Hydrophone Pairs Research Articles

Correlation of Ambient Sea Noise

Space-time correlations of ambient sea noise were measured with vertical pairs of hydrophones in the frequency range of 125–1131 cps and with horizontal pairs in the range of 22–63 cps at various sea states (SS) in the deep ocean. The experimental results are compared with those obtained from theoretical models of a uniform distribution of surface-noise sources, radiating with an intensity pattern of cos2nα, where α is the angle from the vertical. At SS5 and frequencies above 400 cps, a good fit to the experimental spatial correlations as well as to the principal peak of the space-time correlation is obtained for n = 1. At the same sea state, the 250-cps data correspond to n = 12. In the low-frequency range, omnidirectional noise sources explain the experimental data up to and including SS5. The model does not give a sufficient explanation for the space-time correlations at SS3 in the range 400–1131 cps.

Correlative Properties of Deep-Water Ambient Noise at Bermuda

Time-delay correlograms of the ambient noise picked up by various hydrophone pairs of the deep vertical array at Bermuda have been obtained. These show, as a function of time delay, the clipped correlation coefficient of the noise for the selected hydrophone pairs. In the two 1-oct bands 200–400 and 1000–2000 cps, and for wind speeds between 4 and 30 kt, different types of correlograms have been found for different combinations of frequency and wind speed. In the lower-frequency band and at low wind speeds, they show a strong peak at zero delay, consistent with the findings of Fox [J. Acoust. Soc. Am. 36, 1537–1540 (1964)] and Axelrod et al. [J. Acoust. Soc. Am. 37, 77–83 (1965)] that the noise arrives horizontally, probably from distant shipping. In the higher band and at high wind speeds, the correlograms are what would be expected for noise generated at the sea surface. The correlograms permit computation of the array gain of any arbitrary vertical array steered at any angle to the horizontal. Considerable degradation of array gain relative to uncorrelated noise is to be expected under certain conditions. A simple example is given.

Hydrophone-Preamplifier Optimization

In any system, a number of hydrophone-preamplifier combinations are capable of meeting the design parameters; however, the choice of an optimum design is not always obvious. This paper presents an approach by which an optimum design may be obtained. Since the output signal-to-noise ratio of the preamplifier is a function of hydrophone sensitivity and capacity, which in turn are functions of hydrophone dimensions, it is apparent that an optimum hydrophone-preamplifier combination might be arrived at by combining preamplifier characteristics with hydrophone characteristics. This paper outlines a computer approach to this problem and shows the results. A typical case is presented that considers a multielement array where both size and weight are important. A set of equations and curves is generated whereby different sized hydrophones are placed under a common denominator and evaluated on a relative basis. Finally, these results are combined with a preamplifier producing a set of signal-to-noise output contours from which an optimum hydrophone-preamplifier may be selected. This concept has been proved on a recently completed project. One interesting result was the combination of a pair of hydrophones in series at the input of each preamplifier. This is contrary to normal practice of two units used in parallel addition.

Ambient-Sea-Noise Correlation

Measurements of the space-time crosscorrelation of ambient sea noise were made with pairs of hydrophones, separated vertically between 2–40 ft, at a depth of 14 500 ft, in the frequency range from 100 to 1200 cps. Previous results at sea state 5 [J. Acoust. Soc. Am. 38, 146–148(L) (1965)] were extended and now include lower sea states. The crosscorrelations for six frequency bands are given as a function of hydrophone separation and are compared with results by other experimenters. The time delay corresponding to the maximum value of the space-time correlation is given as a function of the separation, and the slope of these lines is shown to increase with frequency. [Hudson Laboratories of Columbia University Informal Documentation No. 84. Work supported by the U. S. Office of Naval Research.]

Correlation of Ambient Noise in the Ocean

Correlation measurements of ambient sea noise have been made with pairs of vertical hydrophones near the ocean bottom at a depth of about 14 500 ft. Typical auto and space-time crosscorrelations are shown at 500 cps for sea state 5. The time delay corresponding to the maximum value of the space time correlation was measured in six frequency bands, with center frequencies of 125, 180, 250, 400, 500, and 800 cps, and is given as a function of hydrophone separation. These curves are almost straight lines. The time delays correspond to steering angles. Maximum correlation is obtained at increasing elevation angles as the frequency increases.

Comparison of Theoretical and Experimental Values of Spatial Correlation

Experimental measurements of spatial correlation were made using the top 12 elements of a 40-element vertical array. The hydrophone outputs were passed through filters in the 200- to 400-, 400- to 600-, 600- to 800-, and 800- to 1000-cps bands. Selected pairs of hydrophones were crosscorrelated. The experimental values of spatial correlation were compared with the theoretical values of spatial correlation. A good agreement of the experimental results with the theoretical results is obtained in the higher passbands. In general, the agreement is better for higher-frequency bands and higher sea states. The results indicate that (1) the noise is predominantly from the horizontal direction in the 200- to 400-cps band, (2) the noise is predominantly from the vertical direction in the other measured bands, and (3) the sources on the surface of the ocean radiate energy similar to that of a dipole source.

Digital-Computer Phase Techniques for Determining Noise-Source Location

The problem of locating submarine self-noise sources is approached from the phase technique rather than the usual method of observing high amplitudes in certain frequency ranges. Individual SONAR hydrophones were simultaneously recorded at sea during various no-target speed events and subsequently digitized in the laboratory. A digital computer was programmed to calculate the phase angle between selected pairs of hydrophones as a function of frequency, using the real and imaginary parts of the crosspower spectral density function. Low ship speeds exhibited broad frequency ranges having linear changes of phase as a function of frequency, indicating the dominance of a coherent source. High ship speeds showed more dispersion in the phase plots, but did display general groupings of points. If a velocity of propagation is assumed, then the slope of this line can be used to determine the bearing of the coherent noise source with respect to the hydrophones.

Vertical Coherence of Explosive Reverberation

The crosscorrelation of the outputs of hydrophones along a vertical array 15 ft long, suspended at depths of 60 and 300 ft from a surface ship, has provided information on the coherence and directional properties of reverberation in the sea. Correlograms of the clipped outputs of various hydrophone pairs in the band 1–2 kc/sec have yielded values for the correlation coefficient and the delay time, equivalent to arrival angle, for the return following the detonation of an explosive charge at a depth of 800 ft. The correlation coefficient is found to decrease with separation of hydrophones, as it should, and to decrease with time as volume reverberation replaces surface reverberation, until the arrival of the bottom reflection. Some correlograms show the presence of a deep scattering layer. The results may have application to the design of reverberation-suppressing arrays.

Measurement of the Effect of Water Correlation

An acoustic experiment was performed to study the effect of path separation on the difference of arrival time of short pulses between a pair of hydrophones. An array of 7 hydrophones was bottom-mounted in 1800 ft of water, and the projector was mounted on a ship held in a 3-point moor. Data were collected over a period of several days, and they exhibited the scatter common to acoustic measurements made in the ocean. Time averages of 11 000 data points, however, show clearly the effect of varying hydrophone separation from 5 to 300 ft. An estimate of the turbulent-cell size is made.

Experimental Setup for Measuring the Amplitude and Phases of 60-kc Acoustic Pulses

Equipment has been developed and installed to measure the phase and amplitude of 2 independent acoustic pulses during a 1-msec overlapping sample. The equipment was designed for a frequency range of 5–100 kc, and has been used extensively at 600 kc. The dynamic range of the apparatus is greater than 40 dB, the dynamic range of the preamplifiers and amplifiers is 80 dB. The phase-angle accuracy is one electrical degree, and the amplitude error less than 1%. This equipment is part of a fixed installation in Dabob Bay. The transmission path used is 3800 ft between a fixed source and 4 hydrophones mounted on a self-leveling array. Hydrophone pairs are separated perpendicular to the transmission path, horizontally at 7, 13, and 20 ft, and vertically at 4 ft. The source and receiving array are at a depth of 200 ft, with a water depth of 400 ft. This arrangement enables phase and amplitude studies to be made either in direct transmission or in the reflection of surface and bottom signals. Results obtained with this installation are discussed elsewhere in the following paper (H5). [This work was supported by the Bureau of Naval Weapons.]

SOUND RANGING IN A MEDIUM HAVING AN UNKNOWN CONSTANT PHASE VELOCITY

A method for the sound ranging of explosions is described, in which the direction‐angle of the explosion is given in terms of the ratio of the differences in the arrival times at two pairs of microphones, geophones, or hydrophones. It is unnecessary to know either the phase‐velocity in the intervening medium or the absolute values of the time differences. However, the paths and the phase‐velocities must be such that the time differences are the same as they would be for a wave‐front traveling from explosion to the microphones at some constant velocity. A rapid graphical method for determining the location of the explosion is described. The precision of the determinations is discussed. Applications of the method to the scoring of bomb hits on bombing practice areas are suggested. The effect of the motion of the medium is considered.