SU1727047A1 - Method of determination of nonlinear acoustic parameter of medium - Google Patents

Description

It is necessary to travel a distance greater than the length of the low-frequency wave.

Another disadvantage of this method is its complexity, due to the need to emit three acoustic waves into the medium under study and receive two acoustic signals.

Another way to determine the nonlinearity parameter is based on measuring the level of the second harmonic generated in the medium when it is irradiated with a plane acoustic wave of frequency f. The non-linear parameter is determined by the ratio of the pressure amplitudes at frequencies f and 2f.

The disadvantage of this method is low accuracy due to the fact that, along with the second harmonic generated by the medium under study and determined by its nonlinear parameter, the second harmonic second harmonic is also formed in the emitter and receiver. The measured amplitude of acoustic pressure at a frequency includes both the useful and parasitic components of the second harmonic, which leads to an error in the measurements and, consequently, to a decrease in accuracy. Accuracy also decreases with a decrease in the linear dimensions of the medium under study, since the level of the second harmonic generated by the medium decreases and the relative level of the parasitic second harmonic increases.

The closest to the invention is a method comprising irradiating a test medium formed in the form of a plane-parallel layer and placed for acoustic matching in an additional fluid medium with a pulse of a normally incident plane acoustic wave and measuring the level of the second harmonic in a wave propagating from the test medium in the opposite direction in relation to the radiated wave. The non-linear parameter is determined by the ratio of the amplitudes of the acoustic pressures at frequencies f and 2f.

The disadvantage of this method is low accuracy, due to the fact that, despite the fulfillment of the acoustic matching conditions of the studied and additional media, the second harmonic reflection coefficient from the plane-parallel layer appears to be small but finite. As a result of this attenuation, the parasitic second harmonic formed in the emitter and the additional medium, reflected from the layer along with the useful signal, also falls on the receiver, which leads to a measurement error of the useful component, and. hence to low

accuracy of measurement of non-linear acoustic parameter.

The purpose of the invention is to improve accuracy.

5 The goal is achieved by the fact that

according to the method for determining the nonlinear acoustic parameter of the medium, which consists in irradiating the test medium with parallel opposite

0 by impulses of a normally incident plane acoustic wave through an intermediate medium, receive acoustic oscillations re-emitted by the medium under study and determine the desired parameter

5 taking into account the amplitude of the pulses emitted through the intermediate medium, a solid medium with an acoustic impedance greater than the acoustic impedance of the medium being studied is chosen as the intermediate medium, the distance between the surfaces of the medium being studied is a multiple of the radiated wavelength in this medium, the distance LOT of the surface of the medium being studied to the boundary of the intermediate environment is chosen from the condition

5L GT / 2 (1)

where C is the speed of sound in the intermediate medium;

T is the pulse duration, and the re-emitted oscillations take on

0 frequency equal to half the frequency of the emitted signal.

The essence of the invention is to create conditions in the acoustic resonator, at which the threshold

5 oscillation generation at frequency f / 2 when the resonator is excited at frequency f.

For this, the test medium, formed in the form of a plane-parallel layer, is placed in an additional solid

The medium through which the test medium is irradiated with a pulse of a plane acoustic wave normally incident on it. It should be noted that the absence of a gap between the studied

5 and additional environments. To eliminate the influence of wave reflections from the system's borders (additional medium), and thereby eliminate the resonator with soft borders in it, where the subharmonic is not excited, the duration T of the pulse of the irradiated wave is chosen from the condition Li, 2% CT / 2, where Li , 2 are the distances from the layer to the outer boundaries of the additional medium (the inner boundaries are the boundaries of the additional medium adjacent to the plane-parallel layer). The impedance of the additional medium is chosen more than the impedance of the test, while the plane-parallel layer is an acoustic resonator with hard boundaries. When such a resonator is excited at one of its even eigenfrequencies

f f2n Co / dn (2)

and when the pump amplitude is exceeded, the critical, so-called threshold amplitude, in the spectrum of its oscillations, appears abruptly (abruptly) the component at the frequency f / 2 f2n / 2 fn, which is also the natural frequency of the resonator. In the spectrum of the wave incident on the layer, the component at this frequency is completely absent (unlike the higher harmonics due to the nonlinearity of the radiator and the additional medium in the prototype).

The threshold (minimal) amplitude of the deformation of the pump wave e0 in the additional medium, in which oscillations appear in the layer at the frequency f / 2, is determined by the expression

g- / g

(3)

 SOS

With -

at p22

-g PC

where Z -fy-, RO o

p is the density of the intermediate medium;

U, / EO, CO - nonlinearity parameter, density and sound speed of the studied medium.

Since the spectrum of the emitted signal is at the frequency f / 2 is absent, in the additional medium where traveling-wave mode is realized, it cannot be born either, its appearance clearly indicates that it appeared in the resonator — the medium being studied — and is caused by nonlinearity the study environment. It should be noted that the wave at the frequency f / 2 can be received by the receiver on either side of the layer.

From the expression (3) follows the formula allowing to calculate the nonlinear acoustic parameter:. SOS

LP Bo Z

(four)

The advantage of the method is that, in contrast to the prototype, in which amplitude measurements are necessary at two frequencies f and 2f, only one measurement is made at pump frequency f.

The drawing shows a block diagram of one of the variants of the device, with which the proposed method can be implemented.

The investigated medium 1, which is a plane-parallel plate (layer), is rigidly fixed in an additional solid medium 2, made, for example, in the form of two identical rods,

To provide acoustic contacts of solid test and additional media, they must be rigidly bonded. This is provided, for example, by gluing or by mechanical fastening. In the study of liquid media, the test liquid fills the space between the additional medium. When used as an extra medium

10 of these rods with liquid fill the gap between them.

For irradiating the test medium 1, an emitter 3 is designed, rigidly attached to the end of one of the rods 2.

15 The emission frequency f is selected in accordance with condition (2). The receiver 4 of the ultrasonic waves is fixed at the end of the same rod 2 as the emitter 3. In other embodiments, the receiver and emitter

20 can be located at the ends of different rods, i.e. on opposite sides of the test medium.

Emitter 3 is electrically connected to series-connected generator

25 5 and a modulator 6 designed to form ultrasonic pulses of duration T. To eliminate the effect of pulse reflections from the ends of the rods, their total length 1 ° C + 1-2 is greater than

30 pulse length (Lp ST). Receiver 4 sub-. keys to frequency selective measuring instrument 7.

The method is carried out as follows.

35 The test medium 1 in the form of a plane-parallel plate is rigidly fixed between two steel rods 2 of the same length. With the help of the emitter 3 through the rod 2 is irradiated investigated

40, the medium formed in the form of a plane-parallel layer 1 by an ultrasonic pulse at a frequency f of duration T. To accurately fulfill the condition (2), the frequency of the radiation 45 is adjusted in small limits so that the level of the signal received by receiver 4 is minimal (if the receiver and radiator are on different sides from the test medium, the level of the received signal

50 should be the maximum). This indicates that the layer of the medium under study is resonant, i.e. condition (2) is satisfied. The choice of the pulse duration in accordance with condition (1) allows one to establish the influence of reflections from the ends of the rods 2 (system boundaries) and thus prevent the implementation of a resonator with soft boundaries in the whole system, in which the subharmonic is not excited in principle. Then, the amplitude of the Co deformation of the pulse is increased by receiving the wave at the frequency f / 2. The measured threshold strain amplitude fc0. The nonlinear parameter is determined by formula (4).

Claims (1)

The method of determining the non-linear acoustic parameter of the medium, which consists in irradiating the test medium with parallel opposite surfaces by pulses of a normally incident plane acoustic wave through an intermediate medium, receives acoustic oscillations re-emitted by the test medium and determine the desired parameter 0 five taking into account the amplitude of pulses emitted through the intermediate medium, characterized in that, in order to increase the accuracy, a solid medium with acoustic impedance greater than the acoustic impedance of the medium is chosen as the intermediate medium, the distance between the surfaces of the medium being studied is selected as a multiple of the radiated wavelength in this medium, the distance L from the surface of the test medium to the boundary of the intermediate medium is chosen from the condition L CT / 2, where C is the speed of sound in the intermediate medium; T is the pulse duration, and the re-emitted oscillations take on a frequency equal to half the frequency of the emitted signal.