JP3668187B2 - Sound reproduction method and sound reproduction apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an acoustic reproduction method and an acoustic reproduction apparatus that can be used for various types of loudspeakers.
[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. In these conference systems, a normal speaker is used as means for reproducing the voice of the other party.
[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 around them, and they can be heard as unnecessary noise around them. There was a problem. As one method for solving this, a method using a parametric speaker in which an ultrasonic signal is amplitude-modulated with an audible sound signal and the audible sound is reproduced by emitting the amplitude-modulated ultrasonic wave can be considered.
[0004]
However, the parametric speaker has a problem that the reproduction efficiency in the high sound range and the low sound range is lower than that in the mid sound range due to the resonance characteristics of the ultrasonic element. The reason will be briefly described below.
FIG. 9 shows a schematic configuration of a generally well-known parametric speaker. The parametric speaker includes an audible sound signal source 1, an amplitude modulation means 2, an ultrasonic signal generation means 3, an amplifier 4, and an electroacoustic conversion unit 5.
In the amplitude modulation means 2, the ultrasonic signal US having a frequency of, for example, about 40 KHz given from the ultrasonic signal generation means 3 is amplitude-modulated with the audible sound signal OD given from the audible sound signal source 1, and the amplitude-modulated ultrasonic wave The signal is amplified by the amplifier 4 as necessary, and the amplified output is applied to the electroacoustic converter 5. The electroacoustic conversion unit 5 includes a plurality of ultrasonic vibration elements EL, drives the plurality of ultrasonic vibration elements EL with an amplitude-modulated ultrasonic signal, and radiates ultrasonic waves into the air.
[0005]
The ultrasonic wave radiated into the air generates an audible sound by the self-demodulation action due to the nonlinearity of the air.
Here, if the amplitude modulation means 2 is a double-sided modulation type amplitude modulator, the amplitude-modulated modulated signal has a frequency ωo of the ultrasonic signal US as a carrier frequency as shown in FIG. An upper sideband SBU and a lower sideband SBD are generated above and below the carrier frequency ωo. When the lower limit frequency of the audible sound signal OD is ωL and the upper limit frequency is ωH, the lower limit frequency of the upper sideband SBU is ωo + ωL, and the upper limit frequency is ωo + ωH. The lower limit frequency of the lower sideband SBD is ωo−ωH, and the upper limit frequency is ωo−ωL.
[0006]
On the other hand, the sound pressure of the audible sound generated by the self-demodulation action of ultrasonic waves has a frequency characteristic that is directly proportional to the square of the frequency ω (ω ² ) within the audible frequency band (ωL to ωH) as shown in FIG. Given. This frequency characteristic acts as a characteristic that suppresses the low frequency components ωo + ωL and ωo−ωL sides as shown by broken lines in FIG. 10 for both the upper sideband SBU and the lower sideband SBD.
Therefore, if the frequency is corrected by a characteristic of 1 / ω ² from the low frequency component ωL to the high frequency component ωH of the audible sound signal OD by an equalizer or the like, this frequency-corrected audible sound signal is input to the amplitude modulation means 2. Conventionally, as one method for solving this inconvenience, the resonance characteristic RS of the ultrasonic vibration element EL corresponds to the square characteristic of the frequency in the audible sound signal OD in the frequency region below the resonance frequency ωo. It is attenuated by the following characteristic f (ω),
f (ω) = {ωL / (ωo−ω)} ² (ωo−ωH ≦ ω ≦ ωo−ωL)
Here, f (ω) represents the attenuation rate with respect to the gain at the frequency ωo−ωL.
In the frequency region above the resonance frequency ωo, a method is adopted in which the frequency characteristic of the sound pressure of the audible sound reproduced without an equalizer is corrected to a flat frequency characteristic by using the following characteristic g (ω). .
[0007]
g (ω) = {ωL / (ω−ωo)} ² (ωo + ωL ≦ ω ≦ ωo + ωH)
However, g (ω) represents the attenuation rate with respect to the gain at the frequency ωo + ωL.
FIG. 12 shows ultrasonic radiation amplitude characteristics of a general ultrasonic vibration element. The frequency region A1 below the resonance frequency ωo has a region DW1 that attenuates with the characteristic f (ω), and the frequency region A2 above the resonance frequency ωo has a region DW2 that attenuates with the property g (ω). . If this ultrasonic vibration element is driven by making the frequency of the ultrasonic signal US shown in FIG. 10 coincide with the resonance frequency ωo shown in FIG. 12, in the frequency regions A1 and A2 within the range of the attenuation regions DW1 and DW2. The frequency characteristic (ω ² ) of the sound pressure of the audible sound generated by the self-demodulation action of the ultrasonic wave is canceled according to the characteristics of the attenuation characteristics DW1 and DW2, and the sound pressure generation characteristic of the audible sound is corrected to a flat frequency characteristic. Can do.
[0008]
However, in an ordinary ordinary ultrasonic transducer, there is a flat portion X that deviates from the characteristics of the attenuation regions DW1 and DW2 in the vicinity of the resonance frequency ωo. The low frequency component of the audible sound generated by the ultrasonic wave due to the presence of the flat portion X is affected by the frequency characteristic (ω ² ) of the sound pressure of the audible sound generated by the self-demodulation action of the ultrasonic wave, and the generation level is The phenomenon of decreasing occurs.
This phenomenon will be described with reference to FIGS. In the resonance characteristic RS of the ultrasonic vibration element shown in FIG. 12, when the frequency up to the upper limit and the lower limit of the flat portion X is ω1, the lower limit frequency of the flat portion X is ωo−ω1, and the upper limit frequency is ωo + ω1. For example, when ω1 is 1 KHz, the audible signal component higher than 1 KHz among the components of the lower sideband SBD and the upper sideband SBU included in the amplitude-modulated ultrasonic signal shown in FIG. Corrected by the slope characteristics of the attenuation regions DW1 and DW2, audible sound is generated with a flat frequency characteristic B shown in FIG.
[0009]
On the other hand, since the audible signal component lower than 1 KHz included in the lower sideband SBD and the upper sideband SBU is outside the range of the attenuation regions DW1 and DW2, the frequency characteristic of the audible signal component lower than 1 KHz is frequency-corrected. First, it exhibits a characteristic that gradually decreases according to the frequency characteristic (ω ² ) given to the sound pressure of the audible sound generated by the self-demodulation action of the ultrasonic wave (curve C in FIG. 13). In order to eliminate this inconvenience, as shown by the broken line in FIG. 12, the resonance characteristic of the ultrasonic vibration element is audible lower than 1 KHz by using the ultrasonic vibration element having a characteristic in which the attenuation regions DW1 and DW2 are extended to the resonance frequency ωo. The signal component is reproduced with the characteristic of the curve D shown in FIG. 13, but it is difficult to obtain an ultrasonic vibration element having such a characteristic.
[0010]
The above has described the inconvenience occurring on the low frequency side, but there is also a disadvantage that the generation level of the high frequency component is lowered on the high frequency side. The reason is that the high frequency component ωH included in the lower sideband SBD and the upper sideband SBU shown in FIG. 10 is located at a frequency farthest from the frequency ωo of the ultrasonic signal US. For this reason, when the ultrasonic vibration element having the ultrasonic radiation amplitude characteristic shown in FIG. 12 is driven so that the resonance frequency ωo matches the frequency of the ultrasonic signal US, the lower sideband SBD and the upper sideband SBU are driven. The included high frequency component ωH is driven in a frequency region where the ultrasonic radiation efficiency is poor. The high frequency component ωH side is emphasized by the frequency characteristic (ω ² ) of the sound pressure of the audible sound generated by the self-demodulation action of the ultrasonic wave, but it is driven in the frequency region where the radiation efficiency of the ultrasonic vibration element is poor. As a result, there arises a second disadvantage that the generation level on the high frequency side also decreases.
An object of the present invention is to propose an acoustic reproduction method and an acoustic reproduction apparatus capable of reproducing both the low-frequency side and the high-frequency side at a level equivalent to that of the middle sound range.
[0011]
[Means for Solving the Problems]
In the present invention, the output amplitude characteristic has a flat portion with a predetermined bandwidth straddling the resonance frequency, and at least one of the regions having a frequency higher than the upper limit frequency or lower than the lower limit frequency of the bandwidth than the flat portion. In an acoustic reproduction method for reproducing an audible sound by driving an electroacoustic transducer having an attenuation characteristic in which an output amplitude characteristic is attenuated by an ultrasonic signal obtained by amplitude-modulating an input ultrasonic signal with an audible sound signal, the audible sound signal is predetermined. Filters the low frequency component lower than the audible sound frequency and the high frequency component higher than the predetermined audible sound frequency, and modulates the amplitude of the ultrasonic signal with each of the low frequency component and the high frequency component with a single-side waveform, and is higher than the upper limit frequency. The attenuation region or the attenuation region lower than the lower limit frequency is the band of the signal obtained by modulating the amplitude of the ultrasonic signal with the low frequency component of the audible sound signal, and the flat region is the amplitude of the ultrasonic signal with the high frequency component of the audible sound signal. Proposes an acoustic playback method being characterized in that the bandwidth of the tone signal.
[0012]
Further, in the sound reproduction method of the present invention, when the amplitude modulation is performed on the low frequency component of the audible sound signal to a single side wave having a lower frequency than the input ultrasonic signal, the high frequency component of the audible sound signal is higher than the input ultrasonic signal. When the amplitude is modulated to a single side wave having a higher frequency than the input ultrasonic signal by the low frequency component of the audible sound signal, the input ultrasonic wave is input by the amplitude modulation means with the high frequency component of the audible sound signal. A sound reproduction method is proposed in which amplitude modulation is performed to a single side wave having a frequency lower than that of a signal.
Further, according to the present invention, the output amplitude characteristic has a flat portion with a predetermined bandwidth across the resonance frequency, and the flat portion is in at least one of a region having a frequency higher than the upper limit frequency or lower than the lower limit frequency of the bandwidth. An electroacoustic transducer having an attenuation characteristic in which the output amplitude characteristic is attenuated, an ultrasonic signal generating means for generating an ultrasonic signal matching a frequency near the lower limit frequency, and an audible sound signal from a predetermined audible sound frequency An ultrasonic signal generated by an ultrasonic signal generating means by a filtering means for filtering a low-frequency component and a high-frequency component higher than a predetermined audible sound frequency, and an audible sound signal filtered by the filtering means. An ultrasonic signal generated by the ultrasonic signal generating means by the first amplitude modulator for amplitude-modulating the single side wave of the lower sideband in the form of amplitude modulation and the high frequency component of the audible signal filtered by the filtering means The upper wave A second amplitude modulator that performs amplitude modulation in an amplitude modulation format that provides a single side wave, and an ultrasonic signal that is amplitude-modulated with a single-side waveform type by the first amplitude modulator and the second amplitude modulator, and A sound reproducing device is proposed which is constituted by adding means for applying the added ultrasonic signal to an electroacoustic transducer.
[0013]
Further, according to the present invention, the output amplitude characteristic has a flat portion with a predetermined bandwidth across the resonance frequency, and the flat portion is in at least one of a region having a frequency higher than the upper limit frequency or lower than the lower limit frequency of the bandwidth. An electroacoustic transducer having an attenuation characteristic that attenuates the output amplitude characteristic, an ultrasonic signal generating means for generating an ultrasonic signal that matches a frequency near the upper limit frequency, and an audible sound signal from a predetermined audible sound frequency. An ultrasonic signal generated by an ultrasonic signal generating means by a filtering means for filtering a low-frequency component and a high-frequency component higher than a predetermined audible sound frequency, and an audible sound signal filtered by the filtering means. An ultrasonic signal generated by the ultrasonic signal generating means by a first amplitude modulator that modulates the amplitude of the audible signal in an amplitude modulation format that can obtain a single sideband of the upper sideband, and a high frequency component of the audible signal filtered by the filtering means Lower side wave A second amplitude modulator that performs amplitude modulation in an amplitude modulation format that provides a single side wave, and an ultrasonic signal that is amplitude-modulated with a single-side waveform type by the first amplitude modulator and the second amplitude modulator, and A sound reproducing device is proposed which is constituted by adding means for applying the added ultrasonic signal to an electroacoustic transducer.
[0014]
According to the sound reproducing method and a sound reproducing apparatus according to the action <br/> this invention, an audible sound signal is separated into low frequency components and high frequency components, these low-frequency components and high-frequency component of the single sideband modulation method Amplitude modulation is performed by the amplitude modulator, and the frequency (carrier frequency) of the ultrasonic signal is matched with the lower limit frequency or the upper limit frequency of the flat portion X of the ultrasonic vibration element, thereby generating a single sideband generated by a low frequency component. It is arranged in the frequency domain that exhibits the attenuation characteristics of the ultrasonic vibration element, and the single sideband generated by the high frequency component is arranged on the flat part of the ultrasonic vibration element to drive the ultrasonic vibration element. All the frequency characteristics are corrected by the attenuation region of the resonance characteristics of the ultrasonic vibration element, and the entire low frequency band of the audible sound signal is reproduced as an audible sound with a flat frequency characteristic.
In addition, since the high frequency component of the audible sound is driven close to the frequency region of the flat portion X existing in the frequency characteristics of the ultrasonic vibration element, the high frequency radiation efficiency is improved and the high frequency component of the audible sound is improved. In addition, there is an advantage that it can be efficiently reproduced.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of the present invention. In the figure, reference numeral 1 denotes an audible sound signal source. In the present invention, the audible sound signal OD output from the audible sound signal source 1 is supplied to the bandpass filters 11 and 12, and the audible sound signal OD is separated into a low frequency component and a high frequency component.
The band filter 11 filters the low frequency component LOW from the audible sound signal OD, and the band filter 12 filters the high frequency component HI from the audible sound signal OD. Assuming that the frequency band of the audible sound signal OD is ωL to ωH and that the resonance characteristics of the ultrasonic vibration element EL used in the electroacoustic transducer 5 are the frequency characteristics shown in FIG. As shown by a curve BP1 in FIG. 3, the wave filter 11 filters the frequency components (ωL to ωM) from the lower limit frequency ωL of the audible sound signal OD to the predetermined frequency ωM, and the low frequency component LOW of the audible sound signal OD. Is output. On the other hand, the bandpass filter 12 filters the frequency components (ωM to ωH) as shown by a curve BP2 in FIG. 3, and outputs the high frequency component HI of the audible sound signal OD.
[0016]
The low-frequency component LOW and high-frequency component HI of the audible sound signal OD filtered by the bandpass filters 11 and 12 are input to the first and second amplitude modulators 13 and 14 of the single side wave modulation method, and an ultrasonic signal is generated. The ultrasonic signal US input from the means 3 is amplitude-modulated by the single side wave modulation method with the low frequency component LOW and the high frequency component HI of the audible sound signal.
Here, the frequency of the ultrasonic signal US inputted from the ultrasonic signal generating means 3 to the first and second amplitude modulators 13 and 14 is set to the flat portion X of the ultrasonic vibration element EL, here the lower limit frequency ωo−ω1 ( Hereinafter, this frequency is expressed as ωC). Further, in this case, the first amplitude modulator 13 that modulates the amplitude of the ultrasonic signal US with the low frequency component LOW of the audible sound signal OD is assumed to be a single side modulation method that generates a lower sideband. Further, the second amplitude modulator 14 uses an amplitude modulator that modulates the amplitude of the ultrasonic signal US having the frequency ωC with the high frequency component HI of the audible sound signal OD and generates an upper wave.
[0017]
FIG. 4 and FIG. 5 show the frequency distribution of the modulated signal amplitude-modulated under the above conditions. FIG. 4 shows the result of amplitude modulation of the ultrasonic signal US with the low frequency component _LOW to obtain the lower sideband SB _LOW . The lower sideband SB _LOW has a frequency component of ωC−ωM on the lower limit side and a frequency component of ωC−ωL on the upper limit side. If ωL = 20 Hz and ωM = 1 KHz, the bandwidth of the lower sideband SB _LOW is 980 Hz.
FIG. 5 shows the result of amplitude modulation of the ultrasonic signal US with the high frequency component _HI to obtain the upper sideband SB _HI . The upper limit frequency of the upper sideband SB _HI is ωC + ωH, and the lower limit frequency is ωC + ωM. In this example, the bandwidth is (ωC + ωH) − (ωC + ωM) = 20 KHz-1 KHz = 19 KHz.
[0018]
The lower side band SB _LOW and the upper side band SB _HI obtained by the amplitude modulation are taken out with band limiting by the filters 15 and 16, added by the adder 17, amplified by the amplifier 18, and amplified by the electroacoustic conversion unit 5. And the low frequency component LOW and the high frequency component HI of the audible sound signal OD included in the lower side band SB _LOW and the upper side band SB _HI are radiated as an ultrasonic wave from the electroacoustic transducer 5. Can be played as.
Here, since the frequency ωC of the ultrasonic signal US is matched with the lower limit frequency ωo−ω1 of the flat portion X of the resonance characteristic RS of the ultrasonic vibration element EL, the ultrasonic signal US is obtained by amplitude-modulating the low-frequency component LOW. Further, the entire lower band SB _LOW can be included in the lower attenuation region DW1 of the resonance characteristic RS of the ultrasonic transducer EL.
[0019]
As a result, the frequency characteristic of the low frequency component LOW of the audible sound signal OD is corrected by the attenuation region DW1 of the resonance characteristic RS, and all bands from ωL to ωM = 20 Hz to 1 KHz are reproduced as audible sounds with flat frequency characteristics. be able to.
On the other hand, the high frequency component HI ultrasonic signal upper sideband obtained by amplitude modulating the US SB _HI is the lower limit frequency .omega.C + .omega.M substantially substantially coincides with the resonance frequency ωo of the ultrasonic vibration element EL, the resonant frequency ωo Since the upper limit frequency ωC + ωH is arranged with a bandwidth of about 19 KHz, the resonance frequency ωo can be made closer to the resonance frequency ωo by the frequency of ω1 than the upper limit frequency ωo + ωH in the case of the conventional double sideband modulation method described in FIG.
[0020]
As a result, the high-frequency component HI can be driven in a frequency region with good responsiveness of the ultrasonic vibration element EL, so that there is an advantage that ultrasonic waves having the same intensity as the conventional one can be emitted with a small driving power. .
FIG. 6 shows a schematic configuration of single sideband modulation type amplitude modulators 13 and 14. The single sideband modulation type amplitude modulators 13 and 14 include an amplifier M, 90 ° phase shifters φ1 and φ2, two multipliers MU1 and MU2, and two adders ADD1 and ADD2. Is done. Consider a case where an audible sound signal cosΩt and an ultrasonic signal cosωot are generated in the audible sound signal source 1 and the ultrasonic signal generation means 3, respectively. Both are 90 ° phase shifted by 90 ° phase shifters φ1 and φ2 to become −sinΩt and −sinωot, respectively, and are multiplied by a multiplier MU1 to obtain an amplitude modulation signal sinΩt sinωot. On the other hand, the multiplier MU2 multiplies the audible sound signal cosΩt and the ultrasonic signal cosωot with each other to obtain an amplitude modulation signal cosΩt cosωot.
[0021]
Here, in order to obtain the component cos (ωo + Ω) {= cosΩt cosωot−sinΩt sinωot} of the frequency ωo + Ω indicating a single sideband having a frequency higher than the resonance frequency ωo, the polarity of the amplitude modulation signal cosΩt cosωot is made positive in the adder ADD2. The polarity of the amplitude modulation signal sinΩt sinωot is made negative.
Here, in order to obtain the component cos (ωo−Ω) {= cosΩt cosωot + sinΩt sinωot} of the frequency ωo−Ω indicating a single sideband having a frequency lower than the resonance frequency ωo, the adder ADD2 uses the amplitude modulation signal cosΩt cosωot and amplitude modulation. Both signals sinΩt and sinωot are positive in polarity. Note that ADD1 is an adder for emitting the ultrasonic signal cosωot together with the upper side wave signal or the lower side wave signal in the air to generate an ultrasonic self-demodulation action.
[0022]
7 and 8 show the upper sideband as the modulation result of the low frequency component LOW when the amplitude of the ultrasonic signal US is modulated by the amplitude modulators 13 and 14 (see FIG. 1) with the low frequency component LOW of the audible sound signal OD. The frequency distribution of each modulated signal in the case where SB ′ _LOW is obtained and the lower sideband SB ′ _HI is obtained as the modulation result of the high frequency component HI is shown.
In this case, the frequency ωC of the ultrasonic signal US is matched with the upper limit frequency ωo + ω1 of the flat portion X of the ultrasonic vibration element EL. Therefore, the lower limit frequency of the upper sideband SB ′ _LOW obtained by amplitude modulating the ultrasonic signal US with the low frequency component LOW is ωC + ωL, the upper limit frequency is ωC + ωM, and the bandwidths are ωL = 20 Hz and ω1 = 1 kHz in this example. Then, 1 KHz-20 Hz = 980 Hz.
[0023]
The lower limit frequency of the lower sideband SB' _HI obtained by amplitude-modulated ultrasonic signal US in the high-frequency component HI is .omega.C-.omega.H, the upper limit frequency .omega.C-.omega.M next, the bandwidth in this example .omega.H = When 20 KHz and ω1 = 1 KHz, the frequency is 19 KHz.
Accordingly, in this case as well, the upper side band SB ′ _LOW including the low frequency component LOW can be within the range of the attenuation characteristic DW2 above the resonance characteristic RS of the ultrasonic vibration element EL when the bandwidth is 980 Hz. That is, even when the low frequency component _LOW is amplitude-modulated with the upper side band SB ′ _LOW , the reproduction frequency characteristic of the audible sound is corrected to a flat characteristic by the attenuation characteristic DW2 of the resonance characteristic RS for the entire low frequency band LOW. Can do.
[0024]
Further, since the lower sideband SB ′ _HI obtained by amplitude-modulating the ultrasonic signal US with the high frequency component HI also has a lower limit frequency of ωC−ωH, ωC in this case is ωo + ω1. Since ωo + ω1-ωH, the lower limit frequency can be made closer to the resonance frequency ωo by the frequency of ω1. As a result, there is an advantage that the ultrasonic vibration element EL can be driven in a frequency band with good responsiveness by the high frequency component HI.
In the above description, the band-pass filters 11 and 12 are filtered with ωM = 1 KHz or less as the low-frequency component LOW and 1 KHz or more as the high-frequency component HI with 1 KHz as the boundary. The value of 1 KHz is the value of the ultrasonic vibration element EL. It can be easily understood that the value is selected according to the resonance characteristics and is not particularly limited to 1 KHz.
[0025]
【The invention's effect】
As described above, according to the present invention, an ultrasonic signal is amplitude-modulated by an audible sound signal, an electroacoustic vibration element constituted by an ultrasonic transducer is driven by the amplitude-modulated ultrasonic signal, In the sound reproduction method for reproducing the audible sound by the above, even if the ultrasonic vibrator has the flat portion X near the resonance frequency, the reproduction frequency characteristic of the audible sound generated by the presence of the flat portion X is low. It is possible to eliminate the disadvantage that the low frequency component sound is difficult to reproduce.
[0026]
In addition, the high-frequency component also drives the ultrasonic vibration element so that the entire band including the high-frequency component is biased in a direction approaching the resonance frequency ω ₀ of the ultrasonic vibration element. Since driving is performed in a region, there is an advantage that high frequency components can be efficiently reproduced.
[Brief description of the drawings]
FIG. 1 is a block diagram for explaining an embodiment of a sound reproducing device for realizing a sound reproducing method according to the present invention.
FIG. 2 is a characteristic curve diagram for explaining a resonance characteristic curve of an ultrasonic vibration element used in the electroacoustic transducer shown in FIG.
FIG. 3 is a characteristic curve diagram for explaining the filtering characteristics of the bandpass filter shown in FIG. 1;
FIG. 4 is a characteristic curve diagram for explaining the frequency distribution of signal components for explaining the operation of the main part of the present invention.
5 is a characteristic curve diagram similar to FIG.
6 is a block diagram for explaining the outline of the single sideband modulation type amplitude modulator used in the embodiment shown in FIG. 1; FIG.
FIG. 7 is a characteristic curve diagram for explaining a modified embodiment of the present invention.
FIG. 8 is a characteristic curve diagram similar to FIG.
FIG. 9 is a block diagram for explaining a conventional technique.
FIG. 10 is a characteristic curve diagram for explaining the distribution of frequency components of a modulated signal obtained by amplitude modulation using a double sideband modulation method used in the prior art.
FIG. 11 is a characteristic curve diagram for explaining frequency characteristics given to sound pressure of audible sound reproduced by self-demodulation action of ultrasonic waves.
FIG. 12 is a characteristic curve diagram for explaining inconveniences occurring in the conventional technique.
13 is a characteristic curve diagram similar to FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Audible sound signal source 14 2nd amplitude modulator 5 Electroacoustic conversion part 15, 16 Filter EL Ultrasonic vibration element 17 Adder 11, 12 Band filter 18 Amplifier 13 1st amplitude modulator

Claims (4)

It has a flat portion of the output amplitude characteristic with a predetermined bandwidth across the resonance frequency, and the output amplitude is higher than the flat portion in at least one of the frequency range higher than the upper limit frequency of the bandwidth or lower than the lower limit frequency. In an acoustic reproduction method of reproducing an audible sound by driving an electroacoustic transducer having an attenuation characteristic whose characteristic is attenuated by an ultrasonic signal obtained by amplitude-modulating an input ultrasonic signal with an audible sound signal,
The audible sound signal is filtered into a low-frequency component lower than a predetermined audible sound frequency and a high-frequency component higher than the predetermined audible sound frequency, and the ultrasonic signal is respectively converted into a single-side waveform by the low-frequency component and the high-frequency component. The amplitude modulation is performed, and the attenuation region higher than the upper limit frequency or the attenuation region lower than the lower limit frequency is set as the band of the signal obtained by amplitude-modulating the ultrasonic signal with the low frequency component of the audible sound signal, and the flat region is the ultrasonic wave. A sound reproduction method characterized in that the signal is a signal band amplitude-modulated by a high frequency component of the audible sound signal. 2. The sound reproducing method according to claim 1, wherein when the amplitude of the low frequency component of the audible sound signal is amplitude-modulated to a single side wave having a frequency lower than that of the input ultrasonic signal, the input ultrasonic wave is detected with the high frequency component of the audible sound signal. Amplitude modulation to a single side wave of higher frequency than the signal,
When amplitude modulation is performed on a single side wave having a higher frequency than the input ultrasonic signal with a low-frequency component of the audible sound signal, the amplitude modulation means has a frequency lower than that of the input ultrasonic signal with the high-frequency component of the audible sound signal. An acoustic reproduction method, wherein amplitude modulation is performed on a single side wave. It has a flat portion of the output amplitude characteristic with a predetermined bandwidth across the resonance frequency, and the output amplitude is higher than the flat portion in at least one of the frequency range higher than the upper limit frequency of the bandwidth or lower than the lower limit frequency. An electroacoustic transducer having an attenuation characteristic that attenuates the characteristic;
Ultrasonic signal generating means for generating an ultrasonic signal that matches a frequency in the vicinity of the lower limit frequency;
Filtering means for filtering the audible sound signal into a low frequency component lower than a predetermined audible sound frequency and a high frequency component higher than the predetermined audible sound frequency;
The first amplitude modulation that modulates the ultrasonic signal generated by the ultrasonic signal generating means by the low frequency component of the audible sound signal filtered by the filtering means in an amplitude modulation format that can obtain a single sideband of the lower sideband. And
A second amplitude modulator for amplitude-modulating the ultrasonic signal generated by the ultrasonic signal generating means by the high-frequency component of the audible signal filtered by the filtering means in an amplitude modulation format in which an upper side band single side wave is obtained; ,
Adding means for adding ultrasonic signals amplitude-modulated with a single-side waveform type by the first amplitude modulator and the second amplitude modulator, and applying the added ultrasonic signals to the electroacoustic converter;
A sound reproduction device characterized by comprising: It has a flat portion of the output amplitude characteristic with a predetermined bandwidth across the resonance frequency, and the output amplitude is higher than the flat portion in at least one of the frequency range higher than the upper limit frequency of the bandwidth or lower than the lower limit frequency. An electroacoustic transducer having an attenuation characteristic that attenuates the characteristic;
An ultrasonic signal generating means for generating an ultrasonic signal matching a frequency in the vicinity of the upper limit frequency;
Filtering means for filtering the audible sound signal into a low frequency component lower than a predetermined audible sound frequency and a high frequency component higher than the predetermined audible sound frequency;
A first amplitude modulator that modulates the ultrasonic signal generated by the ultrasonic signal generating means by the low frequency component of the audible sound signal filtered by the filtering means in an amplitude modulation format that provides a single side wave in the upper side band. When,
A second amplitude modulator for amplitude-modulating the ultrasonic signal generated by the ultrasonic signal generating means by the high-frequency component of the audible signal filtered by the filtering means in an amplitude modulation format in which a single side wave in the lower sideband is obtained. When,
Adding means for adding ultrasonic signals amplitude-modulated with a single-side waveform type by the first amplitude modulator and the second amplitude modulator, and applying the added ultrasonic signals to the electroacoustic converter;
A sound reproduction device characterized by comprising:

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