GB2218593A - Acoustic load device - Google Patents

Abstract

An acoustic transducer intended for underwater use can be tested in air by a load device which simulates the underwater conditions. The load device 1 comprises a body 3 of a lossy polymer composition, means for holding said body in contact with an output surface 6 of the transducer, and a clamp 2,8 to press the body against the output surface 6. This gives a simple and inexpensive way of carrying out some test procedures since the polymer allows rapid dissipation of the acoustic energy produced by the transducer. <IMAGE>

Description

ACOUSTIC LOAD DEVICE This invention relates to an acoustic load device. It relates particularly to a load device of this kind which can be used for testing an acoustic transducer such as one intended to form part of a sonar system.

In testing a range of acoustic transducers for a sonar system it is not always convenient to do this under water immersion conditions and attempts have been made to use "in-air" tests with a dummy load device coupled to the acoustic transducer under examination.

It therefore becomes necessary to provide a dummy load device which behaves in a way closely resembling that which might be expected from a water load and which has loss parameters capable of giving the required level of dissipation of acoustic energy.

The present invention was devised in an attempt to provide a dummy load device which would have these characteristics.

According to the invention, there is provided an acoustic load device for testing an acoustic transducer, the load device comprising a body of a lossy polymeric composition, means for holding said body in contact with an output surface of the acoustic transducer, and clamp means effective to press the body with a mechanical loading against the said output surface.

As indicated, a degree of mechanical loading is applied to the transducer face. Where the device to be tested is an array of transducers, the testing device may comprise an array of individual pad clamps which may be applied by screw and spring means to give the required loading force on the transducers of the array.

Conveniently, the loading force may alternatively be applied by hydraulic means. This technique is particularly suitable as it can enable all of the load pads in an array to be applied with equal pressure.

The device may include a couplant material positioned between the said body and the output surface of the transducer.

Preferably, the said polymeric composition is a polymethylene glycol type polyurethane which has been loaded with an inert filler material.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a sketch showing as an exploded diagram a hydraulic clamping apparatus for holding a matrix of load pads in contact with a transducer array, Figure 2 is a part cross-sectional view showing a single load pad with its hydraulic clamping means, and Figure 3 is a graph showing the dynamic properties of the polymer material, As depicted in Figure 1, the test apparatus has an acoustic load device I which comprises a rigid actuator block 2 supporting a matrix of hydraulically-operable piston actuators 3. The actuator block 2 acts as a manifold for supplying pressurised hydraulic fluid to each of the actuators 3 and the unit is energised by a hand pump 4.The arrangement of the actuator block 2 is such that it is capable of fitting closely against an active front surface 6 of a particular model of a sonar array 7. The actuator block 2 can be clamped up against the array 7 by use of a backing member 8 which is capable of being accurately located against a rear surface 9 of the array. Location pins 11 in the backing member serve to position this accurately against the array 7. The backing member 8 and the actuator block 2 are then connected together by mechanical clamp means 12.

It will be noticed that the actuator block 2 is in fact made up as a number of smaller unit blocks which are connected together by jointing means, in this embodiment, the means are dovetail joints 13.

This construction allows the actuator block 2 to be readily adapted to fit a range of different models of sonar array to allow wider use of the test apparatus.

Figure 2 shows a part cross-sectional view of a single piston actuator 3 of the actuator block 2. The piston actuator comprises a movable platform 14 which is connected by means including rubber ring seals to the rigid block 2. When the hydraulic fluid in the block 2 is pressurised, the platform becomes moved to the left in the arrangement as depicted in Figure 2. The actuator block 2 has internal passages 16 arranged so that all of the piston actuators 3 in the matrix are hydraulically pressurised simultaneously.

The movable platform 14 supports a load pad 17 which is formed of a lossy polymeric material. In the present embodiment, the load pad 17 is of a polymethylene glycol type polyurethane loaded with barium sulphate.

On a left hand side of the load pad 17 a layer of couplant material 18 which is in the form of a soft rubber layer is attached.

In the present embodiment, an oil extended compliant rubber compound has been found suitable. The purpose of this couplant material 18 is to simplify the application of the load pad surface to the sonar array under test and to alleviate any problem of interfacial contact that may arise.

In operation of the test apparatus, the sample of the sonar array 7 which is to be evaluated is mechanically clamped to the rigid actuator block 2 by a suitable positioning and securing of the backing member 8. The hand pump 4 is then used so as to bring all of the piston actuators 3 under pressure and the couplant material 18 surfaces of the load pads will then be pressed into contact with the active front surface 6 of the sonar array 7.

The sonar array 7 could then be put through the usual testing procedures that are required under low and high power drive conditions, at different frequencies, at various stages of drive voltage level, and at different operational temperatures, etc.

Figure 3 is a graph showing the dynamic mechanical response of a particular viscoelastic material and it indicates the dynamic Young's Modulus (Log E in units of Newtons per square metre) on the left hand vertical axis at a constant frequency (10 hertz) and over a temperature range T of from -500C to 50"C. On the right hand vertical axis, the graph shows loss factor (tan delta) over the same temperature range.

The response curve depicted is typical of all types of viscoelastic material in that at low temperature (or high frequency) the materials in this category are in a 'g]assy' type state (that is, high modulus). As the temperature is increased (or the strain frequency reduced), the molecular structure experiences relaxation mechanisms resulting in a transition phase before a 'rubbery' plateau is reached.

The transition phase on the modulus curve is associated with a maximum loss in the material.

The selection of the polymer material for the load pad 17 must be made with regard to the material properties (dynamic mechanical) from which the effective modulus Beff can be derived.

This value enables the mechanical or acoustic impedance of the material to be calculated.

The material thus has a characteristic impedance which will give a reasonable impedance match with water. A compound at a particular frequency and temperature was found to have these properties. For sea water, there is a characteristic impedance over the range from 0 to 450C, which is nominally 1.45 x 106 RAYLS. This value is what the polymer material is required to match.

Use of the acoustic load device of the invention has been found to offer a convenient and cost-effective facility for testing sonar arrays. The device may be coupled to a single transducer or to a multi element array. By changing the arrangement of unit blocks in the actuator block, sonar arrays of different areas can be attached for the testing procedure.

The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, it is not essential that the couplant material for the load pad should be a soft rubber layer, in a different embodiment this could be water or a coupling gel such as a commercially available acoustic gel composition.

Instead of having the unit blocks of the actuator block 2 connected together and pressurised by the hand pump 4, each unit block could be a separate part with its own closed hydraulic system.

The hydraulic system of each unit block could include a pressure gauge and pressurising of each of the unit blocks could then be effected by tightening of the clamp means 12. When the tightening of the clamp means 12 has been done sufficiently to cause each of the gauges on the unit blocks to indicate a predetermined hydraulic pressure this will ensure an accurate distribution of the pressure over the transducer face under test.

The backing member 8 could also be made in separate sections which fit together by joining means if this is required.

Claims (6)

1. An acoustic load device for testing an acoustic transducer, the load device comprising a body of a lossy polymeric composition, means for holding said body in contact with an output surface of the acoustic transducer, and clamp means effective to press the body with a high mechanical loading against the said output surface.

2. A device as claimed in Claim 1, in which the clamp means is an hydraulic clamp.

3. A device as claimed in Claim 1 or 2, including a couplant material positioned between said body and the output surface of the transducer.

4. A device as claimed in Claim 1, 2 or 3, in which the said polymeric composition is a polymethylene glycol type polyurethane which has been loaded with an inert filler material.

5. An acoustic load device substantially as hereinbefore described with reference to the accompanying drawings.

6. A method of testing an acoustic transducer substantially as hereinbefore described.