SU1670425A1 - Method and device for measuring the speed of sound - Google Patents

Abstract

The invention relates to a measurement technique and can be used to determine the speed of sound. The purpose of the invention is to improve accuracy by eliminating the error of sound diffraction at the receiving point. Sound pulses are emitted into the test medium by the emitter, the time they travel for a given distance is recorded according to the instants of the phase change of the light wave focused on the axis of the coherent radiation emitter, which is perpendicular to the direction of the radiation, the speed of sound in the medium is determined by the passage time. 2 sp.f-ly, 1 ill.

Description

The invention relates to acoustic measurements, namely to precision measurements of the absolute values of the speed of sound propagation in liquids, in particular in seawater, as well as in gases.

The purpose of the invention is to improve the accuracy of measuring the speed of sound by eliminating errors from the diffraction of sound at its reception point.

The drawing shows a device that implements the method of measurement

The device consists of a measuring chamber 1, which is, for example, a vessel of corrosion-resistant material, a movable rod 2 with a sound reflector 3 mounted on it. 3. Seals (guides) 4 in which rod 2 moves and mechanism 5 for longitudinal movement of rod 2, for example , a screw pair driven by an electric motor through a gearbox. Stem 2 is connected to meter 6 for longitudinal movement of stem-2; As a meter 6, for example, a commercially available laser displacement meter of the IPL-ZOK type can be used. In the chamber 1, an electroacoustic emitter 7 is installed, for example, made in the form of a disk made of piezo-ceramic PTS or quartz crystal. The device also includes an optical Michelson interferometer 8 with photoelectric registration. The optical interferometer 8, in turn, consists of a source of 9 coherent continuous monochromatic light (for example, a helium-neon laser with a beam expander), a beam splitter 10 (for example, a beam-splitting cube with a semitransparent layer of explosives), a short-focus lens 11 (for example, a photo-lens with a focal distance 50 mm), a concave spherical mirror 12 (for example, with an outer coating) and with the same focal distance and opening angle as that of the lens 11, and the mirror 13 installed in the support arm. For the displacement of the mirror 13 is a block 14, made in

(L

WITH

about

V4

g

YU

cl

as a piezoelectric or magnetostrictive column. The interferometer 8 also contains a photoelectric receiver 15 (for example, a photodiode or a photomultiplier), filters for low 16 and high frequencies for 17 (for example, RC filters), amplifier 18 for low frequencies (operational amplifier), amplifier driver 19 (for example, operational amplifier with comparator ), the meter 20 time interval (between pulses coming from the output of the amplifier-former 19) in the digital code,

The structure of the device input g computing unit 21 and the recorder 22 (for example, digital print or plotter).

Position 23 shows a schematic acoustic pulse propagating from transducer 7 to reflector 3 and back. The position U0 denotes the reference bias voltage of the amplifier 18. The position L denotes the longitudinal movement of the rod 2. The position 24 denotes the medium under investigation — gas or liquid, for example, sea water. The position F is the focus (focal spot) in the working arm of the optical interferometer formed by the lens 11 and the mirror 12. The generator 25 is connected to the radiator 7. The positions a and b indicate the extreme positions of the reflector 3 of the sound.

The photoelectric receiver 15 is connected to the inputs of filters 16, 17, the output of the filter 16 is connected to the input of the amplifier 18, the output of the amplifier 18 is connected to the block 14, and the output of the filter 17 is connected to the input of the amplifier 19, the output of the amplifier 19 is connected to the input of the block 20, the output of the block 20 connected to computing unit 21.

The measuring arm of the optical interferometer 8 is placed in the test medium 24 in the chamber 1 between the transducer 7 and the sound reflector 3 and is formed lying on the same optical axis one against the other by a short-focus lens 11 and a concave spherical mirror 12 so that the foci of the lens 11 and the mirror are inclusive. their common focus F is located on the axis of the acoustic beam, and the optical axis of this arm of the interferometer 8 is oriented normally to the axis of the acoustic beam, while the width of the acoustic BEAM does not increase the distance between the volume tweeted and spherical mirror to eliminate reflections

The position Reset indicates the circuit connecting the first input of the amplifier 18 to the ground, i.e. nullifying its potential at the signal from the computer transmitter 2 .. It is made in the form of a key that controls the input

which is connected to a separate output of the calculator 21.

The method is carried out as follows.

Acoustic pulses are emitted into the test medium and the time of passage or a predetermined distance is recorded according to the instants of the phase change of the light wave focused on the axis of acoustic radiation of coherent radiation perpendicular to the direction of acoustic radiation.

The device works as follows.

The source c) emits a parallel beam of coherent monochromatic light with wavelength AO in vacuum. This beam of light is directed to a beam splitter 10, where it is divided into two parts. One part passes through the beam splitter 10 directly to the short-focus lens 11. By means of which the beam is focused in the test medium at point F on the acoustic axis of the transducer 7 Passing the focus F in the working measuring arm of the optical interferometer 8, the light beam hits the surface of the spherical mirror 12, reflecting which it is again assembled in the focus F, running in the opposite direction

through the lens 11 and returns to the beam splitter 10, where reflected from the semitransparent layer of explosives, is directed to the photodetector 15 Another part of the light beam from the source 9 is directed by the beam splitter 10 to the mirror 13 installed in the reference arm of the interferometer Reflected from the mirror 13 the reference beam of light is directed back through the beam splitter 10 into the photodetector 15. In the plane of the photodetector 15, the reference optical radiation interferes with the light beam coming from the working arm. A photodetector 15 converts the intensity of light into an electrical signal, which is fed to the input

a low-pass filter 16 with, for example, 0 1 kHz. From the output of the filter 16, an electrical signal is fed to one of the inputs of the operational amplifier 18. To the other input of which the reference voltage U0 is supplied. The output voltage of the amplifier 18 through the device 14 controls the position of the mirror 13 in the reference arm of the interfermeter, forming an optical-optical circuit of the negative feedback zi The voltage Uc is set to such a value that when the contact is closed Reset before the start of the measurement cycle the average intensity I of the light recorded by the Photodetector 15. corresponds to the middle of the linear portion of the interference pattern, those 0.5 (lmax + + I min), which corresponds to the case when the interfering reference and working light beams are in quadrature (shift by azeg / 2 or 90). Thus, this negative-feedback optical-electric circuit automatically compensates for slow changes in the difference of optical lengths. ochego and reference interferometer arms keeping it constant at l / 2

At the beginning of a cycle of measurements of sound speed in a test liquid, for example, let water, for example, water, 5 g mechanism, grip rod 2 in guides 4 and set the sound reflector 3 to its original position, for example, the upper one. After emitting, emitter 7 is excited, from the test surface of which medium 24 propagates towards the reflector 3 a short acoustic impulse 23, for example a bi-impulse duration Hz - 1 μs. Spread in medium 24 from the radiator 7 towards the reflector of the chuvka 3 in the forward direction, the sound impulse 23 crosses about it occupied focused light beam in paf Mt-th arm of the interferometer (designated 1 itrilovymi leagues)

With the passage of the focal area F by the sound pulse, the predominant p A p SVTRT changes in the test medium due to the influence of the sound pressure on it. This change is described with the ratio

D., (| LR

where privately produce ER -

Lp 6R

10 10 Pa 1 dl water

-1,4

D P - the amplitude of the sound pressure in the acoustic pulse

The increment of your phase in the working arm of an optical interferometer is determined by the expression

d (n

Yao

d r

AR

gdeiff - effective for the interaction of sound and light in the working arm of the interferometer

order of magnitude & ff 10 m Let the intensity of the sound pulse G be 104 W / m, then in water where the density is p - 10 and the speed of sound C 1.5 103 m / s, the amplitude of the sound pressure will be

 V

2pC G - v 2 10J 1 5 10J 10Ч

0

0

1.7 105Pa use helium-neon

0.63

laser with then

If a

light wave length AO

for eff yu we get

А 2 1 4 1 7 Ю5 -

063-10 b

-7,5-

fraction of the period (or fraction of the interference band)

Conversion to radians and angular degrees, we get 7 5 10 2 360 ° angle 26 ° angle. or p 1 5 Yu2 -2 tg glad -0.5 glad that can be easily registered

If the transverse size of the focal spot of the light is dL where L is the spatial intensity of the sound pulse in the medium, then the increment of phase D of the optical interferometer repeats the shape and duration of the sound pulse When the duration of the sound pulse is gi 1 μs, L with gi 1 5 mm The size of the focal spot

five

0

light d -0,61 A

)

- wave length

light in water n - light refractive index in water F - focal length of the lens (in water) R - hole radius

1.33, AO R 10 mm

(pupil) lens When n 10 3 mm E - 50 mm

About 63

GET

s

0

five

0

five

-z

about -061 - 14 10

-3

ABOUT . 50

Tzz yu So the condition d L is well satisfied

Consequently, when the focal area F pa-6oieiO of the measuring arm of the optical interferometer 8 passes by the sound pulse, an electrical impulse arises at the output of the photoelectric receiver 15 in form and duration of a repetitive acoustic impulse. This short electrical impulse arrives at the high-frequency input 17 with bandwidth, for example, 01- 10 MHz, the post of which is amplified in amplifier 19 at the output of which, at the moment of signal transition through zero (in the case of a bi-pulse), the comparator produces a start impulse that goes into meter 20 Passed focal area sound pulse 23 reaches the surface of the sound reflector 3, from which it is reflected again passes the focal area in the opposite direction, resulting in the output of amplifier 19 forming a stop pulse that goes into meter 20 converts the time interval n between the start and stop 1 pulses into

the digital code that enters the computing unit 21. where it is recorded in the memory.

After that, a command is generated, according to which the mechanism 5 moves the rod 2 with the reflector 3 to the lower position b (dashed lines) by the value L The displacement L is measured by the biv meter in digital form and is fed to the computing unit 21, where it is recorded in a memory device.

Then, a start signal is generated from the generator 25, and in the test medium from the radiator 7 towards the reflector 3, which is in the lower position, a sound pulse begins to propagate. By intersecting the focal area F in the forward and reverse directions at the output of the amplifier former 19, the starting and table impulses are generated with a time interval T2 between them. The time interval T2 converted by the meter 20 in the digital code enters the block 21 and is recorded in its memory. After that, the speed of sound is calculated using the formula C 2L / (t - Tr), and the calculation results are displayed on the recorder 22.,

The advantage of the proposed method and device in comparison with the prototype is to improve the measurement accuracy. This is because a speed measurement is provided that is common1)

Claims (2)

sound in the mode of traveling waves without disturbing the acoustic field at the point of reception Formula of the invention 1 A method for measuring the speed of sound, consisting in that they emit acoustic pulses into the medium under study and record the time they travel a given distance, which determine the speed of sound different from that, in order to increase the accuracy, the registration of the transit time of acoustic pulses is carried out according to the instants of the phase change of the light wave of focused coherent radiation, perpendicular to the direction of acoustic radiation. 2. A device for measuring the speed of sound, comprising a measuring chamber, electroacoustic devices installed in it Emitter and reflector of sound and series-connected time interval meter, computing unit and recorder, characterized in that it is equipped with an optical interferometer Michelson and the high-pass filter connected to the output of his photodetector, the output of which is connected to the input of the time interval meter, and the measuring arm of the interferometer Michelson is made of a short-focus lens and a concave spherical mirror, which have a common focus and are mounted on one otic axis perpendicular to the axis of the electro-acoustic emitter. n I I Editor A.Dolinich Tehred M. Morgenkorrektor S. Cherni Order 2739 Draw 306 Subscription VNIIPI State Committee for Inventions and Discoveries at the State Committee on Science and Technology of the USSR 113035 Moscow, Zh-35, 4/5 Raushsk Nab. f Uo H 13 55 Ј h ÁHL