EXPERIMENTAL DETERMINATION OF THE TRANSDUCER ACOUSTIC VELOCITY IN A SONAR ARRAY USING ACOUSTICAL HOLOGRAPHY TECHNIQUES

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EXPERIMENTAL DETERMINATION OF THE TRANSDUCER ACOUSTIC VELOCITY IN A

SONAR ARRAY USING ACOUSTICAL HOLOGRAPHY TECHNIQUES

C. Giangreco, J. Garcin, C. Audoly

To cite this version:

C. Giangreco, J. Garcin, C. Audoly. EXPERIMENTAL DETERMINATION OF THE TRANS- DUCER ACOUSTIC VELOCITY IN A SONAR ARRAY USING ACOUSTICAL HOLOG- RAPHY TECHNIQUES. Journal de Physique Colloques, 1990, 51 (C2), pp.C2-745-C2-748.

�10.1051/jphyscol:19902173�. �jpa-00230477�

COLLOQUE D E PHYSIQUE

Colloque C2, supplbment au n02, Tome 51, F6vrier 1990 ler Congrgs Franqais dlAcoustique 1990

EXPERIMENTAL DETERMINATION OF THE TRANSDUCER ACOUSTIC VELOCITY IN A SONAR ARRAY USING ACOUSTICAL HOLOGRAPHY TECHNIQUES

C. GIANGRECO, 3. GARCIN and C. AUDOLY

Groupe d'Etudes et de Recherches en Detection Sous-Marine, D.C.A.N.

Toulon Le Brusc, F-83140 Six-Fours, France

Rgsume : On utilik? deux methodes d'holographie acoustique pour determiner expgrimentalement la distribution des vitesses vibratoires complexes sur la surface active d r u n e antenne drBmission sonar. Les r6sultats sont compares 3 des mesures acc~l6rom6triques.

Abstract : Two acoustical holography techniques are used to determine experimentaly the complex vibrating velocity distribution on the active surface of a sonar emission array. Results are compared with accelerometer measurements.

Due to acoustical interactions ~t is very important to know the acoustic velocity of each transducer which composes a sonar array.

The purpose of this study is to use acoustical hologiaphy techniques to measure the velocity distribution on a real array in water, and to compare these results to the acoustical velocity obtained by accelerometers stuck on the transducer heads.

2 - DESCRIPTION OF THE ARRAY AND EXPERIMENTAL SET U P

The array used in this experimental study is composed of three zolumns of four piezoelectric "Fonpilz" transducers, The dimension j f the rectangular transducer head is 0.13x0.12m2. Four accelerometers ere stuck on the transducer heads as illustrated diagrammatically in figure 1.

This array is hung under a carriage which is controlled by an HP computer and can be moved in Y and '2 directions (figure 1).

The hydrophone which is a B K 8 1 0 5 I S fixed and located at 0 . 0 4 m from the vibrating surface of the array.

Pulse sound technique is used to eliminate boundary effects. The measured frequencies are as follows : 2 , 3 , 5 and 6 kHz.

A network analyser is used to measure modulus and phase of the acoustic pressure for each location.

Two types of excitations are studied :

- the input voltage of the three columns are In phase.

- Delays are introduced in the input voltages.

Figure 1 : Diagram of the array.

X : accelerometer Locations.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19902173

COLLOQUE DE PHYSIOUE

3

-

PLANAR NEARFIELD HOLOGRAPHY

3.1

-

Principle of the technique and choice of the experimental parameters This technique which is widely known /1,2/ requires a large number of measured points recorded on a plane parallel to the active surface of the array. The normal vibrating velocities on the transducers are achieved by back propagating the complex acoustic pressure measurements with the help of a specific spatial frequency filter /2/. The size of the measured aperture is chosen to be equal to 0.93x0.93m2 because it was shown /3/ that this size should be at least equal to 1.5 time the largest size of the studied array.

The sample separation is equal to 0.03m and the distance between the vibrating surface of the array and the measured planar hologram is 0.041~ to obtain the expected 0.04m resolution.

3.2

-

Experimental results :

The modulus of the acoustic pressure measured on the planar hologram can be seen on figure 2. The transducer heads can be seen on the figure 3 which shows the modulus- of the vibrating velocity on the active surface of the array.

As we assume that each transducer head acts like a piston, the acoustic velocities which are represented on tables 1 and 2 are worked out by averaging the complex acoustic velocities of the points which belong to the transducer head surface.

It can be seen that this technique underestimates the acoustic velocity because the filter. used in the spatial frequency domain reduces the high spacial frequencies contribution.

Nevertheless this technique leads to coherent estimates of complex velocities of transducer heads.

Figure 2 : Modulus of acoustic pressure

measurements on the planar hologram.

Input voltages with delays.

Frequency : 5000 Hz.

Figure 3 : Vibrating velocities on the array surface.

Input voltages with delays.

Frequency : 5000 Hz

* * ...

* HOLOGRAPHIE * ACCELEROMETER

.*****t****tt****l*1.*******.l*t******,...******************* Table 1 :

* N ' ^MODULUS * PHASE MODULUS PHASE*ERREUR% *DELTAPHA *

.*****t***t***.********.******.****.**********,*.********

* 1 0.2637E-02 * 154.9

8 2 * 0.3221~-02 162.7 t *

:

planar Nearfield Holography

* 3 * 0.3205~-02 * 162.2 * ^t * Transducer heads velocifies.

* 4 * 0.24869-02 * 154.9 ' 0.4800E-02* 164 48.2 * 9.1

* 5 * 0 . 3 7 9 3 E - 0 2 * 174.5*0.5500&-02*174 * 3 1 . 0 * 0.5 *

a 6 * 0.4311~-02 174.6 ' Input voltages in phase.

7 ' 0.4346E-02 * ^-179.7 0.5600E-02*-178 ' ^{22.4 1.7} *

* ⁸ 0.3075E-02 * 171.1

* 9 0.2887E-02 * ^156.6

10 * O . ~ Z I ~ E - O Z * 161.1 0 0.49009-02' 177

:

^{. Frequency : 5000 HZ-}

11 * 0.3501E-02 * 161.3 *

* ¹² 0.2252E-02 150.8 * *

...

HOLOGRAPHIE * ACCELEROMETER *

. . .

N * ^{MODULUS PHASE} * ^MODULUS * PHASEnBRREUR% *DELTAPHA * ...

Table 2 :

1 * 0.1869E-02 47.5 *

* 2 * 0.2723B-02 * 60.7 ¹

3 * 0.2613E-02 * ^59.5 *

:

Planar Nearfield Holography

t 4 * 0.201s~-02 * 47.7 * 0.4030~-02* 57.0- 50.0 9.3 Transducer heads velocities.

* 5 * 0.2844E-02 * 111.6 * 0.4600$-02* 133.0** 38.2 21.4 *

* 6 * 0 3266E-02 118 4 '

* 7

.

O : ~ Z O ~ E - 0 2 1 2 0 : ~ * O . ~ O ~ O E - O Z * 142.0** 20.8 * 21.8

:

Input voltages with delays.

t 8 * 0.2579E-02 * 111.9 '

9 0.3162~-02 162.2 *

10 1 0.3299E-02 * 170.6 * 0.5100~-02* 203.0** 35.3 * 32.4

:

^{~ ~ : 5000 ~ ~ ~ ~ n c ~}

* 11 0.35438-02 * ^171.7 * *

t * * * * * * * * . * * * * * * * * t t * * * * t t * t * ( * * * * * ~ * * * * * * * * . * * * " * * * * ~ * * * * " " * * . * * * * * * * ~ ~ * * 12 * 0.2507E-02 160.9 *

COLLOQUE DE PHYSIQUE

4 - DIGITAL ACOUSTIC HOLOGRAPHY

4.1

-

Principle of the technique and choice of experimental parameters :

The previous technique requires a large number of.measured points.

As some informations of the array are known (the size, shape and positions of the transducer heads, the con'stitution and the shape of the array) the number of the measllred points can be reduced by using digital holography method /4/ based on a Helmoltz integral equation model of the array.

A few well selected measured points located in the near, field of the array is necessary. Then a matrix relationship that links the acoustic pressure of this points to the velocities of the transducer heads leads, by least square resolution, the velocity distribution. Twenty four points among the previous planar hologram were chosen by numerical simulations.

4.2

-

Experimental results :

Digital acoustical holography technique results are very similar to those obtained by planar acoustical holography and are even slightly closer to the velocities obtained by the accelerometerst as shown on tables 3 and 4.

* t * * * * * * * * * * . ~ * * * * t * * t t * * * * * ~ * * * . * * * . * * * * * *

N * MODULUS PHASE ERROR COEF. *

* * t * * t t . t * * t * . * * * t * ~ t * t * * * * * * * ~ . * * * * * * * * * ~

* * * t * * t * * * * . * * * t t * * * * * t t t t * * * t * * * * * * * f t * * *

N * MODULUS * PHASE ERROR COEF. *

.t.~*.****.**....**~L,"L..*~~*.~f**.t**~**

* 1 * 0.2246E-02 * 38.1 * 0.930

* ² 0,298dE-02 68.2 * 0.986 *

* 3 0.3008E-02 57.4 * 0.963 *

* 4 0.2271E-02 * 42.2 1.040

* 5 0.2775E-02 * ^103.4 * ^0.725 *

* ⁶ 0.3208E-02 * ^126.9 * ^0.975

* 7 ' 0.3362E-02 * 121.9 ' 0.911 *

* 8 0,27718-02 * 113.7 1.134 9 0.4034E-02 151.8 * 0.999 10 * 0.3468E-02 * 171.6 * 0.785 *

* ¹¹ 0.3852E-02 165.7 * ^1.019 * 12 ' 0.2922E-02 156.6 0.880

**. **...+~*.****tt**"~~******.*"***tt*t***

AVERAGE ERROR COEFFICIENT: 0.823 AVERAGE ERROR COEFFICIENT : 0.952

Table 3 : Digital Nearfield ~ o l o Table 4 ~ ~ ~ : ~Digital h ~ n ear field ~olography Transducer heads velocities. Transducer heads velocities:

Input voltages in phase. Input voltages with delays.

Frequency : 5000 Hz. Frequency : 5000 Hz.

5 - CONCLUSION

F R E o V E N c y

2000 Hz

5000 HI

6000 Hz

Table 5 sums up the results of this study. As a matter of fact the accuracies are around 30% and the higher the frequencies are the better they become for both methods. However Digital Acoustic Holography, by taking into account "a priori" informations on the array, allows us to drastically reduce the number of measurements and thus the trial time.

REFERENCES

/1/ MAYNARD J.D., WILLIAM E.G. and LEE Y. JASA 79 (1985) 1395.

/2/ ESCHENBERG K.E. and HAYEK S.I. Acoustical Imaging vo1.15 659 /3/ HAYEK S.I. and LUCE T.W. Journal of Vibration, vol. 110/91.

/4/ AUDOLY C. Determination of the transducer velocities in a sonar array using digital acoustical holography. Companion paper.

I N P H A S E , PLANAR H

51.8%

3C%

19.7%

Table 5 :

Error in $ .

The accelerometer results are the references.

D I G I T A L H

27.5%

27%

10,5%

DELAYS PLANAR H

55%

38%

20%

D I G l T A L H 36.8%

35%

15%