Patent GB-1532008-A (GB)
Title: Underwater transient sound generator having pressure compensation
IPC: G01V1/02
Filed: 1975-11-26
Abstract: 1532008 Underwater sound generator HUNTEC (70) Ltd 26 Nov 1975 [20 Dec 1974] 48524/75 Heading H4J A subaqueous sound generator comprises a support 6 carrying at its front an electromagnetic coil 16 (as shown planar), and a piston 26 spaced from and located in front of the coil 16 to define a watertight gas space 56. The piston 26 is moved from a normal position near the coil 16 to a forward position spaced from the coil on energization of the coil (see Fig. 5, not shown) and back to the normal position on de-energization of the coil. A concave surface behind the support 6 and a rear elastic diaphragm 46 define between them a second watertight gas space 54. An aperture 58 in the support member provides communication between the two gas spaces 54, 56. As described the piston 26 is of thin copper plate 28 having honeycomb aluminium 30 as stiffening material. The sound generator is an eddy current device. The support 6 is square and made of concrete reinforced with carbon fibres. It has a circular recess 14 to which the coil 16 is adhered. The coil 16 is energized by a pulse on closing a switch 19. A square gasket 20 has a circular opening 21 (see also Fig. 4) with spacer fingers, about <SP>1</SP>/ 32  inch thick. The piston 26 is circular and is mounted on a square diaphragm 24 sealed in position by a square gasket 32 having a circular opening 34 and by a square clamp plate 36 having a circular opening 38. A rear spacer plate 40 is square in outline but has a circular opening 42 in which is located a fillet 44, circular in plan. The fillet 44 may be hyperbolic, parabolic or smoothly non-uniform but is preferably part-spherical except at its outer rim where it is rounded to reduce tension at the edges of diaphragm 46 and also to reduce chafing. It is made of acoustically lossy material such as small glass or plastics spheres containing air and mixed in a binder, or of soft polyurethane, rubber or concrete. The diaphragm 46 is held by a clamping plate 48. All the parts are secured by corner-nuts and studs 8 and by a circular array of nuts and studs 10. Water is admitted to the rear face of diaphragm 46 through an opening 50 covered by a filter screen 52. In operation, when the device is lowered into water, the ambient water pressure at the piston 26 (transmitted through diaphragm 24) increases. The rear diaphragm 46 deforms (see Fig. 5, not shown) and may eventually lie against the rear surface of fillet 44. When the generator is pulsed, the eddy currents created in the piston 26 produce sudden and powerful displacement forwards of the piston 26, limited by the elasticity of diaphragm 24. Because the piston 26 moves forwardly rapidly and because the passage 58 is very small (e.g. <SP>1</SP>/ 16  inch diameter) the viscosity of the air effectively prevents air (or other gas) from rushing from space 54 to space 56, thereby minimising the amplitude of any negative pulse radiated from the rear of the unit. Diaphragm 24 begins to move piston 26 back before appreciable air can bleed through passage 58. The front gas space 56 together with the passage 58 functions as a non-compressible gas volume and is kept as small as possible whereas the rear space 54 is considered to be a compressible gas volume. The piston 26 and coil 16 need not be planar.