The contamination of acoustic pressure measurements by sensor oscillations

The significance of micromotion (sensor) noise contamination of low frequency, low level, ambient ocean acoustic measurements has been pursued experimentally and analytically. Oceanographic hydrophones are subject to small motions resulting from various phenomena; the present study focussed on a pressure‐sensitive hydrophone exposed to vertical oscillations. While under such imposed motion, the responses from a pressure‐sensitive hydrophone and a collocated accelerometer were analyzed relative to a stationary reference hydrophone. The imposed motion was vertical, colored noise (1 to 50 Hz) of various acceleration amplitudes (10 μg to 10 mg), transmitted through an elastic isolation suspension. Formation of Frequency Response Functions between the measured transducer signals, demonstrated that a three component model of the hydrophone signal predicts the response‐to‐motion contamination of the acoustic signal. In the lower frequency range, the vertical motion through the static head gradient generates a signal similar to the response‐to‐acoustic signal, while in the upper frequency range, the hydrophone responds inertially to the motion. For acceleration greater than 30 μg, these components masked the laboratory ambient sound, except in a narrow frequency band where the two motion related components canceled each other. The in‐water acceleration sensitivity of the hydrophone was found to be higher than the measured in‐air value, apparently due to two hydrodynamic effects: water mass loading predicted by a classical added‐mass term and a greatly magnifying effect from an adjacent moving body. Extrapolating the results to a deep ocean environment, the hydrophone signals would be contaminated below 5 Hz. A spectral technique is demonstrated to remove both forms of motion contamination from laboratory data.