Herman Medwin

Speed of sound in water: A simple equation for realistic parameters

A simple equation is presented for the dependence of sound speed on temperature, salinity, and depth of water. The comparison with Del Grosso’s NRL II shows discrepancies of the order of tenths of m/sec for realistic values of the parameters.Subject Classification: 30.25.

Bubble sources of the Knudsen sea noise spectra

Single coherent bubble contributions to the incoherent underwater noise of spilling breakers have been studied in an anechoic laboratory facility. The waves are generated by a plunger, they propagate 17 m along a 1.2×1.2‐m water waveguide, and ‘‘spill’’ and create bubbles at the surface of a 3×3×3‐m anechoic cube of water. Several species of bubbles have been identified. In general, they act as transient dipoles of duration from 2 to several milliseconds, with peak axial source strength of the order of tenths of pascals, at 1 m. The noise is emitted when the bubble is within hundreds of micrometers or a few millimeters of the surface. Bubbles were observed in the 2 decades of frequency from 500 to 50 000 Hz. The average of the individual bubble events yielded a spectrum that slopes at about 5 dB/oct from 1 to 20 kHz, the same as the Knudsen wind noise spectra at sea. The magnitude of the laboratory breaker noise during continual wave‐breaking events was approximately 80 dB re: 1 μ Pa2/Hz at 1 kHz, which i...

Instrumentation for insitu acoustical measurements of bubble spectra under breaking waves

A floating acoustical resonator has been developed to determine numbers and sizes of bubbles in the region of spilling breakers in the open sea. The change of Q of several modes of the one‐dimensional resonator has been used to infer, simultaneously, bubble populations of nine radii between 30 and 270 μm; smaller radii bubbles can also be studied. To demonstrate the accuracy of the technique, theoretical predictions of resonance broadening due to bubbles were compared with measured broadening for a known bubble population in the laboratory. Statistics of bubble densities under and near spilling breakers were then obtained at a depth of 25 cm below the ocean surface. These ocean data compare very well with recently published laser measurements in a water–wind tunnel over radius range 30<a<270 μm, but agree with older photographically obtained values at sea only for radii 50<a<100 μm.

Acoustical measurements of bubble production by spilling breakers

The surface bubble spectral density of newly created bubbles has been measured under spilling breakers in a laboratory facility. This has been accomplished acoustically by passively identifying the radius, position, and time of creation of several hundred individual bubbles as their shock‐excited pulsations radiate damped spherical waves of sound. The measured bubble spectrum ranges from radius 50 μm to 7.4 mm with a peak of 6 bubbles per square meter in a 1‐μs‐radius increment at radius 150 μs. The rate of bubble production has an exponential fall‐off with time and 97% of the bubbles are produced in the first 500 ms after the breaker passes. The results supplement recent ocean data for low sea states.

Impulse studies of double diffraction: A discrete Huygens interpretation

A closed‐form analytical impulse solution of diffraction by a rigid wedge is interpreted from the point of view of discrete Huygens wavelets emanating from the crest of the wedge. The strengths of these secondary sources and the impulse field diffracted over a second wedge are calculated. Digital Fourier transformation is then used to determine the spectral loss during double diffraction. The technique is applied to a thick plate, an angled wide barrier, and a strip; successful predictions of laboratory experiments are achieved. In principle, the method may be used for any number of diffractions by straight line segments of an extended barrier or scattering surface.

Acoustical determinations of bubble‐size spectra

The exact integration of the equations for acoustical cross sections of bubbles is considered to show when the dependence of the bubble number on radius can be determined by scatter and attenuation experiments in a liquid containing a broad spectrum of bubble radii.

Acoustic fluctuations due to microbubbles in the near‐surface ocean

Within several meters of the ocean surface there are enough ambient bubbles to create a distinctive microstructure with a dispersive acoustic index of refraction. In this region, the variation of total bubble‐volume fraction produces fluctuations of the sound speed for frequencies less than about 25 kHz; the frequency spectra of these speed fluctuations are near Gaussian. However, perturbation of resonance frequencies of predominant bubble populations is the principal cause of fluctuations of the speed between 25 and 100 kHz; these fluctuations have frequency spectra that are imitations of the spectrum of the sea surface height which creates them.

On the failure of the Kirchhoff assumption in backscatter

Diffraction backscatter from a semi‐infinite plate and from a rigid plane wedge is studied in the time and frequency domains. The spectrum of diffraction backscattered sound for a point source, as predicted by use of the Kirchhoff assumption in the Helmholtz–Kirchhoff integral formulation, is compared to experimental results and is found to be substantially incorrect. The errors are very large when the least time path to the diffracting edge is near grazing, for example, 30 dB error at θ=4° to the plate. The Fourier transform of the Biot–Tolstoy closed form, normal coordinate, impulse solution gives accurate predictions of the experimental measurements in the frequency domain.

Chapter 8 – Bubbles

Publisher Summary The immense number of microbubbles per unit volume that have been identified in the ocean are a major factor in near-surface sound propagation. They have also proved to be a unique tool in the study of near-surface ocean characteristics. Most of the bubbles found near the surface of the open sea appear to be continually generated by spilling or plunging breakers or during rainfall. Particularly in coastal regions, the sources of ocean bubbles also include those entrained by continental aerosols that drop into the sea, generated by photosynthesis of marine plants, life processes of marine animals, the decomposition of organic material, or released from gas hydrates on or below the ocean floor. The principal practical techniques for bubble identification and counting at sea have proved to be inversions of linear acoustical measurements. Determinations of bubble densities have been based on acoustic backscatter, excess attenuation, and differential sound speed, as well as on nonlinear behavior and acoustic Doppler shift. The acoustical techniques have also included passive listening at sea to the sound under breaking waves, or during rainfall, to determine the number of newly created bubbles.

UNDERWATER SOUND PRODUCED BY RAINFALL : SECONDARY SPLASHES OF AEROSOLS

Earlier studies have identified three sources of underwater sound production from raindrops: The initial impact, a bubble trapped underwater at the base of the impact crater (type I), and a bubble trapped underwater by a turbulent jet created during the splash canopy formation 50–80 ms after impact (type II). Together these sound sources have been used to predict the underwater sound spectrum produced by rainfall; however, the predictions have underestimated observed sound levels. A new important mechanism of bubble entrapment (sound production) is described—bubbles created during the secondary splashes of drop aerosols thrown up during the initial raindrop impact. These delayed bubbles occur 100–600 ms after the initial impact depending on raindrop size. The average radiated spectral energy incorporating all known sound sources for two large drop sizes (3.2‐ and 4.7‐mm‐diam raindrops) are presented. Improved predictions of the underwater sound produced by rainfall are shown. The predictions form the basi...