GB2317698A

GB2317698A - Acoustic sink for a sonar dome

Info

Publication number: GB2317698A
Application number: GB8329681A
Authority: GB
Inventors: John Richard House
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1983-10-10
Filing date: 1983-10-10
Publication date: 1998-04-01
Anticipated expiration: 2003-10-10
Also published as: GB2317698B; GB8329681D0

Abstract

A sonar dome assembly comprises a glass reinforced plastics dome 101 connected to a frame member 109 of an underwater body, eg. a submarine, via a visco-elastic wedge member 105 acting as an acoustic energy sink to absorb energy which would adversely affect sonar measurements. The visco-elastic material is bonded to an internal flange on an insert ring 104, the dome being bolted (at 105) to the flange and the ring being bolted (at 108) to the frame member. A coating 102 of damping material covers the whole of the external surface of the dome to dampen vibrations in the dome.

Description

Accustic @@@ FOR A SONAR DOME The invention relates to sonar domes and in particular to the joining of the sare to supporting structures.

In attemping to reduce submarine sonar self noise emissions each source of noise must be considered. One sonar system includes an acoustically transparent glass reinforced plastics (GRP) sonar dome encapsulating the array while at the same time forming a hydrodynamically sooth front end of a submarine. The inventor realised that one source of self noise in this dome may be expected to be the joint between the GRP dome and its steel supporting structure. Vibrational energy in the form of copress- ional and/or flexural waves propagate through the GRP until the steel interface is reached when a combination of effects occurs: energy is reflected back into the GRP, mode conversion possibly taking place, with the compressional waves being free to radiate into the array site; b. energy is transmitted through the joint into the steel conversely energy from the steel can propagate into the GRP ere once again it is free to radiate into the array site; and c. accoustic energy will be emitted at the joint into the water and hence the array site.

The acoustic interaction at a joint between GR and steel has been studied and it has been shown that high reflection losses con be obtained provided the joint input impedance is correc'. This however is coupled with a loss transmission loss. Conversely it has been shown that it is possible to obtain high transmission loss, but at the expense of good reflection loss.

In these cases vibrational energy is not lost but is merely redirected or altereted by mode conversion.

The object of the invention is to provide a joint which absorbs energy thereby acting as an acoustic energy sink.

The invention comprices a sonar dome assembly for connection to a body including and an accoustic energy absorbing joint for interposition between dome and the body. Preferably the joint comprises a layer of visco-elastic material.

Advantageously the visco-elastic material is wedge-like, being tapered towards the GRP dome. It has been found that an improvement in performance can result by covering the edge joint internally of the done by a layer of decoupling material5 for example a closed cell foam to prevent radiation of energy from the joint into the dome. A further improvement car be obtained by coating the external surface of t'ne sonar dome with a coating of a wave damping material, for example a polyurethane.

Preferably the visco-elastic material is attached to a metal insert ring by an adhesive, the ring being attachable to the body as by bolts. In one form the visco-elastic material may comprise a wedge portion and G base portion, the base portion having ct therein a slot into which there fits a complementary tongue extending axially frown the insert ring. In an alternative form there is provided an axialby extending tongue at the innermost portion of the insert ring to provide an L section seat on the insert ring to which the visco-clastic material is attached. in the latter arrangement the sonar dome nay be bolted to the outer end of the tongue.

In one form, provided with a decoupling layer, the layer is extended to cover part of a body frame provided for attachment of the dome.

Advantageously the visco-elastic material is a high density nitrile rubber with a density of the order of 170 kgm-3. The adhesive is preferably an epoxy resin.

The invention will now be described by way of example only with reference to the accompanying Drawings of which: Figure 1 is a nart-sectional view through a conventional GRP/steel sonar dome joint; Figure 2 is a part-sectional view through a GRP/steel joint between a sonar dome and a steel support structure, modified to include a visco-elastic izlterlayer; Figure 3 is a sectional view illustrating a tapered visco-elastic joint; Figure 4 is a part-sectional view through a practical visco-elastic joint; Figure 5 is a block diagram of the circuitry and a schematic view cf the apparatus for testing the visco-elastic joint; Figure 6 shows the frequency dependence of the reflection loss measured with different sonar dome interfaces; Figure 7 shows the frequency dependence of the transmission loss measured with the same interfaces as in Figure 6; Figure 8 shows the joint radiation characteristics measured in water at 30 - 60C for the interfaces cf Figure 6 for noise sources n the dome; Figure 9 shows similar characteristics to Figure 8 for noise sources in the steel bodg; and Figure 10 shows a further visco-elastic joint configuration.

A conventional glass fibre reinforced plastics sonar come 1 for enclosing a sonar detector arry is shown in Figure 1 attached to a flange 2 provided there for on a steel supporting structure 3. Also indicated are the mechanismus by which structure-borne acoustic energy is radiated towards the enclosed son=- detector array. GRP-borne acoustic energy 4 can lead directly to self-noise 5 at the site of the sonar detector array, unrelated to the GRP/steel joint. At the GRP/steel interface 6 incident acoustic energy is partially transmitted (7) into the steel structure, partially reflected (8) back into the GRP dome and partially escapes to produce noIse (9) incident on the sonar array. Similarly steel-borne noise 10 incident on the interface 6 is partially reflected (11), partially transmitted (12) and adds to the noise (13) incident on the sonar array. A further source of noise (14) within the dome is the interaction (15) in the GRP dome between noise energy travelling towards the joint and noise energy travelling away from the joint Figure 2 shows an idealised energy absorbing joint between the GRP dome 1 and the steel structure 3 formed by interposing a viscoelastic layer 21. The visco-elastic layer absorbs acoustic energy from the GP.P (22j and the steel (23) and thereby removes the sources of noise 9, 13 and 14 at the sonar array site. The material must be selected to be viso-elastic in the appropriate frequency range and at the ambient temperatures. A material given in the accompanying Table was selected for theoretìcal study using a layer thickness cf 250 mm with approximately 25 air content.

Table

Tablr NBR 465 100 Zinc Oxide 5 Stearic Acid 1 Nonox B 2 Philblack A China Clay 20 DI - CUP 40 8 SR 427 10 Barium Sulphate 9o The transmission loss can be shown to be 17dB for frequoncles above 1kHz. The reflection loss shows narrow band peaks at low frequencies occurring at multiples of #/2 where 2 is the wavelength and thus the material is most useful at higher frequencies where the layer contains many multiples of #/2. Where it is desirable to minimise the layer thickness the best compromise has been shown to be a thickness of 3#/2.

Narrowband reflection loss can be overcome by means of geometric impedance matching le shaping the layer in the form of a wedge so that compressional waves in the GRP dome enter the visco-elastic layer gradually. Figure 3 shows an idealised visco-elastic wedge 31 inserted between a GRP dome 32 and a steel support 33. Theoretical treatment of such a wedge by an approximate method has confirmed that the impedance matching of the joint is improved, and a further advantage of the tapered joint is that it allows more surface area for bonding and therefore makes the joint more structurally sound.

Figure 4 shows a practical implementation of the visco-elastic joint coupling a GRP dome 41 to a metal casting 42 done housing. The visco-elastlc wedge 43 is constructed from a high density nitrite rubber (p ~ 1700 kgm-3) and has a tapered portion 44 150 mms wide and an integrally formed uniformly thick base portion 45 100 mms wide.

The wedge is moulded onto the perimeter of the GRP dome 41 such that the GRP material merges smoothly with the base portion 45 of the wedge. A groove 46 6 mm wide and 135 mm deep is cut into the bas of the wedge and the wedge is bonded to a corresponding steel tongue extending axially from a steel insert ring, 47 using an coy adhesive. The insert ring 47 is bolted to the submarine support frame 42.

An experimental arrangement was devised to test the viscoelastic joint. The arrangement, generally similar to that shown in Figure 5, comprises a GRP beam 51 measuring 2m x 0.07 m x 0.025 m, having a transducer 52 and an accelerometer 53 placed at one end of the beam. The transducer 52 is excited by the tone bursts of 5 - 12 kHz produced by the amplified output from a tone burst generator 51, controlled by an oscillator 55. The accelerometer 53 monitors the tone burst after reflection fro the far end of the bca, he signal from the accelerometer 53 is connected via a pre-amplifier 56 to a spectrum analyser 57 which is also linked to the tone burst generator 54 and the output is connected to a printer 5S. The reflection loss for the visco-elastic joint is established by comparing the signal from the open ended beam (assumed to be perfectly reflecting) to that obtained from the sane beam wnen terminated with the visco-elastic wedge and steel backing. Transmission loss was measured by comparing the signal transmitted to the far end of the beam 51 to that transmitted to the far end of a 1 m long steel bean 59 when bonded to te visco-elastic wedge 60 and the GRP beam 51. The loss of signal due to attenuation along the steel was found to be negligible, being much less than d3. Figures 6 and 7 show respectively the results for airborne measurements of reflection loss and transmission loss with the apparatus in air at 1000. The curves for tile visco-elastic joint 61 and 71 are compared to the measured results for a GRP/steel butt joint (62, 72) and a tapered GRP/steel joint (63, 73) with an apex angle of 10 . The results generally agree with the theoretical predictions, showing reflection losses to be ~ 16.5dB at 12 kHz and > 10dB above 6 kHz and transmission losses to be > 18dB above 1kHz.

Waterborne measurements were taken using the sane apparatus as for the airborne measurements by suspending the GRP beam 51, joint and steel 59 in an anechoic tank until the top of the GRP beam with the transducer 52 is just proud of the water surface 31. h hydro- phone probe 32 mounted 15C mm from the GRP beam 51 is used to monitor the radiated signal at various positions ale the beam and joint when the transducer 52 is excited with 10 kHz tone bursts.

This test was also carried out with the apparatus inverted and the transducer attached to the end of the steel beam 59 to investigate waves propagating from the steel to the GRbP via the visco-elastic joint . Figure 8 shows the results for the case where noise propages from the GRP to steel. The ordinate is the signal strength in dB relative to the signal strength detected at a point alongside the GRP 1500 mm away from the steel interface. Curve 33 shows the results for a butt joint, 84 is for the visco-elastic wedge joint1 85 is for the wedge joint decoupled by means of a layer 87 of closed cell elastic foam material (Figure 5) and 86 is for the wedge joint with the GRP additionally provided with a polyurethane coating 88 (Figure 5) to dampen thickness mode vibrations. As expected the butt joint (8,) is worst and one effect of inserting the viscoelastic wedge is a much smoother transition across the joint in the curve 84. Although it is thought that the wedge joint radiates less energy than the butt joint, its full potential will only be realised when it is covered with a decoupling material. This material 87 should preferably also have anechoic properties to Prevent reflections within the array space. Results for this treatment (35) sho there is little joint radiation although there is likely to be some diffraction from the uncoated side of the joint in the Figure 5 arrangement. When the GRP is coated with a compressional damping material (86) the signal strength is attenuated considerably before reaching the joint at which stage the decoupled wedge acoustically isolates the steel beam area. This is due to the damping effect of the coating on the compressional waves in the GAP and is most effect- ive when the source is a long way from the joint. Figure 9 shows the results for signals propagating fro the steel to the GRP and the curves 95 - 96 respectively cover the sane conditions as curves 83 36. In all four cases z high level oP noise was radiated from the area of steel 89 immediately behind the joint. Inclusion of the wedge (94) considerably reduced the signal strength as the hydrophone is scanned further away from the joint on the GRi? side. Considerable improvements are made by decoupling the wedge (95) and by damping the GRP (96).

The invention acts to terminate compressional waves propagating along the G2P dome thereby preventing reflection of the vibrational energy back into the GRP 14 and so reducing radiation. The wedge joint presents a considerable transmission loss to compressional waves travelling from the GRP to the steel and reciprocally ensures that any steel-borne energy incident on the GRP is also considerably reduced. The effectiveness of the visco-elastic wedge joint is improved by covering it with a layer of decoupling material to suppress any noise radiated by the joint itself. In practice this may mean extending the decoupling layer to any supporting structure at the joint which may also form a likely source of noise radiation.

A further improvement is also obtained by coating the outer surface of the GRP dome with a layer of a suitable damping material.

Although the invention has been described with reference to compress- ional acoustic waves it is envisaged that the same principle may be applied to flexural waves by providing a suitable acoustic sink. A modified tapered joint for a laminar flow dome as shown in Figure 10. The GRP dome 101 is covered with a layer 102 of damping material and is tapered to fit a rasped portion on the outer surface of the visco-elastic materIal 103. The visco-clastic material is bonded to an L-sectioned insert ring 104 along its inner surface 105 and along its lower surface 106. Radially extending bolts 107 secure the GRP done to the insert ring and axially aligned bolts 1C8 secure the ring to a support frame member 1C9 for the dome. Further modifications of the invention will be apparent to those skilled in the art

Claims

Amendments to the Claims have been filed as follows Claims 1. A sonar dome assembly for connection to a body including a dome and an acoustic energy absorbing joint for interposition between the dome and the body, the joint comprising a layer of visco-elastic material.
2. A sonar dome assembly as claimed in claim 1 wherein the joint is so formed that there is acoustic impedance matching between the dome and the visco-elastic material.
3. A sonar dome assembly as claimed in claim 2 wherein the visco-elastic material is wedge-like, being tapered towards the dome.
4. A sonar dome assembly as claimed in claim 3 wherein the wedge joint is covered internally of the dome by a layer of decoupling material to prevent radiation of energy from the joint into the dome.
5. A sonar dome assembly as claimed in claim 4 wherein the covering is a closed cell foam.
6. A sonar dome assembly as claimed in any one preceding claim wherein the external surface of the sonar dome is covered with a coating of a wave damping material.
7. A sonar dome assembly as claimed in claim 6 wherein the wave damping material is a polyurethane.
8. A sonar dome assembly as claimed in any one of claims 2 to 7 wherein the visco-elastic material is attached to a metal insert ring by an adhesive, the ring being attachable to the body as by bolts.
9. A sonar dome assembly as claimed in claim 8 wherein the visco-elastic material comprises a wedge portion and a base portion, the base portion having cut therein a slot into which there fits a complementary tongue extending axially from the insert ring.
10. A sonar dome assembly as claimed in claim 9 wherein there is provided an axially extending tongue at the innermost portion of the insert ring to provide an L section seat on the insert ring to which the visco-elastic material is attached.
11. A sonar dome assembly as claimed in claim 10 wherein the sonar dome may be bolted to the outer end of the tongue.
12. A sonar dome assembly as claimed in any one of claims 4 11, wherein the decoupling layer is extended to cover part of a body frame provided for attachment of the dome.
13. A sonar dome assembly as claimed in any one of claims 2 to 12, wherein the visco-elastic material is a high density nitrile rubber with a density of the order of 1700 kgm 3.
14. A sonar dome assembly as claimed in any one preceding claim wherein the dome is made of glass reinforced plastics material.

IS. Asonar dome assembly as claimed in any one of claims 8 to 14, wherein the adhesive is an epoxy resin.