JPS61118096A - Parametric speaker - Google Patents

Abstract

PURPOSE:To decrease an average level of an ultrasonic wave radiated in the air and to decrease a noise by controlling a level of an ultrasonic wave in accordance with the sound signal level. CONSTITUTION:The output of a sound signal source 3 is inputted in a modulator 4 and used as a control signal of a control device 9. The signal from a carrier oscillating device 5 is controlled by a level of a signal source 3 and inputted to the modulator 4. The signal is inputted through a power amplifier 6 to a speaker 2. An acoustic filter 7 to absorb the ultrasonic wave is installed between the speaker 2 and a listener 8. Thus, the average level of the ultrasonic wave can be decreased and the noise can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a speaker (parametric speaker) that uses the nonlinearity of air with respect to ultrasonic waves.

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 aperture. However, it had the disadvantage of being extremely large in both diameter.

On the other hand, in recent years, speakers that utilize the nonlinearity of air with respect to ultrasonic waves (hereinafter referred to as parametric speakers) have been attracting attention because they can obtain much sharper directivity than conventional speakers. First, conventional parametric speakers will be explained. (For example, Japanese Patent Laid-Open No. 58-119293) FIG. 5 shows the configuration of a conventional parametric speaker. In Fig. 5, 1 is an ultrasonic transducer and 2
is a speaker constructed by arranging these in a honeycomb shape. 3
is an audio signal source, and its output is input to the modulator 4.

Here, the audio signal (modulated wave) is amplitude-modulated using a carrier wave output from the carrier wave oscillator 6 at 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 parametric speakers described above, speaker 2
The carrier wave and the upper and lower sideband waves of the amplitude modulated ultrasonic wave emitted from the oscilloscope cause nonlinear interaction in the air, and a modulated wave with sharp directivity is generated. Here, the amplitude 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, parametric speakers Since the conversion efficiency from primary waves to secondary waves is extremely low (1% or less), powerful ultrasonic waves are required to generate a practical level of secondary waves. For example, to obtain a secondary wave sound pressure of 5 odB, 14
A primary wave sound pressure of 0 dB or more is required. If a listener is directly exposed to such strong ultrasonic waves, it is conceivable that adverse effects such as hearing loss may occur. Therefore, when putting parametric speakers into practical use, it is essential to install an acoustic filter 7 shown in FIG. 6.

Problems to be Solved by the Invention However, with the above configuration, the following problems remain.

(1) Even if the level of the ultrasonic waves is sufficiently lowered below the acoustic filter 7, the ultrasonic waves from the speaker 2 will be directly received outside the acoustic filter 7. Although ultrasonic waves have sharp directivity, this level is not necessarily safe. (2) When using parametric speakers indoors, ultrasonic waves are reflected by surrounding walls and can also be transmitted under the acoustic filter 7. A region reaching a strong ultrasonic level may appear (Ko) If the ultrasonic absorption coefficient of the acoustic filter 7 is increased in order to lower the ultrasonic level, the audible sound level will also decrease.

(→ Even when the level of the signal wave is low (for example, when there is no sound), the level of the ultrasonic wave is constant, so the energy utilization rate is poor, and when there is no sound, the low harmonics generated from the ultrasonic transducer can be heard as noise.

In view of the above problems, the present invention provides a parametric speaker in which the average level of ultrasonic waves is significantly reduced by controlling the level of ultrasonic waves by the level of modulated waves.

Means for Solving the Problems In order to solve the above problems, the parametric speaker of the present invention includes an ultrasonic vibrator and a modulator for driving the ultrasonic vibrator with an ultrasonic wave modulated by an audio signal. and a controller for controlling the level of ultrasonic waves according to the level of the audio signal.

Effect of the Invention The present invention uses the above-described configuration to control the level of the carrier wave in the ultrasonic range input to the modulator by the level of the modulated wave which is an audio frequency, and when the level of the modulated wave is small, the level of the carrier wave is lowered and the level of the carrier wave is reversely modulated. When the wave level is high, the carrier wave level is also raised. As a result, the modulation degree is kept almost constant. The signal in the ultrasonic range modulated by the modulator is radiated into the air as an ultrasonic wave input to an ultrasonic transducer. Then, due to the nonlinearity of the air, the carrier wave and sideband waves interfere, and the original audio signal (modulated wave) is generated in the air. In conventional parametric speakers, the ultrasonic level was set based on the maximum modulated wave level, so the ultrasonic level remained strong even when the modulated wave level was zero, that is, there was no sound. . However, according to the present invention, since the level of the ultrasonic wave changes depending on the level of the modulated wave, the average level of the ultrasonic wave radiated into the air can be significantly reduced.

EXAMPLE An example of the present invention will be described below with reference to the drawings. In FIG. 1, reference numeral 9 denotes a controller for controlling the signal level of a carrier wave, and since the other configurations are the same as those of the conventional example, a description thereof will be omitted.

The controller 9 can be realized with the configuration shown in FIG. The centerpiece is a voltage-controlled amplifier 1o. A modulated wave rectified by a rectifier circuit 11 and amplified by an amplifier 12 is added to the control signal. If necessary, an appropriate DC bias may be added in a superimposed manner.

The output of the carrier wave oscillator 6 is input to the voltage-controlled amplifier 1o, amplified according to the control signal level, and then output to the modulator 4 via the logarithmic/linear conversion amplifier 13.

Next, the operation of this controller 9 will be explained. The output of the audio signal source 3 is input to a modulator 4 and is used as a control signal for a controller 9. The signal from the carrier wave oscillator 6 is controlled by the level of the control signal and input to the modulator 4. A characteristic diagram of the control signal voltage vc and the output voltage/input voltage (V0/V,) of the controller 9 is shown in FIG. With this characteristic, the modulation degree can always be constant (0, 3). As a result, the average level of ultrasonic waves is approximately 20
dB also decreased.

In each figure from FIG. 4 to FIG. 6, dotted line A is conventionally replaced by solid line B.
1 shows the temporal changes in the audio signal level, modulation degree, and ultrasonic level in this example. Conventionally, the ultrasonic output was set so that the modulation depth was 0.3 at the peak of the audio signal, so the ultrasonic level was always 140 dB.
was constant.

However, in this embodiment, since the ultrasonic output is controlled so that the modulation degree is always 1/C0.3, the ultrasonic level can be lowered by about 20 dB on average. has the following effects by providing a controller that controls the level of the carrier wave (ultrasonic wave) according to the level of the modulated wave (audio frequency).

(1) Since the average level of ultrasonic waves radiated into the air can be significantly lowered, it is possible to miniaturize or eliminate acoustic filters, which have conventionally been indispensable for listening at short distances.

(2) Even when an acoustic filter is provided, the amount of attenuation of ultrasonic waves by the filter is smaller than that of the conventional method, so the structure is simplified and a decrease in the audible sound level due to the acoustic filter can be prevented.

('4) When the level of the modulated wave is small or zero, the output of the ultrasonic wave is also small, which eliminates the drawback of conventional methods in which low harmonics generated from the ultrasonic transducer can be heard as noise. .

(萄) The average load on the power amplifier is reduced, saving energy and extending the life of the amplifier.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a parametric speaker according to an embodiment of the present invention, Fig. 2 is a block diagram of the controller, Fig. 3 is a diagram showing voltage attenuation characteristics of the controller, and Figs. 4 to 6 are FIG. 7 is a characteristic diagram showing temporal changes in the audio signal level, modulation degree, and ultrasonic level of the conventional example and this embodiment. FIG. 7 is a diagram showing the configuration of the conventional example. 1... Ultrasonic vibrator, 2... Speaker,
3...Audio signal source, 4...Modulator, 5
... Carrier wave oscillator, 6 ... Power amplifier, 7 ... Acoustic filter, 8 ... Listener, 9 ... Controller. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure [Figure 2 Q Figure 3 $4 FP (f ! (' n Vc (v)
Fig. 4: 7% 1 - Fig. 6 Unexamined patent application -

Claims (1)

[Claims] an audio signal source, an ultrasonic band carrier wave oscillator, and a controller into which the output from the carrier wave oscillator is input and controls the level of the carrier wave according to the output level of the audio signal;
a modulator to which the output of the controller and the output of the audio signal source are input and for modulating the carrier wave of the controller with the modulated wave of the audio signal source; A parametric speaker comprising one or more ultrasonic transducers for radiating sound waves into the air and generating an original audio signal in the air due to the nonlinearity of the air.