摘要:

This is a study of the acoustic properties of various grades (0000, 00, 1, and 3) and densities (0.0046–0.22 g/cm2) of steel wool. A theory that accounts for momentum and heat transfer from the acoustic wave to the wool is presented. Experimental evidence, obtained at frequencies of 173, 480, and 1440 Hz, confirms the theory to the expected accuracy.

ISSN:0001-4966    

DOI:10.1121/1.1912306

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

When a point source radiates steadily in a rectangular enclosure and excites one mode only, its power outputWdepends strongly on its position. Distribution functions forWare found when the source is positioned randomly, and an axial, tangential, or oblique mode is excited. The functions for these three cases form a consistent family of curves, whose shapes are controlled by the mode shapes. The space‐averaged value ofW, i.e., the average for all source positions in the enclosure, does not equal the free‐field valueW0, but depends on the modal damping. The value ofW0can be derived from sound‐pressure measurements, however. The results are derived theoretically, and checked by a Monte Carlo procedure. Some experimental data which confirm the theoretical results are given for a small source driving an axial mode in a reverberation chamber. The work has application to standard methods of measuring the sound power output of a source.

ISSN:0001-4966    

DOI:10.1121/1.1912310

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Vibration waveforms are taken at several points on a vibrating surface and then combined, using a computer with a cathode‐ray‐tube display, to produce several representations of the movement of the surface. An important advantage of this method is that the forms of the data and representations lend themselves readily to manipulation by the computer. A noncontact vibration pickup, developed especially for this application, is briefly described.

ISSN:0001-4966    

DOI:10.1121/1.1912305

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The ability of a likelihood ratio processor to classify similar spherical underwater targets using quadrature components of their acoustic echoes was measured under highly controlled conditions. The five targets used were 7‐in.‐diam, 5‐in.‐diam, and 3‐in.‐diam solid aluminum spheres; a 5‐n.‐diam styrofoam sphere; and a 5‐in.‐diam hollow aluminum sphere. A pulsed sinusoid and a linear FM signal were transmitted, both with the same bandwidth. The performance of the likelihood ratio processor for a specified signal‐to‐noise (S/N) ratio (peak amplitude of echo to rms of noise amplitude) was portrayed with a Receiver Operating Characteristic (ROC) curve. For the pulsed sinusoid with known epoch, the performance was satisfactory for a majority of the target pairs when the S/N ratio was 0 dB or larger. For the pulsed sinusoid, it was found that the S/N ratio was a sensitive parameter that affected performance, and that an uncertainty in epoch over a cycle of the carrier resulted in the same degradation in performance as a 6‐dB decrease in the S/N ratio. The performances obtained with the likelihood ratio processor for the linear FM transmission when the S/N ratio was −6 dB were of the same order as, but better than, that obtained for the sinusoidal pulse when the S/N ratio was 0 dB. Equivalent performances were obtained for both transmissions when the signal energies to mean‐square‐of‐noise amplitudes of the messages for both transmissions were equal.

ISSN:0001-4966    

DOI:10.1121/1.1912307

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A simple small‐scale model of a generalized harbor entrance with rectangular resonators was set up. Using simple electronic equipment and sound waves of the order of 1000 cycles, the geometry for resonance was determined. The results are compared with those obtained from a hydraulic model, using gravity waves of similar wavelength. It was found that the results were not influenced by imperfect boundaries or reasonable changes in frequency. It is concluded that the results are sufficiently accurate to warrant further development of the method as an aid for harbor design, particularly in view of the simplicity and size of the equipment.

ISSN:0001-4966    

DOI:10.1121/1.1912308

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The drilling vessel GLOMAR CHALLENGER has the requirement of maintaining a relatively fixed position in the deep ocean for periods as long as two weeks. The vessel's position is maintained by a dynamic positioning system that uses a sea‐floor acoustic beacon for the position reference. This beacon, which is free falling and nonrecoverable, provides an acoustic signal consisting of a 4‐msec pulse of energy at a frequency of 16 kHz every 2 sec; the beacon is battery operated and can be used in water depths up to 20 000 ft. This acoustic reference system has been used in drilling operations for over 2 yr with very satisfactory performance.

ISSN:0001-4966    

DOI:10.1121/1.1912309

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

An instrument called the Borehole Televiewer has been developed for making acoustic pictures of the walls of a borehole. Very often certain problems in exploration and production of oil can be solved from information gained by looking at the face of the borehole. Owing to the fact that the well is usually filled with an opaque liquid such as drilling mud or oil, optical devices (e.g., cameras or television) cannot be used. The Televiewer, using pulsed 2‐mHz sound, scans the borehole wall. From the amplitude of the reflected signal a pictorial representation of the wall is made. The form of the picture is as if the well were vertically split in the magnetic north direction, unfolded and laid flat. Well features are reproduced in extraordinary detail on the resulting pictures. Azimuthal resolution is on the order of132 in., which is unusual when one considers that the piezoelectric transducer is a disk12 in.in diameter.

ISSN:0001-4966    

DOI:10.1121/1.1975927

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A review is given of information obtained in recent years concerning the effects of atmospheric unsteadiness on sonic boom signatures. These effects include the observed random variations in boom overpressures from those expected for a stratified atmosphere, the anomalously large and variable risetimes, and the occurrence of spiked or rounded waveforms rather than the characteristicNwaves. The extent of the variability in data recorded during actual flight tests is summarized in the form of histograms, representing experimentally obtained probability density functions. The physical mechanisms believed to be responsible for the variations and the anomalous features in the signatures are described. These include refraction and subsequent wavefront rippling by turbulence, the possible focusing or defocusing of rays, the formation of caustics and the phenomenon of wavefront folding, and diffraction. Recent statistical theories of shock propagation through a turbulent atmosphere proposed by Crow, Plotkin and by George, Pierce, and Horning are reviewed.

ISSN:0001-4966    

DOI:10.1121/1.1975941

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Previous studies showed that cavitation noise and the half‐order subharmonic occurred at much lower thresholds than required to produce macroscopic visible bubbles in the superfluid helium II. It might be supposed that this low “audible” threshold is associated with the presence of subvisible resonant bubbles. Accordingly experiments are in progress at lower frequencies in anticipation that any such bubbles will be larger in size with consequently less distinction between audible and visible thresholds. One investigation employed a sandwich‐type transducer coupled to the liquid through a velocity transformer operating at 9.93 kHz. Measurements of particle velocity were made by including the transducer as one arm of a Wheatstone bridge and monitoring the off‐balance voltage. Pressure thresholds could be deduced from this information assuming progressive wave radiation. In helium II, at a temperature of about 2.0°K, pressure amplitude thresholds were found to be as follows: for subharmonic generation 0.4 mbar; for white‐noise generation, 3.4 mbar; and for visible bubble generation about 30 mbar; i.e., there was still a large difference between audible and visible thresholds. A device designed to operate around 100 Hz is presently under construction.

ISSN:0001-4966    

DOI:10.1121/1.1975951

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

It is known that transient cavities, generated by cavitation in an acoustic stationary wave, travel at high velocities outward from the region of high acoustic pressure. An analysis of the motion of these cavities generated in a cylindrical resonator has been made. Cavity velocities, on the order of 100 cm/sec, were obtained by photographing the cavitation event under stroboscopic illumination. Owing to the short duration of the stroboscopic flash, considerable detail can be seen in the photographs of individual cavities moving outward from the acoustic pressure antinode. Acoustic pressure amplitudes were near but above threshold and varied from approximately 3 to 8 bars. The cavities varied in size, were roughly spherical in shape, and tended to break up as they collapsed. Some photographs show several tracks diverging from the same point, apparently originating simultaneously. It is found that if the cavities are treated as spherical bubbles, then the observed motion agrees roughly with that predicted analytically. [Work supported by the Office of Naval Research and the Naval Academy Research Council.]

ISSN:0001-4966    

DOI:10.1121/1.1975955

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP