MEASUREMENT OF ACOUSTIC MATERIALS IN AN IMPEDANCE TUBE : INFLUENCE OF THE INTERSTICE OF WATER AROUND THE SAMPLE

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MEASUREMENT OF ACOUSTIC MATERIALS IN AN IMPEDANCE TUBE : INFLUENCE OF THE INTERSTICE OF WATER AROUND THE SAMPLE

C. Giangreco, C. Audoly

To cite this version:

C. Giangreco, C. Audoly. MEASUREMENT OF ACOUSTIC MATERIALS IN AN IMPEDANCE

TUBE : INFLUENCE OF THE INTERSTICE OF WATER AROUND THE SAMPLE. Journal de

Physique Colloques, 1990, 51 (C2), pp.C2-1029-C2-1032. �10.1051/jphyscol:19902241�. �jpa-00230570�

ler Congres Frangais d'Acoustique 1990

MEASUREMENT OF RCOUSTIC MATERIALS IN AN IMPEDANCE TUBE : INFLUENCE OF THE INTERSTICE OF WATER AROUND THE SAMPLE

C. GIANGRECO and C. AUDOLY

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

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

Abstract : Impedance tubes filled of water can be used to measure the reflection and transmission coefficients of acoustic materials. Some experimental results often show disagreements with models and other measurement techniques. A finite element code is used to model the

impedance tube and we show the relationship between these phenomena and the presence of an interstice of water around the sample. The difficulty of finding a practical mounting for the sample giving good results is put forward.

1. INTRODUCTION

An impedance tube filled of water is used to measure the reflection and transmission coefficients of a sample of acoustic material. The sample diameter is slightly smaller than the inner diameter of the impedance tube.

Hence an interstice of water exists and perturbs the experimental results. The purpose of this study is to understand these phenomena and to show the difficulty of finding an appropriate practical mounting for the sample.

2. DESCRIPTION OF THE MEASUREMENT TECHNIQUE

The impedance tube is used to measure the reflection and transmission coefficients of a sample of acoustic material as if the sample was an infinite plane of same thickness. The main advantage of this technique by comparison to the test panel measurement is to require only a small sample of material.

Pulse sound technique is used to eliminate the boundary effects and to separate the incident and the reflected waves. For the transmission coefficient, two measurements are necessary: the incident wave is first determined without the sample, then the transmitted wave is measured with the.

sample located between the projector and the hydrophone, the measurements can be made between 2 and 17 kHz. Below 2 kHz, the pulse sound technique cannot give accurate results due to the length of the tube. 17 kHz corresponds to the cut-off frequency above which other modes than the plane wave can propagate.

3. EXPERIMENTAL RESULTS

First a few samples whose acoustic properties are well known were tested.

For example, Fig.l shows the reflection and transmission ceofficients obtained on a sample of steel with a thickness of 8 cm. We observe two resonances at 5.5 kHz and 11 kHz which do not agree with the infinite plane theory. This phenomenon appeared for all the metallic samples tested, and the position of the resonances varied with the material and the thickness of the sample. For damped rubber type materials, such resonances did not appear, but strong discrepancies with the test panel measurements were obtained.

Résumé : L'utilisation d'un tube d'impédance rempli d'eau permet la mesure des coefficients de réflexion et de transmission de matériaux acoustiques.

Des résultats expérimentaux montrent qu'il y a souvent désaccord avec les résultats de modèles théoriques ou d'autres méthodes expérimentales. On utilise un code par éléments finis pour modéliser le tube d'impédance et l'on montre que ces phénomènes sont liés à la présence d'un interstice entre l'échantillon et le diamètre interne du tube. On conclut sur la difficulté à trouver un montage mécanique de l'échantillon donnant des résultats corrects.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19902241

COLLOQUE DE PHYSIQUE

Fig.1 : Experimental reflection and transmission coefficients of a 8 cm thick steel sample;

(---) Reflection; (-) Transmission.

4. MODELING OF THE IMPEDANCE TUBE MEASUREMENTS USING A FINITE ELEMENT CODE

A finite element code is used to understand these phenomena. The modeling of the portion of the tube containing the sample is done using the finite element code ATILA, which is mainly used to model underwater acoustic projectors.

Because of the symmetry of the tube, the axisymmetric elements can be used.

The inner steel tube boundary is assumed perfectly rigid and a cylindrical piezoelectric ceramic provides the incident wave. An example of finite element mesh is given on Fig.2. The reflection and transmission coefficients are obtained by using the acoustic pressure calculated by ATILA code at 3 cross sections located in the fluid region. First the sample is assumed to be well adjusted in the tube, so the radial displacement of the sample points which are in contact with the inner impedance tube diameter is imposed to be zero.

Fig.3 shows that this condition gives the desired result, i.e. the infinite panel result. Then, the influence of an interstice of water was studied. Fig.

4 shows that with an interstice of water between the sample and the tube, a resonance appears. The analysis of the deformations of the sample shows that the resonance is related to a radial displacement of cylindrical boundary of the sample. The position in frequency of the theoretical resonance is 8 kHz (instead of 5.5 kHz). The mismatch with the experimental results may be due to the fact that in reality the geometry is not perfectly axisymmetric. The water interstice creates some disturbances even for a soft material (acoustic impedance much smaller than water). See Fig.5.

Fig.2 : Example of finite element mesh of impedance tube with sample

0.01

.

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0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 FREQUENCY (kHz)

Fig.3 : Transmission coefficient of a 8 cm thick steel sample;

(-)

. .

( . ) Finite element, radial displacement blocked.

Fig.4 : Transmission coefficient of a 8 cm thick steel sample computed using the finite element model;

(-1 Radial displacement blocked;

(*.--..-*') Interstice 1 mm;

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Fig.5 : Transmission coefficient of a soft material sample computed using the finite element model;

(- ) Radial displacement blocked;

(-..-...) Interstice 1 mm;

(---) Interstice 2 mm.

COLLOQUE DE PHYSIQUE

5. PRACTICAL MOUNTING FOR THE SAMPLE

After these results, a special mounting for the sample was designed to try to improve the results. The difficulty is to prevent the radial displacement.

of the sample points which are in contact with the the inner diameter (in fact the order of magnitude of the acoustic displacements is about 10-1°m). This practical mounting was tested with steel samples of thickness 2 cm, and the results are shown on Fig.6. The first sample had a diameter slightly larger than the tube diameter and was introduced by force. Under these conditions, the transmission coefficient is slightly below the theoretical one. With a sample with diameter smaller than the tube diameter, but adjusted with an accuracy of 1/100 of mm, the transmission coefficient is above the theoretical one. These mountings do not give much better results than the previous ones.

Fig.6 : Transmission coefficient of a 2 cm thick steel sample;

(- ) ~ n f i n i t e plane model;

( - - - ..-..) Previous results;

(---) Sample introduced by force;

(---) Sample precisely adjusted.

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2 3 .

s

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0.0 8.0

6. CONCLUSION

This study has shown that the interstice of water pertubs the measurement of the reflection and transmission coefficients of acoustic materials using an impedance tube filled of water. Special mountings for the sample are tested and show the difficulty of finding a practical mounting which would provide good experimental results.

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