JPH06106000B2 - Parametric speaker - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a parametric speaker that reproduces an audio signal by using the nonlinearity of air with respect to ultrasonic waves.

2. Description of the Related Art Conventionally, in a loudspeaker, the demand for sharpening the directivity of sound like a spotlight and allowing only a certain range of people to be heard without being affected by ambient noise has been demanded at exhibitions and the like. When you want to give a separate explanation for each exhibit,
There were some strong uses such as information broadcasting at station platforms.

Conventionally, a horn speaker has been mainly used for such an application, but the directivity of the horn speaker strongly depends on its length and caliber. In particular, to obtain a sharp directivity in the low frequency range like voice, However, there was a drawback that the diameter would be extremely large.

On the other hand, in recent years, a speaker that utilizes the non-linearity of air with respect to ultrasonic waves (hereinafter referred to as a parametric speaker) has attracted attention because it can obtain a much sharper directivity than in the past. First, a conventional parametric speaker will be described (for example, JP-A-58-119293).

FIG. 5 shows the structure of a conventional parametric speaker. In FIG. 5, reference numeral 1 is an ultrasonic transducer, and 2 is an ultrasonic source configured by arranging the ultrasonic transducers 1 in a honeycomb shape. Reference numeral 3 is an audio signal source, and its output is input to the modulator 4. Here, the audio signal is amplitude-modulated with a carrier wave of 40 KHz, and is input to the ultrasonic wave source 2 via the power amplifier 6.
Reference numeral 7 denotes an acoustic filter for absorbing ultrasonic waves, which is installed between the ultrasonic source 2 and the listener 8.

In the parametric speaker described above, the ultrasonic source 2
The carrier of the amplitude-modulated ultrasonic wave radiated from the antenna and the upper and lower sidebands cause a nonlinear interaction in the air, and a modulated wave having a sharp directivity is generated. Here, the amplitude-modulated ultrasonic wave emitted from the parametric speaker is called a primary wave, and the original audio signal (modulated wave) generated as a result of the nonlinear interaction of the primary wave is called a secondary wave.

By the way, since the conversion efficiency from the primary wave to the secondary wave is extremely low (1% or less) in the parametric speaker, strong ultrasonic waves are required to generate the secondary wave at a practical level. For example, 140 dB to obtain a secondary wave sound pressure of 90 dB
Alternatively, a higher primary wave sound pressure is required.

In addition, the usable frequency is limited to about 30 to 70 KHz in terms of absorption coefficient and conversion efficiency of sound waves in the air, and a band of several KHz is required to modulate with a sound signal. In order to meet the above requirements, an ultrasonic source is one in which hundreds to thousands of ultrasonic transducers using a bimorph piezoelectric ceramic are arranged.

Next, the drive system will be described. All the oscillators are connected in parallel to drive in the same phase. By the way, the impedance characteristic of each vibrator is several hundred ohms at the resonance point and KΩ at the anti-resonance point as shown in FIG. Therefore, if a large number of vibrators are connected in parallel, the impedance will drop and it will not be possible to drive with a normal amplifier. Therefore, the whole vibrator is usually divided into a large number of channels, and the impedance of each channel is matched with the output impedance of the amplifier.

By the way, a cone-shaped resonator is adhered and fixed to the center of the bimorph in the vibrator in order to expand the amplitude, improve the sound pressure level, and lower the Q of resonance to widen the band. Therefore, the sound pressure frequency characteristic of the oscillator changes more gently than the impedance characteristic as shown in FIG. 2b.

Focusing on this point, the power required to generate a constant secondary wave sound pressure level (75 dB / 2 m) was measured using an ultrasonic source in which 552 ultrasonic transducers were connected in parallel. Came to show. From this result, it can be seen that the optimum driving frequency is near the anti-resonance frequency.

However, the impedance of the ultrasonic transducer changes not only greatly with frequency near the resonance point but also with temperature. For example, the resonance frequency, which was 40 KHz at room temperature, drops to about 38 KHz at 65 ° C. Therefore, when a large input is applied to the ultrasonic transducer, it is necessary to gradually increase the input. The reason will be described with reference to FIG. In FIG. 4, a is an impedance characteristic at room temperature of an ultrasonic source in which 550 ultrasonic transducers are connected in parallel. b shows the impedance characteristics of the same ultrasonic source two hours after inputting 200 W of input, at which time the temperature of the oscillator had risen to about 65 ° C. By the way, the optimum drive frequency at the time of inputting 200 W is 40 KHz, and the impedance B at that time is about 8Ω. However, the impedance A at 40KHz at room temperature is only about 2Ω, so suddenly inputting a large input will destroy the amplifier. for that reason,
In the past, the input was gradually increased, or at first it was driven at around 42 KHz and the frequency was gradually decreased.

Problems to be Solved by the Invention However, in the above method, not only is it troublesome, but also the drive frequency must be changed every time the input is changed, and further, the impedance is changed by the temperature change after changing the input. It took a considerable amount of time to change, and it was difficult to always drive at the optimum frequency. Moreover, the driving method as described above is not only uneconomical because the number of channels, that is, the number of amplifiers is increased, but also has a problem of heat generation of the amplifier and the vibrator.

In view of the above problems, the present invention provides a parametric speaker capable of obtaining a high sound pressure level with low power consumption.

In order to achieve this object, the parametric speaker of the present invention comprises an audio signal source, an ultrasonic wave carrier oscillator, and a modulation for modulating the output of the carrier oscillator with the output of the audio signal source. And an ultrasonic source that is driven by the output of the modulator and emits a modulated ultrasonic wave in the air, and one or more ultrasonic transducers for reproducing an audio signal due to the nonlinearity of air, and an ultrasonic source. A power meter that measures the input power to the sound source,
A controller for controlling the carrier wave frequency of the carrier wave oscillator so that the output of the wattmeter becomes minimum.

Action The present invention, by the above-described configuration, first measures the input power to the ultrasonic source, then changes the drive frequency little by little with the controller, compares the power at that time with the previous one, and successively performs this process. By repeating, the frequency is converged to the lowest power frequency. By constantly driving at this optimum frequency, a parametric speaker with high conversion efficiency is realized.

Actually, not only such electrical feedback but also maximizing the electroacoustic conversion efficiency is measured, so that the secondary wave sound pressure is measured to measure the power under a constant secondary wave sound pressure. However, as a result of an experiment in which the output voltage of the power amplifier was fixed and the frequency was changed, the sound pressure level of the secondary wave was almost constant. Also, even if the secondary wave sound pressure is measured, the level of the audio signal always changes,
In reality, it is difficult to judge whether the secondary wave sound pressure is constant. Therefore, it is possible to realize driving at a sufficiently optimum frequency by the above method.

Embodiment An embodiment of the present invention will be described below with reference to FIG. FIG. 1 shows the configuration of a parametric speaker in one embodiment of the present invention. In the conventional example shown in FIG. 5, the ultrasonic transducer 1, the ultrasonic source 2, the modulator 4, and the power amplifier 6 are used.
Is the same as that of the embodiment, so in this embodiment,
FIG. 1 shows only the drive unit.

In FIG. 1, R ₁ and R ₂ are sufficiently larger than the impedance of the ultrasonic transducer 1, and R ₃ is a sufficiently small resistance.
The input voltage and current to the ultrasonic transducer 1 are measured. These outputs are input to the multiplier 9 to obtain electric power. Next, the output of the multiplier 9 is the carrier frequency controller (hereinafter referred to as controller) 10
To enter. In the controller 10, first, the output of the multiplier 9 is compared with the reference value stored in the memory of the multiplier 9, the smaller value is newly stored in the memory, and the positive or negative pulse is sent to the carrier oscillator 5. send. The oscillation frequency of the carrier wave oscillator 5 is adapted to be ± 0.1 KHz each time a positive or negative pulse is applied.

In this embodiment, by repeating the above process,
It is always possible to drive at the most efficient frequency. As described above, the optimum frequency is greatly affected by temperature, so if the temperature of the ultrasonic transducer 1 changes due to changes in the amplification factor of the amplifier, ± 1 KHz changes, but such a configuration According to the method, it is possible to always drive at the optimum frequency even if the amplification degree is changed.

EFFECTS OF THE INVENTION As described above, according to the present invention, by providing the controller capable of setting the oscillation frequency so that the input power to the ultrasonic transducer is minimized, the electroacoustic conversion efficiency can be always driven in the maximum state. A parametric speaker can be realized. As a result, the amplifier can be cheaper and cheaper than the conventional one.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a parametric speaker in one embodiment of the present invention, FIG. 2 is a characteristic diagram showing a relationship between sound pressure frequency and impedance of an ultrasonic transducer, and FIG. 3 is an ultrasonic transducer. 4 is a characteristic diagram showing the relationship between the driving frequency and input voltage, current and power, FIG. 4 is a characteristic diagram showing the difference in impedance due to input, and FIG. 5 is a block diagram showing the configuration of a conventional parametric speaker. 1 ... Ultrasonic transducer, 2 ... Ultrasonic source, 3 ... Audio signal source, 4 ... Modulator, 5 ... Carrier oscillator, 6 ... Power amplifier, 7 ... Acoustic filter, 8 ... Reception Listener, 9 ... Multiplier, 10 ... Controller.

Claims (1)

[Claims] 1. An audio signal source, an ultrasonic wave carrier oscillator, a modulator for modulating the output of the carrier oscillator with the output of the audio signal source, and a modulated super-wave driven by the output of the modulator. An ultrasonic source comprising one or a plurality of ultrasonic transducers for radiating sound waves into the air and reproducing an audio signal due to the non-linearity of air, and a power meter for measuring the input power to the ultrasonic source. And a controller for controlling the carrier frequency of the carrier oscillator so that the output of the power meter becomes minimum.