Ambient Sea Noise Research Articles

Verification of Attenuation and Ambient Noise in the Ocean

The attenuation of acoustic energy in the ocean at a frequency of 40 kc was measured by means of a bottom-mounted array of 7 hydrophones in 1800 ft of water. The source of short pulses was a ship-mounted projector. Measurements were made by determining the peak amplitude of the received pulses as a function of range. Spherical transducers were used to minimize the effect of patterns on the measurements. These measurements agreed with the unpublished values obtained by APL, Washington. In addition, the ambient sea noise as a function of sea state was measured. The values obtained, when corrected for the attenuation over the depth of 1800 ft, agree with published values within 12 dB from sea state 3 down to below sea state 1, where system noise became predominant.

Ambient Sea-Noise Directivity

This paper reports on the Fixed Acoustic Buoy (FAB), an acoustic noise-measuring system, which was installed on the ocean floor on 10 December 1960 in 2300 fathoms of water. The system utilized a vertical array of 21 hydrophones which could be electrically steered to any one of 13 vertical angles ranging from 0° (horizontal) to 90° (vertical upwards). The array was omnidirectional in azimuth. The objective of the FAB program was to investigate ambient sea noise over the spectrum from about 800 to 5000 cps at deep depths to determine noise-spectra levels as a function of sea state and angle of arrival in the vertical plane. The system was operated for a period of 11 months, during which data were recorded on magnetic tape for processing with narrow-band filters, A-to-D converter, IBM 7090 computer, and automatic readout equipment. Spectrum levels, directivity characteristics, and statistical characteristics were determined. A mathematical model for the surface-generated sea noise was constructed from the resulting data, and is reported on in the paper “Deep Ocean-Noise Model Determined by a Deep-Line Array” by Jack G. Watson (L5).

Directional Distribution of Ambient Noise in the Ocean

The directional distribution of ambient noise in the ocean has not been measured previously. The conventional, trainable, searchlight transducer is unsuitable for measurements; observations in several directions in succession are inadequate owing to the required assumption of a stationary process. In addition, the single transducer fails to permit investigation of the correlation of envelope fluctuations in several directions. Accordingly, the system employed is based upon a 32-element receiving array employing 32 beams, simultaneously distributed uniformly over all space [V. C. Anderson, J. Acoust. Soc. Am. 30, 470 (1958)]. The signal reception and processing system is described briefly. Because of the finite beam widths (low resolving power) and beam side lobe structure, an analysis technique converting observed relative beam responses to the true field distribution has been developed. Data obtained at depths to 1000 ft are discussed in light of departures of ambient noise from an isotropic distribution. Preliminary deductions concerning the sources of sea ambient noise found in the distribution are presented. (This work represents one of the results of research carried out under contract with the Office of Naval Research.)