Ira Dyer

Statistics of Sound Propagation in the Ocean

The statistics of transmission fluctuations in the ocean are examined theoretically for both multipath and scattering processes. It is shown that, when both are present, multipath propagation dominates the fluctuations and leads to quite broad statistical distributions. In such cases, the mean and standard deviation for logarithmic measures such as transmission loss differ considerably from corresponding mean‐square measures. Results for a single tone have been generalized to include multitones, as a model of ambient noise. Statistics of a signal combined with such noise have been derived; the standard deviation of the combination may be several decibels higher than the 5.6 dB of signal alone. The distributions of logarithmic quantities are generally found to be log transformations and combinations of chi‐square distributions, and not log‐normal distributions as commonly supposed.

Response of Plates to a Decaying and Convecting Random Pressure Field

Following the methods of Lyon, an analysis of the vibratory response of a plate to a random pressure field is given. The pressure correlation of the random field is assumed to have a scale small compared to the plate size, to decay exponentially, and to convect with constant speed over the plate. Two eases are considered, one in which the convection speed is much less than the speed of free flexural waves in the plate, the other in which the convection speed is the same order as the flexural wave speed. The mean square plate displacement is shown to be relatively independent of convection for speeds much less than the flexural wave speed, and to increase significantly for speeds in the order of the flexural wave speed. It is shown that damping is usually, but not always, an effective means of vibration reduction. In the case of convection speeds much smaller than the flexural speed, the use of hysteretic damping for reduction of the displacement response is shown to be limited by the decay of the assumed ...

Environmental correlates of pack ice noise

Low‐frequency ambient noise under pack ice of the central Arctic Ocean has long‐term variations (periods greater than 1 h) which correlate highly with composite measures of stress applied to the ice by wind, current, and drift. These composites are the horizontal ice stress and the stress moment, and are derived from meteorological and oceanographic data observed simultaneously with the noise. Atmospheric cooling, a known high correlate of midfrequency noise under the ice, is not important at low frequencies.

Statistics of distant shipping noise

In an earlier paper by Dyer, a rudimentary analytical model was proposed for distant shipping noise in the ocean. This model has been extended to reflect actual ocean conditions better. With this extension, the dependence of statistical parameters on the disposition of ships at sea and on the observational beamwidth and bandwidth may be more readily understood. Predictions of the newer model compare favorably with measurements and are believed to be valid in the frequency range within which the noise of distant shipping dominates wind‐generated sea surface noise or noise of other mechanisms. The dependence on depth and the temporal correlation of the noise envelope are not explicitly considered in the model, but will be discussed.

Moment Impedance of Plates

The moment impedance of an infinitely extended plate is calculated with the use of plate equations that allow for transverse shear deformation and rotatory inertia. It is shown that the effects of transverse shear deformation may be important even for thin plates, a result that is not contained in classical theory. Classical theory is shown to be inadequate when the distance over which the moment is applied is smaller than plate thickness. The moment impedance for geometries of greatest practical interest is found to consist of a resistance connected in series with a stiffness reactance, both roughly inversely proportional to frequency, with the reactance of greater magnitude. Both the resistance and the reactance decrease with decreasing distance over which the moment is applied, because of the increasing importance of transverse shear flexibility. The practical implications of the results are discussed, and some thoughts are given concerning other problems where classical theory for flexural waves in th...

Environmental correlates of Arctic ice edge noise

Temporal variations of low‐frequency, broadband ambient noise measured in early summer under drifting ice floes of the marginal ice zone (MIZ) are cross correlated with local environmental forces and ice field descriptors. Surface gravity wave forcing is the primary correlate of the noise; its interaction with ice floes generates sound, most likely via flexural floe failure, and unloading motion, within a few kilometers of the ice edge. Ice concentration is also well correlated with the noise, most likely parametrically. That is, as ice concentration increases, so does the density of potential sound sources in the ice field. Ice stress, ice moment, and wind stress magnitude, while highly correlated with low‐frequency noise in the fully ice‐covered Arctic, are poor correlates in the MIZ. Lateral melt rate, as a surrogate for thermally induced ice stress, is also poorly correlated with low‐frequency MIZ noise. When surface wave forcing is weak, about 1/2 the time in this experiment, episodes of high noise a...

Measurement of Noise Sources in Ducts

Recently Kerka, and earlier Beranek, Reynolds, and Wilson, discussed the measurement of sound power radiated by a fan in a straight duct, by measurements of the sound pressure inside the duct. A difficulty associated with this technique is that above a certain frequency the sound pressure varies markedly across transverse section of the duct and the relation between mean square pressure and power is not simple. The transverse variation can be understood by application of mode theory to propagation of noise in the duct. In the theory it is assumed that the noise source is purely random and of low internal impedance. Propagation in the various duet modes is shown to be statistically independent, and to give rise to equipartition of energy in the limit of high frequencies. The theory agrees well with measurements reported by Kerka. With the use of the theory, it is possible to select measurement positions inside the duct such that the mean square pressure is related to the power flow by the simple plane‐wave...

Distribution of Sound Sources in a Jet Stream

It is shown that the axial distribution of sound sources in a turbulent jet stream can be found simply in terms of (1) the spectrum of the total radiated power and (2) the frequency of the sources as a function of location along the jet axis. With the use of existing data on the sound power spectrum and recently reported but preliminary data on the frequency dependence of the most probable source location, an approximate axial distribution is derived. The derived distribution is in agreement with calculations by Sanders based on turbulence measurements, and with calculations by Ribner based on Lighthills theory.

Boundary Layer Induced Flow Noise

A discussion is given on the magnitude of flow noise in underwater devices associated with turbulent boundary layers. Measurements by Willmarth and Harrison of boundary layers in air are used to determine a phenomenological form for the boundary layer pressure field. In calculating the resulting noise, attention is restricted to geometries and conditions of interest in underwater devices. In essence the work is an extension and improvement of that presented previously (Dyer, Second Symposium on Naval Hydrodynamics). [Supported by the Office of Naval Research under Contract Nonr‐2321(00), Task NR 062‐205.]

On Noise of Aerodynamic Origin

An attempt is made in this paper to classify noises of various aerodynamic origin by means of an efficiency of conversion from mechanical to acoustical energy, and also by means of representative spectra associated with corresponding characteristic frequencies. The classification has been tried successfully on measurements of wind tunnel noises, turbojet noises, and air jet noises.