Acoustics – Geophysical or subsurface exploration – Seismic wave generation
Patent
1987-02-27
1989-03-14
Steinberger, Brian S.
Acoustics
Geophysical or subsurface exploration
Seismic wave generation
367146, 181402, H04R 102
Patent
active
048118150
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a process and a device for hydrodynamic sound pulse generation in a liquid, in particular for sound pulse generation at sea for marine geophysics measurements.
2. Description of Related Art
Although the invention is used preferably for sound pulse generation at sea for marine geophysics measurements, it is also suitable for other applications requiring hydrodynamic sound pulses, namely water depth measurement, using the echo sounding procedure, or even therapeutic and medical underwater treatments.
At the present time, sound pulses in liquids are generated through pulsed input of sound energy. This is achieved mostly by means of spark discharges or detonation of special explosives within the liquid. Generally speaking, however, piezoelectric and magnetostrictive sound generation cannot deliver strong sound pulses due to their limited amplitudes. Additional possibilities exist for aerodynamic sound pulse generation. A quantity of compressed air is introduced into the surrounding liquid. The increase in pressure in the liquid is due to the expansion of the compressed air and the resultant displacement of the liquid generates a sound wave. Sound pulse generation can be also generated through the collapse of cavitation bubbles.
Using known processes, however, a strong waved pressure signal which causes interference from multiple pulses is created. Moreover, the frequency range of the sound signal is, for the most part, limited to a few hertz and, therefore, is only marginally suited to examination of frequency-dependent dimensions such as, for example, the attenuation of sound waves at the bottom of the sea. Furthermore, the energy requirement for generating a desired sound intensity is high.
This invention meets the requirement of creating a process and a device by means of which strong sound pulses, sharply delimited in time, can be generated in the liquid, with higher bandwidths from the sound frequency spectrum and relatively low energy consumption.
In the process according to the invention: In a pipe open on one end, which surrounds a liquid column of a specified length that makes contact with the liquid at the open end of the pipe, the end of the liquid [column]opposite the open end of the pipe is subjected to sudden bursts of acceleration or deceleration in such a way that a sound pressure pulse is generated at this end of the liquid column; this pulse is transmitted through the liquid column to the open end of the pipe and from there it is radiated, on the one hand, into the liquid as a sound pulse, and is reflected, on the other hand, into the liquid column as a rarefaction wave.
According to the invention, the sound pulses are generated through the use of the well-known water hammer, which results from a very rapid change of flow inside a pipe within a time interval, which, according to Joukowski, must be shorter than twice the length of the pipe divided by the velocity of sound in the liquid. Under this condition, the liquid in the vicinity of the rapid change of flow becomes compressed, creating a sound wave, which causes, for example in pipes and other conduits of a pipe system, a knocking sound which can be very bothersome. In contrast, according to the invention, the water hammer is used for hydrodynamic sound pulse generation in a liquid, with the sound pressure pulse generated using the water-hammer principle radiated through the open end of a pipe into the liquid.
Simultaneously, a reflection occurs at this pipe end, and since it is an acoustically open pipe end, the pressure wave caused by the water hammer is reflected with a 180 degree phase angle shift, i.e., as a rarefaction wave, to the liquid column. Since this causes the pressure to fall sharply after arrival of the reflected rarefaction wave at the open end of the pipe, the rear flank of the emitted sound pulse drops steeply and this sharply delimits the radiated sound pulse in time. The duration of the transmitted sound pulse is thus determined by the lengt
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Laake Andreas
Meier Gerd
501 Max-Planck-Gesellschaft Zur Forderung der Wissenchaften E.V.
Steinberger Brian S.
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