摘要:

A theoretical study of the cross correlation of noise from a horizontal surface is presented. Only spatial separations in the vertical direction are considered. The ratio of the depths of the receivers is a parameter considered in the analysis so that the results might apply near the surface of the deep ocean, in the regions of shallow water where there is no bottom reflection, or where the receivers are not sensitive to signals from below. In previous work only receiver depth ratios close to unity were considered, so the results applied only to special cases such as surface‐noise cross correlation in very deep water. The results indicate that the cross‐correlation function depends very strongly on spatial separation of the receivers but very weakly on the ratio of the depths of the receivers. It is suggested, therefore, that the literature which applies to vertical correlation of surface noise in deep water can also be applied when the receivers are close to the surface.

ISSN:0001-4966    

DOI:10.1121/1.1912634

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Monostatic and bistatic backscattering from the ocean bottom was measured at frequencies from 0.5 to 6.3 kHz at four stations in the North Atlantic and North Pacific Oceans. The data can be approximately described by Lambert's law of diffuse reflection at grazing angles below 50°. The average of the scattering constants 10 logμ measured for all frequencies is −29 dB.

ISSN:0001-4966    

DOI:10.1121/1.1912635

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The averaged impulse response for sound transmission in a shallow‐water channel is estimated by a theory based on ray acoustics with lossy specular reflection from the boundaries. Averaging is introduced primarily by strict averaging in depth of the square of the received pressure. Rays need be traced over but a single cycle of their paths. The impulse response contains useful and readily accessible information about environmental loss parameters. The rate of decay of the reverberant tail, which is characteristic of channels with only moderate variations in sound speed, appears to be a particularly valuable measure. Theoretical and experimental data are compared.

ISSN:0001-4966    

DOI:10.1121/1.1912636

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Icebergs seem to have escaped serious attention as sources of underwater sound. The noise of icebergs has been measured by means of sonobuoys dropped by an aircraft at distances between 200 and 10 000 yd. Two isolated icebergs 130 to 150 ft high were measured at a location northeast of Newfoundland. Noise apparently originated by the bergs was found to have a spectrum flat to about 10 kHz, the limit of measurement, with spectrum levels of −37 and −42 dBre1 dyn/cm2at 200 yd from the two bergs. Ice sizzle is readily observed with a cube of cloudy ice and a hydrophone in a container of water. It is surmised that this noise is caused by the explosion of tiny air bubbles entrapped in the ice under pressure and released as melting occurs. Alternatively, at deep depths, the process may be one of implosion of the cavities by inrushing water. The level of the noise of icebergs doubtless depends on many factors, such as size, depth, air content, and rate of melting in the surrounding water.

ISSN:0001-4966    

DOI:10.1121/1.1912637

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

In the ray theory of acoustics, there has been some confusion regarding the variables to be held constant in the evaluation of partial derivatives of the ray position vector in the calculation of geometric spreading loss. It is shown that the general formula for spreading loss remains unchanged when the parameter along a ray is changed from travel time to a new parameter, provided any variable that occurs in the expression for the position vector is regarded as a function of the new parameter and the initial ray angles.

ISSN:0001-4966    

DOI:10.1121/1.1912638

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Transverse horizontal cross‐coherence lengths as a function of signal frequency were measured by a hydrophone array 80 m long under homogeneous winter conditions with heavy sea state. The decrease of the normalized coherence length with the acoustic wavenumber is shown as an example of sound propagation parallel to the wave crests and is compared with published reverberation measurements. The cross‐coherence length is interpreted in a simple model computation as the reciprocal of a medium‐dependent angular uncertainty.

ISSN:0001-4966    

DOI:10.1121/1.1912639

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The generation of impulsive sound, commonly called blade slap, due to blade‐vortex interaction for helicopter rotors is discussed. The unsteady lift on the blades is calculated using linear unsteady aerodynamic theory for an oblique gust model of the blade‐vortex interaction. A theoretical model for the radiated sound due to the transient lift fluctuations is presented. Expressions for the directivity, frequency spectrum, transient signal, and total power of the acoustic signal are derived. Typical results are presented and discussed. Calculations of the transient signal are presented in comparison with recent experimental results. The agreement is very good.

ISSN:0001-4966    

DOI:10.1121/1.1912640

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Experiments to investigate the fluid mechanics of whistling are reported. A model, consisting of a cylindrical cavity with rounded holes at each end, is used to simulate human whistling. It is found that the frequency is very near the Helmholtz resonator frequency, and that the resonator can be excited by flow through the smooth‐edged orifices bounding the resonant cavity. Furthermore, it is found that the flow velocity of the jet which excites the resonator must lie between limits that are proportional to frequency and that increase with both diameter and thickness of the orifice. It is concluded that whistling can be included in the same class of sound sources as the Rayleigh bird call and the Pfeifentöne, since the essential mechanism for exciting them depends on the instability of a jet to the formation of vortex rings and the interaction of the rings with a rigid boundary in the flow.

ISSN:0001-4966    

DOI:10.1121/1.1912641

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

This work shows the effect of the drive‐point impedance of a finite acoustic system on the power delivered by a realistic source. At low frequencies when the modal overlap of the system is small, the frequency‐averaged power delivered by the source can be much less than the power which the source would deliver to an equivalent semiinfinite system. An analysis of the power flow into a finite one‐dimensional wave tube is used to explain the observation that broad‐band high‐intensity acoustic drivers deliver less low‐frequency power to small reverberation chambers than they deliver to progressive wave tubes. The analysis indicates that the modal overlap also plays a central role in the determination of the ratio of space‐averaged to drive‐point response.

ISSN:0001-4966    

DOI:10.1121/1.1912642

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Previous measurements of sound absorption due to vibrational relaxation inCO2/D2Smixtures indicate a vibrational‐vibrational (v‐v) coupling between the bending mode ofCO2and the stretching modes ofD2S. A theory by Tanczos was used to analyze the results, but this theory did not allow for quantum changes greater than two in either one of the colliding molecules. Theoretical curves were calculated, therefore, for a v‐v coupling in which a two‐for‐one quantum exchange took place between the bending mode inCO2and the stretching modes inD2S. The theory has now been extended to treat cases where the v‐v coupling involves a three‐for‐one quantum exchange between modes. Using this theory, there is a slight improvement in the theoretical fit to the experimental absorption data onCO2/D2Smixtures. This is to be expected because of the near resonance between the 0310 level ofCO2and the 100 and 001 levels ofD2S.

ISSN:0001-4966    

DOI:10.1121/1.1912643

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP