GB2339617A

GB2339617A - Underwater acoustic sensor

Info

Publication number: GB2339617A
Application number: GB8826004A
Authority: GB
Inventors: Michael Arthur Ralph; Peter Watkinson
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1988-11-07
Filing date: 1988-11-07
Publication date: 2000-02-02
Anticipated expiration: 2008-11-07
Also published as: GB8826004D0; GB8924437D0; GB2339617B

Description

2339617 UNDERWATER ACOUSTIC SENSOR This invention relates to an underwater

acoustic sensor for deriving unambiguous information as to the bearing of an acoustic source.

An existing sonobuoy employs a sensor to provide unambiguous information of a bearing source, the sensor utilising a pressure gradient hydrophone to infer acoustic particle velocity together with a pressure sensing hydrophone. This is achieved in one available device by the reaction of pairs of "wings" against a central seismic mass with the resulting motion being sensed by a piezoelectric ceramic strip. This arrangement has the disadvantage that below the resonant frequency of the "wings" the acoustic particle velocity is not measured directly.

To avoid these problems, the present invention provides an underwater acoustic sensor employing a velocity hydrophone wherein an ultrasonic wave field is established and the particle velocity within the field is measured in more than one direction, and the field is exposed to acoustic radiation from an underwater source which modulates the ultrasonic wavefield, whereby the modulated particle volocities measured provide an indication of the direction of the source relative to the sensor.

Thus the invention is based on the fact that an ultrasonic wave will travel faster in a fluid flow in the same direction as the sound propagation and slower in a fluid flow of the opposite direction. In an acoustic field the particle velocity acts as an oscillating fluid flow.

2 If c is the normal propagation velocity for sound in still water, its velocity in water flowing with velocity v is c+v in the same direction and c-v in the opposite direction.

If a pair of ultrasonic transducers radiating with constant amplitude and frequency are placed close to each other the particle velocity of the sound field will act as the oscillating fluid flow whose velocity may be determined from; (c+v) - (c-v) = 2v For frequencies where the wavelength is significant compared with the transducer separation measurements taken in two opposing directions cancel the dependence of the acoustic velocity on the pressure field.

A direct measurement of acoustic particle velocity is therefore possible from DC to an upper frequency dependent on transducer separation.

Opposing pairs of ultrasonic transducers (transmitter and receiver) are positioned some 100mm apart on a suitable frame which may consist wholly or in part of material having an acoustic impedance close to that of seawater.

A second similar set of pairs of transducers are positioned in the same or a different plane at right angles to the first set to provide ornnidirectional beamforming capability. Figure I illustrates the arrangement.

The measurement assembly may or may not form an integral part of flownoise reducing techniques such as; Flow-cage assemblies Acoustic encapsulation (potting) Sonar-type domes and may or may not be operated in conjunction with a acoustic pressure sensor to provide unambiguous bearing information.

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Claims

1. An underwater acoustic sensor employing a velocity hydrophone wherein an ultrasonic wavefield is established and the particle velocity within the field is measured in more than one direction, and the field is exposed to acoustic radiation from an underwater source which modulates the ultrasonic wavefield, whereby the modulated particle volocities measured provide an indication of the direction of the source relative to the sensor.

2. A sensor as claimed in claim 1, including frame means mounting a first set opposing pairs of ultrasonic transceiver and a second set of opposing pairs of ultrasonic transceivers disposed at right angles to the first set.

3. An underwater acoustic sensor substantially as described with reference to the accompanying drawing.

I

1-01 Amendments to the claims have been filed as follows CLAIMS 1. An underwater acoustic sensor comprising an acoustic particle velocity hydrophone including means for establishing an ultrasonic wavefield and means for measuring the particle velocity within the field in more than one direction, wherein in use the field is exposed to acoustic radiation from an underwater source which modulates the ultrasonic wavefield such that the modulated particle velocities measured provide an indication of the direction of the source relative to the sensor.

2. A sensor as claimed in claim 1, including frame means mounting a first set of opposing pairs of ultrasonic transducers and a second set of opposing pairs of ultrasonic transducers disposed at right angles to the first set.