JPH0458758B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field 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 both methods produce air compression waves by vibrating a diaphragm in the air, which generates mechanical vibrations. It converts energy into acoustic energy.

Ultrasonic waves modulated in amplitude by a signal in the audio frequency band are radiated into a medium such as air or water at a finite amplitude level, and the self-demodulation effect based on the nonlinear effect of the medium causes Various methods have already been reported for parametric speakers that use demodulated sound waves generated in the field as a communication means.

However, the above method has the disadvantage that there are many second harmonic distortion components, especially the second harmonic distortion component.
The harmonic distortion rate is directly related to the modulation depth during amplitude modulation, and becomes worse as the modulation becomes deeper.

Purpose The present invention solves the above-mentioned drawbacks by emitting ultrasonic waves amplitude-modulated by a signal in the audio frequency band into the air at a finite amplitude level and generating an audible signal that is self-demodulated by the nonlinear effect of the air. (referred to as parametric speakers)
This was done for the purpose of preventing the deterioration of the second harmonic distortion characteristics, which is considered a drawback.

Configuration The configuration of the present invention will be described below based on Examples.

Parametric speakers that utilize the nonlinear phenomenon of sound waves are characterized by their sharp directivity, which is because they amplitude-modulate a high-frequency carrier wave with an audible signal wave and transmit it as a finite-amplitude wave. Due to the nonlinear interaction of sound waves, secondary waves related to the signal are distributed in a vertical array in space, resulting in sharp directivity and small side lobes.

Suppose that a circular transmitter with radius a emits a finite amplitude sound wave P ₁ =P ₀ f(t)sinω ₀ t (1) with envelope f(t). Here, P ₀ is the sound source pressure, ω ₀
is the angular frequency of the carrier wave. If we assume that this primary wave is a plane wave and is sufficiently collimated, then 2
The next wave P ₂ is on the sound axis P ₂ = βP ₀ ² a ² /16ρ ₀ C ₀ ⁴ αr ∂ ² /∂t ² f ² (t−Z/C ₀
)...(2) becomes. Note that β is the nonlinear parameter of the medium, ρ ₀
is the medium density, C ₀ is the speed of sound, and α is the linear absorption coefficient of the first-order wave. According to equation (2), P ₂ is proportional to f ² . That is, a secondary wave is generated by undergoing a nonlinear operation called the square of the envelope. This squaring operation is a direct result of the second-order nonlinearity of the sound wave, and is a cause of distortion in the reproduced signal. Therefore, if we now set f(t)=√1+(t)...(3), P ₁ will be proportional to the signal S(t), and no distortion will occur.

The present invention utilizes a completely different means from conventional acoustic transducers such as speakers, that is, the parametric effect of finite amplitude sound waves due to air nonlinearity. and reproduced sound waves (referred to as secondary waves)
is characterized by having a directivity pattern equivalent to that of carrier sound waves in the ultrasonic range.

Generally, as the frequency of ultrasonic waves increases, the sound waves emitted from the transducer become beam-shaped and travel straight.

Now, assuming that amplitude-modulated ultrasonic waves are radiated in the form of a beam from a transducer array with radius a, the sound pressure P at a distance x from the array can be expressed by the following equation.

P=P ₀ {1+m・g(t-x/C ₀ ))e ^- 〓 ^x sin(ω
₀ t−k ₀ x)……(4) where C ₀ is the speed of sound, α is the attenuation coefficient of the sound wave at each frequency ω ₀ , P ₀ is the initial sound pressure, m is the modulation degree, and g(t) is the modulation wave. It is. The sound pressure of the secondary wave generated when the ultrasonic wave with a finite amplitude level expressed by equation (3) is demodulated in air by nonlinear parametric action is expressed by the following non-homogeneous wave equation.

▽ ² Ps−1/C ₀ ²・∂ ² Ps/∂t ² =−ρ ₀ ∂q/∂t……(5
) In equation (5), Ps: sound pressure of secondary wave, ρ ₀ : density of air, q: virtual sound source density of secondary wave generated in the primary wave beam, where q can be expressed by the following equation.

q＝β／ρ ₀ ² C ₀ ⁴・∂／∂tρ ² ...(6) Therefore, from equations (4) and (6), the distance x from the array (on the axis)
The following equation is obtained by calculating the virtual sound source density at the point.

q=βP ₀ ² /ρ ₀ ² C ₀ ⁴ e ^-2 〓 ^x ∂／∂t [m・g(t−
x/C ₀ )+1/2m ² g ² (t-x/C ₀ )]...(7) The first term on the right side of equation (7) above represents the virtual sound source density based on the signal component, and the second term represents the virtual sound source density based on the signal component. The term represents the virtual sound source density of the distortion component.

As explained above, the present invention relates to a modulation method for removing distortion components generated in an acoustic transducer using nonlinear parametric effects. That is, this is a modulation method in which a certain DC component is added to the modulation signal, the signal is converted into √, and then the product with the carrier signal is calculated.

In this case, the modulated signal can be expressed by the following equation.

V=√1+.(t)sinω ₀ t...(8) Therefore, the sound pressure of the primary wave (modulated ultrasound) at a distance x from the transducer array is becomes. In this case, the virtual sound source density of the secondary wave is calculated using equation (6) as q=βP ₀ ² /2ρ ₀ ² C ₀ ⁴ e ^-2dx・m・∂/∂tg(t−x
/C ₀ ) ...(10). Therefore, when this modulation method is used, it is expected that the distortion component will be reduced, as shown in the second term on the right side of equation (7), and the quality of reproduced sound will be significantly improved.

Embodiment 1 An example of a basic configuration for carrying out the present invention is shown in FIG. In Figure 1, 1 is a modulation signal source (audio frequency band), 2 is a coefficient multiplier, 3 is a DC source, 4 is an adder, 5 is a √ converter, 6 is an ultrasonic band oscillator, 7 is a multiplier, 8 is a power amplifier, and 9 is an ultrasonic transducer array.

Embodiment 2 A modified embodiment of the present invention is shown in FIG. In the figure, 10 is a double integrator, and the other parts are the same as in FIG. The reproduced sound pressure obtained in the nonlinear parametric speaker based on this modulation method is finally given by the following equation at the point x on the axis of the array.

Ps=βP ₀ ² a ² m/16ρ ₀ C ₀ ⁴ αx ∂ ² /∂t ² g(t−x/C
₀ )...(11) In other words, the reproduced sound pressure is proportional to the second-order differential of the original modulation signal. Therefore, as shown in Figure 2, by passing the modulation signal through a double integrator before modulation and then applying modulation, the reproduced sound pressure proportional to the original modulation signal, that is, Ps = βP ₀ ² a ² m / 16ρ ₀ C ₀ ⁴ αxg(t-x/C ₀ )……(12
) can be obtained.

Effects As is clear from the above description, according to the present invention, harmonic distortion of reproduced sound is improved, and high-quality reproduced sound can be obtained. In the conventional method, the modulation depth (m≦1)
Although the strain rate deteriorated significantly as the depth of m increased, the present invention can basically reduce strain regardless of m, so the effect is significant when m is large (however, m≦1). Here, since the reproduced sound pressure is proportional to m, being able to use a large m is a very desirable direction for improving the efficiency of the acoustic transducer.

[Brief explanation of the drawing]

1 and 2 are electrical circuit diagrams for explaining embodiments of the present invention, respectively. 1... Modulation signal source, 2... Coefficient unit, 3... DC source, 4... Adder, 5... √ converter, 6... Ultrasonic band oscillator, 7... Multiplier, 8... Power Amplifier, 9... Ultrasonic transducer array, 10... Double integrator.

Claims (1)

[Claims] 1. An ultrasonic vibration element that converts an electrical signal from a signal source in an audible frequency band into an acoustic signal, emits finite amplitude ultrasonic waves into the air, and generates an audible signal by parametric action due to nonlinear characteristics. In an electroacoustic transducer for obtaining an input signal, a first converting means for adding a DC component to an input signal at a predetermined ratio and performing 1/2 power conversion;
A second conversion means is provided for power amplifying the output signal of the first conversion means by multiplying it by a carrier having a frequency sufficiently higher than that of the output signal, and converts the electric signal into the first and second conversion means. After converting by means,
An electroacoustic transducer characterized in that the electroacoustic transducer is configured to supply the ultrasonic vibration to the ultrasonic transducer. 2. The electroacoustic transducer according to claim 1, wherein the electrical signal is temporally integrated twice through a double integrator.