized in that the frequency is Ultrasound composed of an ultrasonic signal oscillator, an amplitude modulator that modulates the ultrasonic signal generated by the ultrasonic signal oscillator with an audible sound signal, and an electroacoustic transducer that emits the amplitude-modulated ultrasonic signal In the playback device,
The electroacoustic transducer has a plurality of resonance frequencies, and the amplitude-modulated ultrasonic frequency is sandwiched between the plurality of resonance frequencies, and the amplitude output characteristic for the ultrasonic signal that drives the electroacoustic transducer is minimal. An ultrasonic reproducing apparatus having a frequency of   3. The ultrasonic reproduction apparatus according to claim 2, wherein the electroacoustic conversion unit has a plurality of piezoelectric elements having different resonance frequencies mounted on a common substrate.

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