Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1997-04-14
1998-07-21
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367159, 367167, 367172, 310337, H04R 1700
Patent
active
057843414
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to underwater acoustic transmitters used at considerable submersions, capable of reaching 1000 m for example. These acoustic transmitters can be used to carry out underwater charting according to the art of sonars.
It is known to construct underwater acoustic transmitters which make it possible to obtain a transmission pattern which is omnidirectional in a plane, generally in bearing. To do this, use is made of a stack of annular piezoelectric ceramics which vibrate radially. To obtain good acoustic efficiency, the transmission frequency is fixed at substantially the resonant frequency of the annuli. Usual operational values are a diameter of around 20 cm for a transmission frequency of the order of 5 KHz.
For relatively small submersions, corresponding for example to those of a hull sonar, the hydrostatic water pressure has a negligible influence on the operation of such a transmitter.
There is known from the U.S. Pat. No. 3,444,508 an underwater acoustic receiver suspended by a cable from a buoy. This device does not make it possible to reach large submersions, by virtue of the limited length of the cable. It includes a support tube terminating in two plugs whose structure does not appear to be particularly adapted so as to be resistant to the effects of submersion at large depth. Furthermore, being a receiver, this device does not have to support large mechanical loads stemming from the acoustic waves, as is the case in a transmitter.
Also known from patent U.S. Pat. No. 5,099,460 is the use of seals to protect acoustic ceramic annuli from shocks.
When it is desired to undertake explorations at larger depths, by placing for example the transmitter in a fish towed at a considerable submersion, the influence of the hydrostatic pressure on this transmitter becomes greater and greater and ultimately excessively disturbs the operation thereof. This may even in certain cases involve damage to, or even destruction of the transmitter by virtue of the superposition of the hydrostatic stresses and dynamic stresses stemming from the vibration required for transmission of the acoustic wave. Thus, to obtain adequate acoustic transmission power, the piezoelectric ceramic needs to be loaded by a considerable electric field which gives rise to internal stresses which may be very high, to the point of causing fractures of the ceramic, this making it necessary therefore to limit the power radiated.
At large depth, the ceramic annuli of diameter R and thickness e are subjected to a hydrostatic pressure whose radial component generates in the ceramic a stress which is itself amplified by a factor R/e. By way of example this amplification factor is of the order of 10 for a depth of 1000 m and hence a stress of radial origin of the order of 1000 bar is obtained.
Furthermore, the axial force due to the hydrostatic pressure on the ends of the transmitter reaches a value of 300,000 newtons (30 tonnes) for a depth of 1000 m and a 20 cm diameter transmitter. This force applied to the rim of the ceramic annuli gives rise to a further additional stress of the order of 600,000 hectopascals (600 bar). Apart from the risks of fracture, the resultant of these two additional stresses entails serious consequences by modifying the piezoelectric coefficients of the ceramics, resulting in a drifting of performance with regard to sound level and the impedances of the antenna. This drifting exhibits at least partially an irreversible character which may become aggravated in the course of successive submersions. Compensation of all these effects is if not impossible at the very least difficult and expensive to implement.
Furthermore, for well-known strictly acoustic reasons, there is benefit in mutually mechanically decoupling the ceramic annuli stacked one above the other to form the antenna in such a way as to be able to obtain the desired performance with regard to the transmission pattern of the acoustic transmitter. Such mechanical decoupling between the ceramic annuli, by means usually employed, is rendered
REFERENCES:
patent: 3890591 (1975-06-01), Bocquillon et al.
patent: 5068834 (1991-11-01), Fromont
patent: 5099457 (1992-03-01), Giannotta et al.
patent: 5517467 (1996-05-01), Fromont et al.
Adda Josette
Bocquillon Pascal
Fromont Bernard
Sernit Eric
"Thomson-CSF"
Eldred J. Woodrow
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