Flutter Noise in Suspended Hydrophones

Abstract

Telephone wires have long been known to “sing” in a wind, creating aeolian tones attributed to periodic vortex shedding reinforced by the vibrational resonance of the wire. When a hydrophone is suspended from a cable in a water current, a similar quasi-sinusoidal noise, caused by cable flutter transmitted to the hydrophone, may be expected at the hydrophone terminals. Observations of what was apparently noise of this type were made in Chesapeake Bay at current speeds up to 1.3 knots. Strong sinusoidal noise voltages, at frequencies between 10 and 30 cps, depending upon current speed, were measured at the hydrophone terminals. The amplitude of this noise varied irregularly with cable length and cable tension but often was equivalent to a sound pressure level of +35 db above 1 dyne/cm2. Several remedies were tried and found effective: using a streamlined cable fairing, reducing cable tension by suspending the hydrophone by rubber bands just above its attachment to the cable, and using a hydrophone less sensitive to acceleration. Noise of this type should be expected in all low-frequency work with suspended hydrophones but apparently can be reduced greatly by one or more simple control methods.