JPH0231919B2 - SUPIIKANOKUDOHOHO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a speaker.

Recently, speakers that utilize the nonlinearity of air have been developed, and it has become possible to obtain sound sources with sharp directional characteristics. Generally, such a speaker is composed of an ultrasonic element, and is driven by applying a signal outside the audible frequency band. In particular, in order to emit an audible sound, a carrier wave frequency is selected to be approximately equal to the resonant frequency of the ultrasonic element, and a signal amplitude-modulated by an audio signal is applied to a speaker. FIG. 1 shows the waveform, and FIG. 2 shows the spectrum arrangement, assuming that the audio signals are g and t and the carrier frequency is ωc. Even if such a waveform (FIG. 1 c, FIG. 2 c) is applied to a speaker, it cannot be heard using a normal method because it does not have a frequency component in the audible band. However, if the sound pressure emitted from the ultrasonic element is large, it will be detected due to the nonlinearity of the air, and the sound waves shown in FIGS. 1, a, and 2 a will be emitted into the air.

Here, consider the signal to be reproduced as a single frequency, and let that frequency be ωs. The sound pressure f(t) emitted from the speaker is f(t)=(1+mcosω _S t)cosω _C t (1). A virtual sound source G generated in the air by this
is G∝∂/∂tf ² (t) ...(2), and the sound pressure P ₁ we hear is P ₁ ∝∂/∂tG ...(3). Calculating (3) from equations (1) and (2) reveals that Ps∝mcosω _S t Pd∝m ² /2cos2ω _S t. Ps and Pd represent the sound pressure of ωs and 2ωs components, respectively. In other words, the desired signal and twice its harmonic components are generated, and the magnitude of the sound pressure is m.
It can be seen that it is determined by m ² . Therefore, in order to efficiently obtain a loud audible sound Ps using the conventional method, m
Although it is desirable that m be large, there is a drawback that increasing m increases strain Pd. m=1
In the case of , the distortion Pd becomes 1/2 of the desired signal Ps.

The present invention aims to eliminate distortion and obtain efficient sound production.

In order to achieve the above purpose, a modulator that amplitude modulates a carrier wave outside the audio frequency band with a signal to be reproduced in the audio frequency band, and a filter that removes one of the sideband waves of the modulated signal. The amplitude modulation index m=
2 and input it to an ultrasonic element for electroacoustic conversion, which is detected by air nonlinearity and reproduced sound in the audible frequency band. Single sideband detection reduces distortion of the reproduced sound. can do.

This will be explained using a spectrum diagram. FIG. 3 is a spectrum diagram when the signal of formula (1) is generated using the conventional method. Drive signal spectrum ωc−ωs, ωc, ωc+
It has a ωs component, and the ωs component and strain component from the air.
Emit 2ωs. In particular, the figure shows the case where m=1. While the carrier component ωc and the component of ωc±ωs, which is 6 dB lower than the carrier component, interfere with each other to generate the ωs component, the interference between the sideband waves of ωc−ωs and ωc+ωs generates a distortion component of 2ωs. Therefore, one of the sideband waves is removed here by using a filter or the like having steep cutoff characteristics.

Furthermore, when the modulation index is set to m=2, the levels of the carrier wave component and the sideband component are equal, allowing efficient sound generation. This will be explained using FIG. 4. FIG. 4a shows a case where a high band sideband is used, and d shows a case where a low band sideband is used. Both use interference between two waves, a sideband wave and a carrier wave, so there is no generation of harmonics 2ωs. Furthermore, if the resonance characteristics of the ultrasonic element are asymmetrical with respect to the resonance frequency and there are undesirable resonance peaks, only the desirable characteristics can be used. Furthermore, the conventional method has the following drawbacks. If the frequency band of the reproduced signal is 15 KHz, only ultrasonic elements that resonate at frequencies 15 KHz or more higher than the audible limit frequency can be used. However, if only the upper sideband is used in the implementation of the present invention, it is possible to select an element with a wide band as long as the resonant frequency of the ultrasonic element is outside the audible band.

According to the present invention, single sideband reproduction is performed, so even if an ultrasonic element with a narrow reproduction band is used as an electroacoustic transducer, reproduced sound with little distortion can be obtained.

[Brief explanation of drawings]

FIG. 1 is a waveform explanatory diagram, FIG. 2 is a spectrum explanatory diagram, FIG. 3 is a spectrum diagram according to the conventional method, and FIG. 4 is a spectrum diagram according to the present invention.

Claims (1)

[Claims] 1. Modulation means for amplitude modulating a carrier wave outside the audio frequency band with a signal to be reproduced in the audio frequency band, and one sideband wave of the both sideband waves of the signal modulated by the modulation means. , a means for determining a modulation index so that the level of the carrier wave and the sideband wave are approximately equal to each other, and an ultrasonic element for electroacoustic conversion of the modulated signal. A method for driving a speaker characterized by obtaining a reproduced signal in an audible frequency band. 2. The method of driving a speaker according to claim 1, wherein the filter means removes one sideband wave by utilizing cut-off characteristics of the ultrasonic element.