RU1768319C - Method for parametric radiation of acoustic vibrations - Google Patents

Abstract

The invention relates to hydroacoustics, in particular to methods for parametric sound emission, and can be used to create systems for acoustic sounding of the thickness and bottom of the ocean. The method consists in exciting two concentrated forces in a solid-state waveguide in the form of a plate of bending vibrations. By varying the excitation frequencies while ensuring that the reduced value of the sound pressure amplitude in the far zone matches the specified mathematical expression, the radiation pattern of the parametric emitter is controlled. 2 ill.

Description

The invention relates to hydroacoustics, in particular to methods of parametric radiation, and can be used to create technical systems for acoustic sounding of the thickness and bottom of the ocean.

The closest in technical essence and the achieved effect to the invention is the method of parametric radiation, implemented in a parametric emitter on surface waves.

This method involves the formation of two high-frequency pump fields with frequencies w and un by excitation in the solid-state waveguide of two surface Rayleigh waves with phase velocities exceeding the speed of sound in the medium.

The disadvantages of this method are:

the inability to control the radiation pattern that

due to the constant angle of the pump radiation and is a direct consequence of the lack of dispersion in Rayleigh waves;

the difficulty of implementation is the introduction of an additional element — a periodically irregular portion on the surface of the waveguide to ensure the transformation of body waves into surface waves, while reducing efficiency due to energy loss during transformation.

The aim of the invention is to simplify the method and to provide the ability to control the pattern.

The method is implemented using a solid-state waveguide (Fig. 1), made in the form of a plate of thickness h and length C in which oscillations biharmonic in time are excited by a concentrated source.

The material and thickness h of the plate are selected from the condition of excitation in the plate

(L

WITH

vj o

00

00

Yu

bending waves in a given frequency range with phase velocities Cph exceeding the speed of sound in a liquid C0. And the length L and the width of the plate S should be much longer than the lengths of the excited bending waves.

In a plate, when a concentrated source acts on it, waves are excited that are described by the Tymoshenko-Mindlin equations,

In this case, due to the dispersion of the bending waves and the dispersion of their phase velocity, the conditions of phase (spatial and temporal) synchronism cannot be satisfied.

0) 1 - 0) 2 WBp4, K1-K2 Kvr4, (1)

When these conditions are met, energy is transferred efficiently from the waves and pumped into the wave of the difference frequency.

The problem of finding the sound field of the difference frequency is solved by the method of successive approximations. Waves of the first approximation - pump waves are assumed to be weakly damped and the diffraction effects are not taken into account. In the far zone (KaR 1), the sound pressure field of the differential frequency waves has the following expression:

 Ј FiF2Koi Ko2. (.

2 (2 lessons &

R

I / 4 l (v) /

Vi (v) V2 (v) sin v dv;

o + | Ky (SOZOZOZU-1) 2 + К2ггз п2У51 П2 /

Moreover, expression (3) determines the radiation pattern of the parametric emitter. Under the integral is the product of three sharp functions: Vi (v), V2 (v) - pump radiation patterns and 1 / V7 ... The sharpness of the last factor

due to the appearance of phase synchronism between the pump waves and the wave

differential frequency propagating in the direction of radiation v. Obviously, such a resonant behavior takes place under the condition that many wavelengths cr К K fit in the damping length and in the most typical case of not very sharp diagrams of the pump wave and weak attenuation a Ј KЈ, the result is as follows

P ЈQFrF2-KorKo2 V1 (v) w (v) e “Kf -ad

  R

(4)

G

5

10 15

20 25

thirty

35

4h

fifty

Figure 2 shows the numerical calculations of the radiation pattern of a parametric emitter of a particular plate material - steel. The quantity (e QKoi Ko2 h2 / 2 / CbCo3} l Vi V21 was calculated, illustrating the angular distribution of the difference frequency wave as a function of the pump frequencies.

The calculations were carried out at a frequency of 5 and a frequency of Ш2, varying from the CMS to

1.9 s (4 0 Co2 / Ci2) (1 - Co2 / C22)

threshold frequency corresponding to the occurrence of directional radiation from the plate into the medium). At (graphs 1,2), the direction of the maximum of the radiation weakly depends on the frequency, and at 0% a) - (graphs 3,4), the emission angle increases noticeably. This behavior is explained by the fact that since the value and acuteness of the function V (CD, v) increase with increasing ω, the direction of radiation and its level are determined by that factor V (o), v), which corresponds to a higher frequency ω. Since the frequency 0) remained fixed when wi (sh ai) changed, there were no changes in the radiation pattern. When the frequency hell became greater than yl, the behavior of the radiation pattern began to be determined by the value V (ar, vJ.To the result confirms the possibility of controlling the radiation pattern of the parametric emitter by changing the pump frequencies. Calculations also confirm the presence of a narrow radiation pattern of parametric radiation, so for the case under consideration steel plate, it can be 2-4 °.

The amplitude of the difference-frequency wave is proportional to the product of the amplitudes of the interacting waves. The main factor limiting the maximum permissible radiation level of a difference frequency wave (and hence the efficiency) of a parametric antenna is the cavitation threshold,

Claims (1)

SUMMARY OF THE INVENTION A method for parametrically emitting acoustic vibrations, comprising generating two interacting high-frequency pump fields by exciting elastic waves in a solid-state waveguide, characterized in that for the purpose of forgiving it and forgiving the radiation pattern, a plate in which a solid-state waveguide is used excite bending waves and change the excitation parameters in accordance with the expression L, eQFrF2-KorKQ2Vi (0) V2 (0) P (C01,, 0) V4 Soro r2 iiEv Vu {. , (L- where P (, o &, c) is the amplitude of the sound pressure in the far zone, reduced to a distance of 1 m from the source for the waveguide in the form of a plate; b is the angle between the normal to the surface of the waveguide and the direction of radiation; ol.2 excitation frequencies; Fi, 2 - amplitudes of exciting concentrated forces; Q | u) i is the difference frequency; 5Ko 1.2 al, 2 / Co - wave numbers B medium; Co is the speed of sound in the medium; PO is the density of the medium; e is a nonlinear parameter of the medium; h is the thickness of the waveguide; 10С Eh / {m (1- о2)) 17 is the velocity of the quasilongitudinal wave in the plate located in vacuum;., „ C2 Eh / (2m (1 + 0} r) 1 / z - modified shear wave velocity; 15E - Young's modulus; a Poisson's ratio; m is the mass of the plate coming from per unit area; t - correction factor. twenty 2.0SSH by i .M 02 &