JPS60201275A - Electroacoustic transducer - Google Patents

Abstract

PURPOSE:To obtain the value of an optimum parameter for obtaining high reproduced sound pressure of a secondary wave by selecting the ultrasonic wave frequency of a carrier within a specific range and setting the initial sound pressure level of an unmodulated carrier ultrasonic wave within a fixed range. CONSTITUTION:The maximum frequency of a modulated wave which is considered as to a parametric speaker is fm=20kHz, and fm=8kHz is sufficient for only a voice wave containing no instrument sound component. The range of the primary wave frequency optimum for the parametric speaker is f=25-40kHz. The optimum value of the primary wave initial sound pressure in this case is >=150dB. Cavitation, however, occurs almost at 160dB, so the propagating wave motion equation of a finite amplitude wave does not hold any more. Therefore, the optimum value of the initial sound pressure is 150-155dB. The maximum sound pressure of the secondary wave is therefore about 110-120dB.

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION The present invention relates to an electroacoustic transducer for radiating an electrical signal in an audible frequency band into the air as an acoustic signal.

Prior Art Currently, electrodynamic direct radiation speakers and horn-loaded speakers are the mainstream electroacoustic transducers, but in both systems, air compression waves are created by vibrating a diaphragm in the air, which generates mechanical vibrations. It converts energy into acoustic energy.

The present invention uses a completely different means from conventional acoustic transducers such as speakers, that is, it utilizes the parametric effect of finite amplitude sound waves due to air nonlinearity. Sound waves (2
(referred to as the next wave).

It is characterized by having a directivity pattern equivalent to carrier sound waves in the ultrasonic range.

Ultrasonic waves that have been amplitude-modulated by signals in the audio frequency band are radiated into a medium such as air or water at a finite amplitude level, and a demodulated sound wave is generated in the medium by a self-demodulating action based on the nonlinear effect of the air. Various methods have already been reported for using parametric speakers as communication means. Parametric speakers that utilize this nonlinear phenomenon of sound waves are characterized by their sharp directivity, but in general, as the frequency of ultrasonic waves increases, the sound waves emitted from the transducer become beam-shaped. Go straight.

Now, if we assume that amplitude-modulated ultrasonic waves are emitted in the form of a beam from a transducer array with radius q, then
The sound pressure P at a point of distance can be expressed by the following equation.

(Where, Go is the speed of sound, α is the attenuation coefficient of the sound wave with angular frequency ω0, po is the initial sound pressure, m is the degree of modulation, and g is the modulated wave.) A finite amplitude level expressed by equation (1) The sound pressure of the secondary wave generated when the harmonic wave of is demodulated in air by nonlinear parametric action is expressed by the following non-homogeneous wave equation.

In equation (2), Ps is the sound pressure of the secondary wave, ρ0 is the air density, and q is the virtual sound source density of the secondary wave generated in the primary wave beam, and this q can be expressed by the following equation.

Therefore, by calculating the virtual sound source density at a point at distance X (on the axis) from the array from equations (1) and (3), we obtain the following equation. The second term represents the virtual sound source density of the distortion component.

Furthermore, there is an rAM modulation method as a modulation method for reducing distortion components of secondary waves.

This FAM modulation method is a modulation method in which a certain DC component is added to a modulated signal, F-modulated, and then the product with a carrier signal is calculated. In this case, the modulated signal can be expressed by the following equation.

V= 5l-17Li 〒) sin (11゜t: Strike・-(5) Therefore, the sound pressure of the primary wave (modulated ultrasonic wave) at a distance of X from the transducer array is. In this case Using equation (3) for the virtual sound source density of the secondary wave, we get: Therefore, if the single modulation method is used, the distortion component shown in the second term on the right side of equation (4) will be reduced, and the quality of the reproduced sound will be improved. is significantly improved.

However, when an electroacoustic transducer using the parametric effect as described above is put into practical use, low conversion efficiency is a major problem. On the other hand, the reproduced sound pressure level of the secondary wave is determined by various parameters. Among these parameters, parameters such as ρ02CO+β have fixed values depending on the medium used, but the ultrasonic frequency (f
), initial sound pressure (Po), array diameter (0), etc. are parameters whose values can be selected at the time of setting H1.

The present invention has been made in view of the above-mentioned circumstances.
In particular, when putting into practical use electroacoustic transducers for audible sound reproduction that utilize parametric effects in the air, we aimed to obtain optimal parameter values to obtain high reproduction sound pressure of secondary waves. It has been done.

BEST MODE FOR CARRYING OUT THE INVENTION The structure of the present invention will be described below based on examples.

Figure 1 shows the effects of the primary wave frequency (f) and the initial sound pressure (Po) of the primary wave on the secondary wave sound pressure, which are parameters related to the primary wave, determined by computer simulation. As is clear from the figure, the lower the primary wave frequency, the better, but it must be at least twice the maximum frequency fm of the modulated wave. At frequencies below this, as shown in FIG. 2, spectral folding occurs during modulation. Furthermore, when the frequency is low, the attenuation of the primary wave in the air is small, which poses a problem of physiological effects on the listener, which is not desirable in practice. Also, the primary wave frequency is 2
If the frequency is below 0 kHz, there is a drawback that the listener can hear the primary wave together with the secondary wave.

The maximum frequency of the modulated wave that can be considered now is fm = 20kll
z, and in the case of only audio waves without instruments, fm = 8
About kHz is sufficient. From the above considerations, the optimal primary wave frequency range for parametric speakers is f = 2
5 kHz to 40 kl (z). In this case, the optimum value of the initial sound pressure of the primary wave is approximately 150 dB or more as shown in Fig. 1. However, cavitation begins to occur near 160 dB, so in this case, the finite amplitude The wave propagation wave equation no longer holds true. Therefore, the optimal value of the initial sound pressure is 150 to 155 dB. Therefore, f = 25kllz
The maximum sound pressure of the secondary wave obtained in the range of ~40 kllz and Po=150 to 155 dB is approximately 110 dB to 120 dB.

Layer---US As is clear from the above description, according to the present invention, optimal conditions for the primary wave can be obtained to obtain a high secondary wave sound pressure at a practical level.

[Brief explanation of drawings]

Figure 1 shows the results of a computer simulation of the influence of the initial sound pressure of the primary wave on the secondary wave sound pressure using the primary wave frequency as a parameter. It is a figure which shows the spectrum in the case of twice or less of a frequency. Figure 10 Initial sound JLP. Figure 2 Procedural amendment (May 11, 1980 Director General of the Patent Office Kazuo Wakasugi 1, Indication of the case 1983 Patent Application No. 58042 2, Title of invention Electro-acoustic transducer 3, Make amendments) Relationship with the Patent Applicant Ota Ri Nakamagome Address 1-3-6 Nakamagome, Ota-ku, Tokyo Name (Name)
(674) Ricoh Co., Ltd. Representative Hama 1) Hiro 4, Agent Address 1-2-77 Furo-cho, Naka-ku, Yokohama 231 Contents of amendment (1), page 1, line 16 of the specification The description of “R” and “rice”
is corrected to ``string ■ history''. (2) The "nonlinear effect of air" described in the 16th line of page 2 is corrected to "nonlinear effect of medium." (3), amended as stated on page 4, line 3 of the same.

Claims (1)

[Claims] A carrier signal in the ultrasonic frequency band is modulated by a signal from a signal source in the audible frequency band, the power is amplified, and then guided to an ultrasonic transducer, where the modulated wave is converted into a sound wave with a finite amplitude level and transmitted into the air. In the electroacoustic transducer which reproduces the original audible sound by the nonlinear effect of the air, the ultrasonic frequency of the carrier is set to 25 kHz to 40 kHz.
An electroacoustic transducer characterized in that the initial sound pressure level of the carrier ultrasonic wave when not modulated is set in the range of 150 to 155 dB.