Measurements of acoustic phase stability in Loch Linnhe, Scotland

The temporal phase stability of the acoustic environment is vital to the formation of a synthetic aperture. Acoustic phase stability measurements were made of the water in Loch Linnhe, Scotland over a period of several days. The phase stability was estimated from a sequence of phase measurements made over periods of 1 and 15 min. Over periods of a minute, it was found that the standard deviation of the fluctuations around the mean phase was 10°. Moreover, for frequencies in the band 15–30 kHz, the standard deviation was independent of frequency. This implies that the standard deviation measured is limited by the measurement accuracy of the experiment rather than a measure of the actual acoustic stability of the medium. Consequently, it is concluded that the acoustic phase stability is better than 10° for all frequencies and that the path-length variation due to medium fluctuations is smaller than 6 mm in a total acoustic path of 130 m.