| 摘要 |
<p>A low frequency sound source has a radiating piston 3 of the order of a few metres across backed by a gas spring 13, 15 containing a fixed mass of gas. The gas pressure in the spring is kept at levels for which the natural frequency of the piston 3 loaded by the fluid 41 lies in the seismic band and may be as low as 0.5Hz. The piston 3 is given an initial displacement and begins to oscillate. Its oscillations are sustained by an actuator 27, 29 whose drive signal is derived froth the velocity of the piston 5 via a velocity or displacement sensor. The sound source is caused to perform a frequency sweep by gradually compressing the gas in the gas spring 13, 15 so that the spring becomes stiffer both because of the rising pressure and because of the reducing volume of the gas spring spaces 13, 15. This double effect allows large changes in stiffness to be produced and hence allows the source to operate over at least three octaves of frequency.</p> |
