Electroacoustic transducer for the parametric generation of ultrasound

Abstract

The construction has two piezoelectric transducers with radiating apertures shaped as sections of spherical surface one is concave, the second is convex, of sufficient wave sizes D/λ>10, where D is the diameter of the aperture, λ is the wavelength of the emitted pump signal). For each piezoelectric transducers radii of curvature R.sub.0, focal lengths F.sub.0 and focal spots radii r.sub.0 with a wave length of radiated signal are the same and are related by r.sub.0×R.sub.0=0.61×λ×F.sub.0. Piezoelectric transducers with radiating apertures shaped as sections of spherical surface are provided with shielding elements, hydro-, electric- and noise insulation and one of them with convex aperture is made with an open axial hole with radius r=(2÷3)×r.sub.0 in the central part of the convex spherical surface of the aperture.

Claims

1. An electro-acoustic transducer for parametric generation of ultrasound comprising: two generators of electrical oscillations, the two generators having their outputs connected via a linear adder to an input of a pulse modulator; the pulse modulator controlled by a pulse generator; a power amplifier; and a notch filter having an output connected to an input of a first piezoelectric transducer, the first piezoelectric transducer having a radiating aperture which is a section of a convex spherical surface; a second piezoelectric transducer with a concave spherical aperture surface, the second piezoelectric transducer being connected to the output of the notch filter; and a supporting structure of a cylindrical shape supporting the first and the second piezoelectric transducers, wherein for both of the piezoelectric transducers a diameter D of the apertures, an average wavelength λ for a range of pump signals emitted by the piezoelectric transducers, radii of curvature R.sub.0, focal lengths F.sub.0, radii r.sub.0 of focal spots are chosen identical and are related as r.sub.0×R.sub.0=0.61×λ×F.sub.0, and wherein the first piezoelectric transducer is provided with an open axial hole radius r=(2÷3)×r.sub.0 in a central portion of the convex spherical surface of the radiating aperture.

2. The electro-acoustic transducer for parametric generation of ultrasound according to claim 1, the supporting structure of the cylindrical shape is configured to provide a change in a distance between the first and the second piezoelectric transducers.

3. The electro-acoustic transducer for parametric generation of ultrasound according to claim 1, wherein the first and the second piezoelectric transducers are equipped with shielding elements, hydro-, electro- and noise insulation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated by drawings, with

(2) FIG. 1 showing a block diagram of an electro-acoustic transducer for parametric generation of ultrasound;

(3) FIG. 2. showing distribution of sound pressure amplitudes for a difference frequency signal F=25 kHz on the acoustic axis of an electro-acoustic transducer for parametric generation of ultrasound;

(4) FIG. 3. showing transverse distribution of sound pressure amplitudes for a difference frequency signal F=50 kHz of an electro-acoustic transducer for parametric generation of ultrasound.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) The electro-acoustic transducer for parametric generation of ultrasound contains (FIG. 1) the first 1 and second 3 electric oscillation generators, the outputs of which are through a linear adder 2, a pulse modulator 4, controlled by a pulse generator 5, as well as a power amplifier 6 and a notch filter 7 are connected to the inputs of the first a piezoelectric transducer 8 with a convex spherical surface of the aperture and a second piezoelectric transducer 9 with a concave spherical surface of the aperture, and the supporting structure of the cylinder 10, which combines piezoelectric transducers 8 and 9. The supporting structure 10 provides the possibility of a centered change in the distance between the piezoelectric transducers 8 and 9, moreover, for both piezoelectric transducers, the diameters D of apertures, the average wavelength λ for the range of emitted pump signals, the curvature radii R.sub.0, focal lengths F.sub.0, focal spot radii r.sub.0 are the same and are related by the relation r.sub.oxR.sub.0=0.61×λ×F.sub.0 (see Rosenberg L. F. Focusing ultrasound emitters. In the book: Sources of powerful ultrasound. Part 3. M Nauka, 1967. —321 s), and the first piezoelectric transducer 8 with a convex spherical surface of the aperture is made with open axial hole with a radius of r=(2÷3)×r.sub.0 in its central part. The input of the second piezoelectric transducer 9 with a concave spherical surface of the aperture is connected to the output of the notch filter 7. For phasing both acoustic beams of powerful pump waves propagating in aquatic environment both in the saturation region (near zone, stabilization distance) and in the working region of spherical propagation formed calibration signals, the supporting structure of the cylindrical shape 10 provides the ability to change the distance between the first 8 and second 9 piezoelectric transducers (the principle of constructing load-bearing structures providing alignment of the acoustic system, and methods for precision changing the distance between transducers are known and are used in methods of ultrasonic interferometry (see Special Physical Workshop, Part 1 3rd ed., M, Moscow University Press—ta, 1977, pp. 309-317, Ultrasound, Small Encyclopedia, Editor-in-Chief of IP Golyamin, Moscow: Sov. Encyclopedia, 1979. P. 151-153), the latter being equipped with elements of shielding, hydro, electrical and noise insulation.

(6) The claimed invention allows to expand the functionality of an electro-acoustic transducer for parametric generation of ultrasound, which consists in increasing the amplitude of the sound pressure generated in a nonlinear aqueous medium of acoustic signals of operating frequencies used as calibration signals during hydroacoustic measurements. This increases the reliability of the measurement results and reduces the complexity of obtaining them by reducing the level of masking noise with increasing amplitudes of sound pressure formed in a nonlinear aqueous medium of calibration signals.

(7) Electro-acoustic transducer for parametric generation of ultrasound (FIG. 1) works as follows. Generators 1, 3 generate electrical signals with frequencies f.sub.1, f.sub.2, coming through a linear adder 2 to the input of a pulse modulator 4, controlled by a pulse generator 5. From the output of the pulse modulator 4, a radio pulse with biharmonic filling (beat of electrical vibrations of close frequencies f.sub.1, f.sub.2, located in the passband of piezoelectric transducers 8, 9 with convex and concave spherical surfaces of the apertures), through a power amplifier 6 and a notch filter 7 is fed to the inputs of piezoelectric transducers 8, 9 with both convex and concave spherical surfaces of apertures equipped with elements of shielding, hydro, electrical and noise insulation. The half-wave piezoceramic active elements—the apertures of the piezoelectric transducers 8, 9 are sections of a convex (concave) spherical surface that form in the aquatic environment 11 a directivity characteristic for acoustic pump waves having circular symmetry about an axis passing through its center and perpendicular to the middle of the convex (concave)) surface. Due to the piezoelectric properties, the piezoceramic apertures of the piezoelectric transducers 8, 9 will change their half-wave thicknesses with frequencies equal to the frequencies of the applied voltage, i.e. will oscillate. All points of the surfaces oscillate in phase and with the same amplitude. These vibrations are transmitted to the aqueous medium 11 and propagate in the form of condensations and discharges, and, in some directions, the resulting amplitude of coherent oscillations with the frequencies of the pump signals increases (the oscillation phases coincide), in others, they weaken to one degree or another (the oscillation phases do not coincide). These disturbances create a distribution of the sound pressure level of powerful pump signals in space, having circular symmetry about an axis passing through the center and perpendicular to the middle of the convex (concave) surfaces and determined by the direction to the observation point from the location of the transducer, which is called its directivity characteristic. The relative position of piezoelectric transducers 8, 9 with convex and concave spherical surfaces of the apertures is fixed by a cylindrical supporting structure 10, and it is thus chosen that the electro-acoustic transducer 8 with a convex aperture is located on the acoustic axis of the electro-acoustic transducer 9 with a concave aperture at a focal distance F.sub.0 from it, wherein, the focal spot is located in the through hole of radius r=(2×3)×r.sub.0 in the central portion of the electroacoustic transducer 8 with convex spherical surface aperture. With this arrangement, the predominant part of the focused emitted acoustic energy of the pump waves passes through the main diffraction maximum of the focal spot with a radius r.sub.0, as a result of which the piezoelectric transducer 8 with a convex spherical aperture surface located in the focal plane has practically no effect on the radiation mode and parameters of the piezoelectric transducer 8 with convex aperture.

(8) A converging acoustic wave from a piezoelectric transducer 9 with a concave spherical surface of the aperture is transformed in focus into a diverging spherical wave, the phase of which differs from the phase of the initial wave by π (see Rosenberg L F Focusing ultrasound emitters. In the book: Sources of powerful ultrasound. Part 3. —M.: Nauka, 1967. —321 p.). Therefore, the wave front after focusing coincides with the wave front emitted by the piezoelectric transducer 8 with a convex spherical surface of the aperture, and when the initial phases of the oscillations coincide, the acoustic vibrations of both diverging spherical waves are added, which in turn leads to an increase in the sound pressure amplitude level of the difference signal, used for graduation in the aquatic environment 11.

(9) The aquatic environment 11 has a nonlinearity of its elastic properties, which leads to the appearance of nonlinear effects of both self-interaction and interaction during the propagation of an intense ultrasonic wave pulse (see Muir T. J. Nonlinear acoustics and its role in the geophysics of marine sediments//Acoustics of marine sediments/Translated from English; Edited by Yu. Yu. Zhitkovsky. —M.: Mir, 1977. p. 227-273). These effects can be considered as the result of the nonlinear change in the elastic properties of water 11 on the characteristics of a powerful pulse pump signal in the propagation region, as a result of which, in particular, the pump signals interact with frequencies acTOT f.sub.1, f.sub.2, the result of which is the parametric generation of calibration ultrasonic signals as a difference F=|f.sub.2−f.sub.1|, and the total f.sub.+=f.sub.2+f.sub.1 frequencies, second harmonics 2f.sub.1,2f.sub.2 of pump waves.

(10) The claimed construction that implements a method of adding acoustic power while maintaining the initial characteristics of electro-acoustic transducers 8, 9 with convex and concave spherical surfaces of the apertures has peculiarities. Since in this case we use two electro-acoustic transducers 8, 9 of sufficient wave sizes D/λ>10, where D is the diameter of the aperture, λ is the wavelength of the emitted pump signal, when choosing a specific distance between them, the beat of two frequencies should be used as the pump signal and phasing the pump signals mechanically, the result of which is fixed by the supporting structure 10. Operation in the pulsed mode with this method of adding acoustic signals of difference frequency imposes a condition on the value of the duration of the emitted pulse τ.sub.u=F.sub.0/c, where c is the speed of sound.

(11) The stated principle of constructing an electro-acoustic transducer for parametric generation of ultrasound was implemented in the design of a pump transducer for the non-linear acoustic emitter NAI-9 (resonant frequency f.sub.0=1380 kHz, focal length F.sub.0=47 mm, segment diameter 2a=47 mm, emitter depth h=6 mm), experimental results were obtained for both low- and high-frequency components of the spectrum.

(12) FIG. 2 shows the distribution of amplitudes of sound pressure for a difference frequency signal F=25 kHz on the acoustic axis of an electro-acoustic transducer for parametric generation of ultrasound:

(13) 1) it emits only the first piezoelectric transducer 8 with a convex spherical surface of the aperture;

(14) 2) emits only the second piezoelectric transducer 9 with a concave spherical surface of the aperture;

(15) 3) both the first and second piezoelectric transducers 8, 9 are emitted with convex and concave spherical surfaces of the apertures, which in the far zone leads to an increase in the amplitude of the sound calibration signal by (4-5) dB.

(16) FIG. 3 shows the experimental transverse distribution of the amplitudes of sound pressure for a difference frequency signal F=50 kHz of the clamed electro-acoustic transducer for parametric generation of ultrasound, where:

(17) 1) emits only the first piezoelectric transducer 8 with a convex spherical surface of the aperture;

(18) 2) emits only the second piezoelectric transducer 9 with a concave spherical surface of the aperture

(19) 3) jointly emit both the first and second piezoelectric transducers 8, 9 with convex and concave spherical surfaces of the apertures. From a comparison of the curves (FIG. 3), it follows that the third option in the far zone leads to an increase in the amplitude of the sound calibration signal by 6 dB.

(20) Analysis of the above experimental results allows us to draw the following conclusions—the inventive electro-acoustic transducer for parametric generation of ultrasound satisfies at least the following special operational requirements:—a large dynamic range of amplitudes of calibration sound pressure; —a wide range of operating frequencies; the directivity characteristic of the measuring emitter contains the minimum number of additional petals in the formed calibration acoustic field. It should be noted that both the axial and transverse distributions of the amplitudes of sound pressure (FIGS. 2, 3) for the acoustic fields of difference-frequency signals generated by an electro-acoustic transducer for parametric generation of ultrasound are in compliance with another operational requirement for the formed calibration field:—monotonies and the uniformity of changes in the amplitudes of sound pressure both in the longitudinal and transverse directions of the water volume of the hydroacoustic pool, which allows to place calibration receivers close enough to the device. Based on this, it can be assumed that the practical use of the proposed electro-acoustic transducer for parametric generation of ultrasound as a measuring one in the conditions of hydro-acoustic pools of limited sizes, in addition to the above advantages, will make it possible to reduce the weight and dimensional parameters of the hydro-acoustic pools.

(21) It must be emphasized that an increase in the amplitude of sound pressure of the components of the calibration signal of the resulting ultrasonic field in the measuring volume of the sonar pool leads to an increase in the reliability of the measurement results. It should be noted that in acoustic measurements, in addition to the useful signal, the receiving channel is affected by signals that make it difficult to take measurements, since they distort or mask the useful signal. Thus, as a rule, in acoustic measurements, the signal level (measured with disturbance interference) should be (10-15) dB higher than the noise level (measured with no signal), and it is most difficult to eliminate the reverberation noise, i.e. noise generated by a scattered useful signal. However, the inventive electro-acoustic transducer for parametric generation even in this case benefits due to the increasing of the directivity and the absence of side radiation.

(22) Claimed invention can find wide application in the field of acoustic measurements, in particular in measuring sound pressure emitters, which in a hydro-acoustic pool can be used as a source of sound vibrations with a high amplitude of sound pressure, which improves the reliability of the measurement results and reduces the difficulty of obtaining them for by reducing masking noise.