摘要:

An underwater acoustics laboratory course has been instituted at the University of Miami using a natural body of water as the experimental medium. The experiments are conducted in Bear Cut, which connects Biscayne Bay to the Straits of Florida. The 600‐ft acoustic range from the Institute of Marine Sciences pier to an instrument tower in Bear Cut is an almost flat channel with an average depth of about 13 ft. The majority of the equipment employed by the students consisted of piezoelectric projectors and hydrophones, and standard electronic laboratory apparatus such as oscillators, oscilloscopes, power amplifiers, filters, etc. The sequence of the experiments were planned for equipment familiarization and results needed for future tests. During the initial offering of this course in the Spring 1970 semester, experiments were conducted on the measurement of a piezoelectric transducer equivalent circuit, ambient‐noise spectrum, spreading loss for several distances, reciprocity calibration, and waveguide effect. For the next offering, additional experiments are planned on arrays including shading and electrical steering, pulse modulation transmission, reflection, sound‐velocity measurement, and longer range propagation. The laboratory's facilities are also available for student research.

ISSN:0001-4966    

DOI:10.1121/1.1912405

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Ultrasonic absorption and velocity measurements have been made in high‐purity liquid sodium, potassium, rubidium, and cesium at temperatures from their melting points to 250°C. Corresponding classical absorption values have also been computed over these ranges of temperature. It has been found that the measured absorption values exceed the classical values by 3%–30%. These excess absorptions are attributed to structural relaxation. The ratio of bulk viscosity to shear viscosity was found to be approximately 2.3 in sodium and potassium, 3.4 in rubidium, and 4 in cesium. Sound velocities are presented as linear functions of temperature. Both observed and classical absorptions are given as quadratic functions of temperature.

ISSN:0001-4966    

DOI:10.1121/1.1912406

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The complex transmission coefficient for a plastic specimen immersed in water (or other suitable liquid) is determined through use of a phase meter and a calibrated attenuator. Heterodyning the received rf pulses directly to video signals with a 10‐Hz beat frequency makes possible (1) the use of a commercial low‐frequency phase meter, and (2) an increase in signal‐to‐noise ratio of approximately 20 dB [H. J. McSkimin and T. B. Bateman, J. Acoust. Soc. Amer.45, 852–858 (1969)]. By way of illustration, measurements for attenuation and phase velocity are listed for a number of specimens, including Plexiglas (acrylate and methacrylate resin), Epon adhesive, epoxy foam, and several composite structures. The frequency and temperature ranges covered were 0.4–1.1 MHz and 20°–60°C, respectively.

ISSN:0001-4966    

DOI:10.1121/1.1912407

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The elastic constants ofD2Oice have been measured between 0° and −140°C. From these data and the elastic constants ofH2Oice, the molecular force constants—Kfor change of bond length andGfor change of 0⋯0⋯0 angle—have been calculated.KandGshow a similar temperature dependence as the elastic constants; their absolute values enlarge on deuteration. For comparison, values ofKandGcalculated from infrared data are given.

ISSN:0001-4966    

DOI:10.1121/1.1912408

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

When sound waves are generated in a homogeneous rod many wavelengths in diameter by a transducer of smaller diameter affixed coaxially to its end, the pressure received by the same transducer in a pulse‐echo mode or by a similar transducer on the other end of the rod is not a monotonic function of the length of the rod. There are strong fluctuations in sound level with length. The fluctuations are caused by waveguide effects. These effects have been made inoperative in the past by techniques such as cutting screw threads on the surface of the rod. In this experiment the material of the rod itself was made inhomogeneous in a radially symmetric manner by heat treatment. It was found that the waveguide fluctuations were extinguished and the wave died out exponentially in the inhomogeneous rods. Much background noise, however, was generated through the spatial diffusion of the wave.

ISSN:0001-4966    

DOI:10.1121/1.1912409

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The pressure signatures of sonic booms measured in the field often show considerable variations from the idealNwaveshape. Peaked and rounded versions of theNare commonly observed. Pierce [J. Acoust. Soc. Amer.44, 1052–1061 (1968)] has proposed that peaking and rounding are the result of refraction and diffraction caused by atmospheric inhomogeneities. To test this explanation, we have carried out a model experiment in the laboratory.Nwaves about 1 cm long were produced by an electric spark, refracted and diffracted by a gas‐filled soap bubble, and received by a very wide‐range condenser microphone. The microphone output was displayed on an oscilloscope. The spark‐microphone distance was held fixed, and measurements were taken with the bubble at various points in between. The bubble acted as a converging acoustic lens when it was filled with argon and as a diverging lens when filled with helium. It was found that the converging lens caused peaking of theNwave, while the diverging lens caused rounding. These results qualitatively support Pierre's theoretical model. The data do not serve as a quantitative test of Pierre's specific numerical examples, however, because the particular conditions he assumed were not reproduced in our experiment. A discussion given of the effect of some of these conditions, such as the order and relative time of arrival of the refracted and diffracted waves, on the waveform. Another factor that had a rather strong influence on the waveform, particularly when a helium‐filled bubble was used, was the finite size of the microphone. The waveforms that would have been observed with a true point probe are deduced.

ISSN:0001-4966    

DOI:10.1121/1.1912410

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

As an example of a previously described theory for plane and gently curved radiators, the farfield of a rectangular pistonlike radiator is shown to be composed of four components of equal magnitude, each of which behaves as if generated at one of the corners. The magnitude of each component varies monotonically with increasing angle from the acoustic axis and is independent of the dimensions of the radiator. For the steady state, the vectorial superposition of components, taking account of the separations between the four effective sources, yields the familiar CW field, containing lobe structure. In the two normal central planes parallel to the radiator sides, the four components reduce to two, and on the acoustic axis they reduce to one. If instead of a CW signal a pulse is applied to the radiator, the field components arrive sequentially, according to the travel times from each of the corners to the field point. If the applied pulse is sufficiently short, these field pulses may be fully resolved. Successful experimental verification of the theory is described in which resolution was obtained of the two pulses in a central plane of a rectangular transducer.

ISSN:0001-4966    

DOI:10.1121/1.1912411

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

In this paper, the speeds of propagation of acceleration waves in a two‐constituent mixture are developed. The mixture is assumed to behave elastically, and no reaction is assumed to take place. The amplitude of the waves does not have to be small.

ISSN:0001-4966    

DOI:10.1121/1.1912412

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A dynamic moment Green's function for plate and shell structures is defined as the spatial gradient of the normal load dynamic Green's function. With the moment Green's function known for a particular structure, its response to dynamic moments of any type of distribution and time characteristic can be evaluated. The technique is applied to the technically significant problems of a rotating moment acting on a rectangular plate and a circular cylindrical shell segment.

ISSN:0001-4966    

DOI:10.1121/1.1912413

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

An infinite elastic cylindrical shell, submerged in a semiinfinite acoustic medium with a free or rigid surface, is acted upon by a concentrated harmonic line force. By using Flügge's shell equations and the appropriate radiation condition, a boundary‐value problem is formulated. In order to make numerical integration feasible, the discontinuities in the solutions of the shell equations due to the concentrated load are removed by subtracting a closed‐form static response, and the semiinfinite physical domain is mapped into a finite (rectangular) mathematical one by using a bipolar transformation. Extensive numerical results are presented for the shell displacements and fluid pressure field for steel shells in water.

ISSN:0001-4966    

DOI:10.1121/1.1912414

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP