Fixed‐System Studies of Underwater Acoustic Propagation

Sound waves of 420 Hz continuously transmitted across the Florida Current from a fixed source to fixed hydrophones are used to investigate relationships between temporal fluctuations of the waves and changes of Florida Current transport and water‐column temperatures. The amplitude and phase of the received waves show variations with periods in the range from seconds to weeks and longer‐term trends. Characteristically, variations of acoustic phase correspond to changes of transport and temperature, but fluctuations of acoustic amplitude are noiselike and not readily identifiable. Long period changes of transport and temperature result in slowly changing phases of the multipath arrivals and varying patterns of interference and amplitude fading. Spectra of the amplitude fluctuations show components over a wide frequency range which mask components corresponding to transport and temperature changes. The above characteristics also appear in calculations with ray‐propagation models in which sound speed versus depth was varied with time in accordance with measured variations. Since the models have plane parallel boundaries, characteristic features of the variations appear to be independent of surface and bottom irregularities. This raises questions of modeling amplitude fluctuations from transport and temperature changes obtained from acoustic phase measurements. The paper summarizes relevant observations and analyses.