The scattering of sound from a spherical fluid obstacle of size comparable to a wave-length is considered, neglecting dissipation. Calculations of the acoustic pressure and the total energy in the scattered wave are presented graphically; sound velocities and densities of the sphere lie between 0.5 and 2.0 times that of the external medium. The limiting cases of Rayleigh scattering and scattering from a fixed rigid sphere are also shown for comparison. In the region where the diameter of the sphere is comparable to a wave-length, the scattering is a complicated function of frequency, showing in some cases large maxima and minima. The amplitude of the scattered wave in the backward direction from a fluid sphere a few wave-lengths in diameter exceeds twice that from a rigid sphere of the same size for the case of the sound velocity 0.8 and density equal to that of the surrounding medium.

A remote instrument has been used to record the sound and environment of small surface spills in light winds from a depth of approximately 1 m in the open ocean. Recordings from the instrument indicate that these small breaks have no correlation with the amplitude or phase of long-period swells moving faster than prevailing winds. The sound from the spills, which is composed of a number of distinct resonant bubble oscillations, is very similar to that described by Medwin and Beaky [J. Acoust. Soc. Am. 86, 1124-1130 (1989) ] for windless artificial wave breaks. Peak oscillation source pressures range up to 1.2 Pa. The average of several acoustic spectra from a single energetic spill has shown a slope of --5 dB per octave over the frequency range of the instrument, roughly 500-8000 Hz. The unique frequency for each oscillating bubble within a spill indicates that bubbles are "rung" as they are formed during entrainment, die out exponentially within milliseconds, and then no longer contribute actively to the acoustic record. Analysis of the acoustic energy generated by a number of bubbles versus frequency suggests that the --5 dB per octave wind-dependent ambient noise slopes of the Knudsen curves [J. Mar. Res. 7, 410-429 (1948) ] are caused by the shorter lifetimes of highfrequency bubbles, rather than significantly lower peak pressures.

A large aperture, high-frequency array with sound source has been used to detect returns from sound velocity microstructure. Returns were infinitely clipped and numerous beams were formed by steering for curved wave fronts. The processing was a unique application of digital array phasing that allowed discrimination between returns from reflectors and discrete point scatterers. Microstructure reflections were found to be highly directional with beamformer outputs dropping more than 15 dB as the angle of incidence varied to 2 ø off from normal. Detection of microstructure returns with conventional echosounders will be masked by biological reverberation for almost all typical oceanic thermoclines.

A common receiver structure for the detection of sinusoidal signals obscured by a noise background is the filter, square, and integrate processor. For this processor, an interpretation of postdetection integration gain is given in terms of a low-pass filtering of the noise power time series. Although a processing gain of 1.5 dB per integration interval doubling is predicted, subsequent analysis on the highly colored envelope spectrum of ambient ocean acoustic noise data in the kilohertz region shows that its nonstationarity significantly alters realizable processing gains for integration intervals of tens to hundreds of seconds.