It is shown that the efficiency of the parametric transmitting array may be improved over that obtained where two equal amplitude-frequency components are employed in the primary wave. Through suitable periodic shaping of the envelope of the primary wave, available improvements in the output at the ’’difference’’ frequency of between 2.1 and 6.0 dB are possible in principle. The degree of available improvement depends upon whether the transmitter is peak power or average power limited and whether finite-amplitude attenuation of the primary wave is significant. It is proposed that the on–off modulation of a single-frequency carrier provides a simple, practical method through which near optimum efficiencies may be obtained under most circumstances. A bandwidth greater than the ’’difference’’ frequency is required at the transducer in order to achieve this improvement. Subject Classification: 25.35; 30.75.
Bottom reverberation was measured at five locations in the North Atlantic using a hydrophone and explosive charges suspended near the ocean floor. Measured backscattering strengths in the frequency range from 800 Hz to 12.8 kHz show a tendency to rise moderately with frequency. The dependence on grazing angle was found to vary between a sine to the one-half and a sine-squared relation. Measurements made at several depths indicate that a portion of the reverberation originates in the subbottom layer structure directly beneath the point of measurement. At frequencies above 400 Hz, this reverberation from the subbottom appears to contribute significantly only at the steeper grazing angles.
A method is described wherein the endfire array treatments of Westervelt [P. J. Westervelt, J. Acoust. Soc. Amer. 35, 535–537 (1963)] and Berktay [J. Sound Vibration 2, 439–461 (1965)] may be modified to include the effects of a high-intensity primary wave. An approximate empirical expression for the total acoustic intensity of a finite-amplitude plane wave is employed to determine the source function along the array. It is shown that in the limit of a very intense primary wave, the scattered sound waveform in the forward direction does not depend upon the parameter of nonlinearity The beam pattern (calculated for the case of periodic on/off modulation of the primary wave, only) is shown to narrow as the parameter of nonlinearity is reduced. Various effects related to harmonics of the “difference” frequency are discussed briefly. [This work was carried out at the University of Birmingham under the sponsorship of the Admiralty Underwater Weapons Establishment, Ministry of Defence (Navy).]
Ambient noise in the frequency range 1–20 Hz was measured for 1-day periods at each of four widely separated sites in the North Atlantic Ocean with freely drifting, surface-suspended hydrophones. The acoustic data were recorded aboard an attending research vessel following transmission over a radio link. Narrow-band spectral analysis was performed at 1-h intervals to form time series of noise spectral level for each site. Statistical analyses of the power-spectral time series included the determination of cumulative probabilities, standard deviation, skew, kurtosis, decorrelation times, and the wind-speed dependence of mean spectral level. Comparisons with data from bottom-mounted hydrophones and studies of inter- and intra-array coherence indicate that the data were, with few exceptions, uncontaminated by self-noise down to 1 Hz. A significant dependence of noise spectrum level upon local wind speed was observed in the 1.5- to 3-Hz frequency band at each site. At 4 Hz, the dependence upon wind speed was much weaker, suggesting that the noise sources at this frequency are more distant. Estimates of noise directionality and tests for normality were also made on the data between 1 and 5 Hz.
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