US3185868A - Acoustic absorber pad - Google Patents

Description

May 25, 1955 F. E. CQYLEETAL 3,185,868

ACOUSTIC ABsoRBER PAD Filed May 28, 1962 IN V EN TOR.

564,557 fv- 76 l BY ffm/f 27m: 65.5

/g' mum United States Patent O 3,185,868 ACGUSTIC ABSRBER PAD Forrest E. Coyle, Glen Burnie, and Frank P. Hodges,

Baltimore, Md., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed May 28, 1962, Ser. No. 198,382

. 3 Claims. (Cl. S10-8.2)

This invention relates to transducer devices and particularly to sound absorbing medium for such devices. When electrical energy is applied to a piezoelectric crystal of a transducer, 'equal amounts of sound energy radiate from each side of the crystal. In a sonar transducer it is often desired to have the energy radiate from vone side to 4obtain directivity and therefore radiated unwanted encountered energy must be absorbed. A situation where this is encountered is a sonar transducer used at substantial depth to obtain information relative to the wave motion in order to predict the trajectory of a missile fired from an underwater position.

It is, therefore, an object of this invention to provide a transducer with improved directivity.

Another object is to provide a sonar transducer with a new and improved sound absorbing pad to enable it to obtain better directivity.

A further object is to provide a new material for absorbing high frequency sound waves.

Still a further object is to provide a method of making a high frequency sound absorbing pad for use in transducers.

Other objects will become apparent from the specicat-ion and drawing in which the characteristics of thev invention are set out.

In the drawing,

FIG. 1 is a plan view of a transducer employing the invention.

FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG. 1.

FIG. 3 is -a cross-section through the sound absorbing pad.

The invention is based upon the discovery that high frequency sound waves are absorbed by a batt of loose polyester bers containing a proper proportion of copper chips `distributed through it. A batt of this type formed into a pad and placed to one side of a wave emitting transducer element will absorb the emissions from that side and, consequently, directivity is obtained in that emission only from the free side.

Referring to FIGS. 1 and 2, 10 indicates a transducer having a hollow cylindrical body 12, externally threaded at 14, and with a bottom or end wall 16. Transducer is adapted to be joined by threads 14 with an unshown stuiiing tube extending from an underwater craft; an O-ring 18 in end wall 16 seals the joint between them. Conductors connect t-he transducer through the stuing tube to the apparatus within the craft. The position of the transducer on the stuling box is in a upward looking direction toward the waters surface. The construction of .a receiving transducer used in conjunction with the transmitter, for the inventions purposes, is similar and may, if so desired, employ the sound absorbing pad, shown in FIG. 3, to be described hereinafter. Description, however, is limited to a transmitter transducer.

rThe body 12 of the transducer is generally cylindrical and has an axial cavity 20 opening towards its top. Two apertures, 21, penetrating the end wall 16 and longitudinal flutes 24 in the peripheral wall dening cavity 20 provide a passageway for leading electric energy to the oscillating element within the transducer body.

Seated within the transducer body is a co-axial stack of cylindrical elements shaped to tit the bodys cavity rice 20. The lowermost of these is a coil supporting cartridge 26. Its lower end is externally reduced by an annular groove 28 in order not to cover the end wall apertures, 21, and thereby close t-he passageway between them and flutes 24. The upper end of coil supporting cartridge 26 has a bore 30 and there is a radial groove 32 in the bottom wall 34 defining the bore 30 which receives a coil 36. The combined depth of bore 30 and groove 32 is equal to the height of coil 36. A disk-like coil cover 37 seated on coil support cartridge 26 keeps coil 36 in its desired position. Radial groove 38 in the upper end face of coil cartridge 26 furnishes access for conductors from utes 24 to coil 36.

A sound absorbing pad 40 tted into a ring 42, having radial apertures 43, is seated on coil cover 37. Each of the elements, ring 42, coil cover 37 and coil support cartridge 26 has Yan external longitudinal groove, designated .as 35, 35 and 35", respectively, for receiving a dowel 43 mounted in one of the flutes 24. This secures them against rotation within the transducer body cavity 20.

The piezoelectric oscillator element 48 is a ceramic disk of barium titanate encircled by a spacer ring 50. It .is disposed between lower and upper electrical contact rings, 44 and 46, and seated on the sound absorber assembly, ring 42 and pad 40. Contact rings 44 and 46 are provided with external, radial ta-bs 47 for entering a ute 24 and securing them against rotation. A conductor 51, passing along a flute 24, connects contact ring 44 to a connector 45 in transducer body aperture 21; whereas, a conductor 52 similarly connects, not shown, contact ring 46 through coil 36 to a connector 47 in the other aperture. Connectors 45 and 47 are adapted to seal the apertures against tiuid passage and to receive alternating energy from a source, not shown, for transmission of the energy to the oscillator element by the conductors inthe utes.

The elements thus far described are held against axial movement in transducer body 12 by lan externally threaded ring nut 54 which is threaded into the bodys cavity 20 and seated on a shim ring 56. Shim 56 is secured against rotation by a tab 57 entering a flute 24.

Transducer body 12 contains oil, not shown, and is closed by a cap assembly 58. The latter includes a rubber disk or sound transmitting window `60 secured at its ends between upper and lower clamping rings, 62 and 64, which are seated on the end face 66 of the trans- -ducer body. Rings 62 and 64 are carried in an encircling nut 68 having an inward flange 70 that engages an end face of the upper ring 62 yso that they move axially with it. Nut 68 is threaded to the external threads 14 on the transducer body and further secured with a set screw 70. An O-ring 72 under the lower ring 64 in the end face of the transducer body is furnished as a seal against fluid leakage.

In operation, alternating electric energy, applied at the faces `of oscillator element 48 through the contact rings, 44 and 46, cause it to vibrate and emit a sound signal in both axial directions of the transducer body 12. 'Ihe emission from the upper face passes through the oil, the window 60, and the water in which the transducer is immersed toward the waters surface. An echo results which is rellected to suitable equipment for resolution. The energy radiated from the lower face toward the bottom of the transducer body, however, is confronted by the lsound. absorbing pad 40 and absorbed. Emission is thus in one direction, upward from the transducer. For the purposes set out, oscillation is one megacycle or ten times normal frequency which is more than a standard acoustic material can cope with.

The use of various types of lossy rubber and cork to absorb unwanted acoustical energy that is radiated from transducer elements, may be satisfactory for depths and pressure experienced by submarines of World War II vintage. However, at higher pressures, such as those resulting from dept-hs within the capabilities of present day submarines, these materials lose their lossy properties until this pressure is reduced. In luid filled transducers, the problem is more severe because the high pressure forces the iluid into the material and then causes a permanent change.

Absorbing pad 40, FIG. 3, which has proven to perform excellently, comprises a lluffiy batt 76 of polyester fibers of ethylene glycol and terephthalic lacid, known as Dacron in the art and supplied by the E. I. du Pont Company, having interspersed throughout a multitude of curled copper chips 78. A nylon mesh covering 80 encases it to contain the metal chips and yet allow passage Vof the transducer fluid. The metal chips are used to disperse acoustical energy. The fibers of the Dacron are not bound to each other and proved a pad which is firm and of stable dimension form for suspension of the metal chips are used to disperse acoustical energy.

It will be understood that various changes in the details which have been herein described and illustrated, in order to explain the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed -in the appended claims.

What is claimed is:

l. In an electromechanical transducer, a disk-shaped piezoelectric element, and a fibrous sound absorbing pad, said sound absorbing pad comprising a batt of fibers of polyester of ethylene glycol and terephthalic acid with curled copper lings distributed therethrough.

2. The device of claim 1 wherein said copper filings are curled and there is three and one half times their weight per unit weight of said fibers.

3. The device of claim 2 wherein said pad is enclosed in a nylon mesh casing.

References Cited by the Examiner UNITED STATES PATENTS 2,427,348 9/ 47 Bond et al.

2,707,755 5/55 Hardie et al. 3l08.2 2,881,336 4/59 Elion 310-8.2 2,972,068 2/61 Howry et al.

MILTON O. HIRSHFIELD, Primary Examiner.

ORIS L. RADER, Examiner.

Claims (1)

1. IN AN ELECTROMECHANICAL TRANSDUCER, A DISK-SHAPED PIEZOELECTRIC ELEMENT, AND A FIBROUS SOUND ABSORBING PAD, SAID SOUND ABSOIRBING PAD COMPRISING A BATT OF FIBERS OF POLYESTER OF ETHYLENE GLYCOL AND TEREPHTHALIC ACID WITH CURLED COPPER FILINGS DISTRUTED THERETHROUGH.

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