Underwater Sound Measurements Research Articles

The hydroacoustics of a drop impact

Rainfall has consistently been one of the most difficult geophysical quantities to measure, especially in oceanic regions. Recently, it has been demonstrated that rainfall can be detected at sea by monitoring for the unique underwater sound generated by the rain impacting onto the ocean surface [Nystuen and Farmer, Atmos‐Ocean (1988), in press]. The goal of quantifying rainfall rate through underwater ambient sound measurements requires a more complete understanding of the hydroacoustics of the drop splash and the influence of wind upon these physics. A numerical model is used to explain the hydroacoustics of a drop impact as an acoustic waterhammer modified by the emergence of a fluid jet at the base of the drop. The drop is an acoustic source until the radius of the contact circle between the drop and the underlying water surface reaches the maximum cross‐sectional radius of the drop. The duration of the pulse depends on drop size, shape, and speed. Using realistic raindrops, the observed unique underwater acoustical signature of rain, an increase of ambient sound energy at 15 kHz, is explained.

Low frequency underwater sound propagation measurements west of the NOSC oceanographic tower

Acoustic signals from 50 SUS charges detonated at ranges from 1 to 50 mi west of the NOSC tower were recorded in December, 1978 on both hydrophones and three-component geophones buried about 1 ft in the sediment and cable connected to the tower. An additional hydrophone was placed on the bottom. Both ground and water arrivals were observed for shots within the 400 fa curve (at ranges up to 10 miles) with only water arrivals noted for shots in deeper water of the San Diego trough. Propagation paths in the water were SR/BR (Surface Reflected/Bottom Reflected) throughout, with pronounced up-slope propagation near the receivers. Spectrum analysis of the various arrivals shows that the S/N ratios of the ground and water arrivals are similar below 20 Hz for shots within the 400 fa curve. The ground arrivals, however, have a longer duration (several seconds) than the water arrival (<1 s) because recordings were obtained of both compressional and shear waves refracted in the sediment and rock layers below the sea floor. [Work supported by ONR.]

Acoustic absorption by MgCO03 ion-pair relaxation

UNDERWATER sound propagation measurements in natural bodies of water frequently reveal attenuation anomalies that cannot be ascribed to known processes. When chemical relaxations are suspected, the acoustic resonator–decay method has proved an effective means of investigation in laboratory conditions. By stepwise synthesis of an artificial medium of similar composition and measuring changes in decay rates, the responsible chemical constituents can be identified. Then by combined stoichiometry and acoustic measurements with different concentrations, the relaxation kinetics of the reacting species can be determined. We have shown previously by resonator measurements that the acoustic absorption in Lake Tanganyika is the result of a chemical relaxation involving magnesium and carbonic acid1. The same relaxation is also believed to be responsible for one of the absorption components in seawater2. We show here that the relaxation kinetics follow a two-step association of the magnesium carbonate ion-pair.

The effect of fluid loading on acoustic pressure measurements near structures

Some of the difficulties encountered in underwater sound pressure measurements near structures are discussed. A computation is made of the sound field generated by a monopole acoustic source in the vicinity of an infinite thin elastic plate. To demonstrate the effect of fluid loading on pressure measurements, results are given for the upper half-space above the plate filled, alternately, with air and water, while a vacuum fills the lower half-space. Computed results for this model indicate that no energy bearing acoustic field can be propagated near an elastic plate when fluid loading is heavy. In such circumstances, sound pressure measurements near structures in air are not analogous to measurements that would obtain in water.

Sources of geoacoustic noise in the ocean

Very low-frequency (<50 Hz) underwater sound measurements made from fixed hydrophone arrays have revealed several major sources of geoacoustic noise in the deep ocean. For example, sounds from undersea volcanic eruptions are detected at transoceanic distances in the deep ocean sound (SOFAR) channel [Northrop, J. Acoust. Soc. Am. 56, 837–841 (1974)]. Underwater explosions cause about 20 000 hydroacoustic signals in the Pacific per year [Spiess, Northrop, and Werner, J. Acoust. Soc. Am. 43, 640–641 (1968)], and acoustic waves radiated from ocean margin earthquakes account for another 10 000 per year. Sounds from microearthquakes on the midoceanic ridges, however, generally are not well coupled with the SOFAR channel, but account for considerable local seismic noise in the vicinity of the ridges. Recent measurements in the eastern Pacific using sonobuoys, modified to record these very low-frequency sounds, have shown an average of one microearthquake per hour on the Rivera fracture zone at 19°15′N, 108°40′W, and 41 per hour on the Galapagos spreading center at 1 °N, 86°W. These sources of ambient noise in the ocean should be incorporated in environmental acoustic models. [Work Supported by Office of Naval Research.]

Long-range propagation in the shallow Bering Sea

Long-range underwater sound propagation measurements were conducted in the shallow Bering Sea during the summer of 1960. The region, which had been acoustically surveyed, has vast 20- to 25-fathom areas of flat bottoms, and is therefore ideal for shallow water propagation research. The program was implemented to investigate some complex aspects of shallow propagation by acquiring extensive sets of continuous wave data at several frequencies, as well as with explosives. Supporting environmental measurements were more controlled and complete than for most such efforts, with many sound speed profiles determined from bathythermographs. Sound was received by an anchored string of 12 hydrophones spaced vertically at intervals of 10 ft. The transmitting ship, anchored at ranges of 5, 10, 20, 40, and 80 kyd sent 15-minute CW signals of 58, 350, 700, and 1300 Hz. Single-frequency signals exhibited frequency spreading and amplitude fluctuations of 50 dB. Range and frequency dependence of relative pressure level spectra are described by empirical equations. Fifty-eight sound pressure level profiles, each based on an average of 88 000 data points, yielded accurate propagation loss data and demonstrated depth dependence with very little evidence of mode-stripping.

International standardisation in underwater sound measurements: Tratt, W.J., Acustica, Vol 20, No 3, pp 169–181

International standardisation in underwater sound measurements: Tratt, W.J., Acustica, Vol 20, No 3, pp 169–181

INTERNATIONAL ROUND-ROBIN CALIBRATION IN UNDERWATER SOUND

A round-robin program of hydrophone calibrations was organized by a committee of the International Electrotechnical Commission to determine the sperad in values obtained by participating international agencies in calibrating the same group of hydrophones. The results were expected to permit specifying a practical figure for accuracy of measurement and to provide information that could be used to design an international standard hydrophone for underwater sound measurements. The average deviation of all data is ±0.6 dB.

An Accurate and Simple Means of Extending the Range of Underwater Sound Measurements to Include Infrasonic Frequencies

The extension of hydrophone sensitivity measurements to frequencies below 40 cycles per second is usually done by reverting from free field measurements to a stiffness controlled system. This requires an elaborate set-up not obtainable at most laboratories. The method described presents by comparison a relatively inexpensive and simple means for hydrophone sensitivity measurements in the infrasonic range. Limitations are discussed, and data are given showing the comparison of the method to a stiffness controlled system for instruments of various sensitivity levels.