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Mar 1971

Volume 49, Issue 3B, pp. 697-936

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Development of an Underwater Acoustics Laboratory Course

N. L. Weinberg and W. G. Grantham

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 697-705 (1971); (9 pages)

Online Publication Date: 03 Aug 2005

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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.

Ultrasonic Measurement in Liquid Alkali Metals

M. G. Kim, K. A. Kemp, and S. V. Letcher

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 706-712 (1971); (7 pages) | Cited 7 times

Online Publication Date: 03 Aug 2005

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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.

A Water Immersion Technique for Measuring Attenuation and Phase Velocity of Longitudinal Waves in Plastics

H. J. McSkimin and P. Andreatch, Jr.

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 713-722 (1971); (10 pages) | Cited 1 time

Online Publication Date: 03 Aug 2005

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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.

Elastic Constants of D2O Ice and Variation of Intermolecular Forces on Deuteration

U. Mitzdorf and D. Helmreich

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 723-728 (1971); (6 pages) | Cited 3 times

Online Publication Date: 03 Aug 2005

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The elastic constants of D2O ice have been measured between 0° and −140°C. From these data and the elastic constants of H2O ice, the molecular force constants−K for change of bond length and G for change of 0⋯0⋯0 angle−have been calculated. K and G show a similar temperature dependence as the elastic constants; their absolute values enlarge on deuteration. For comparison, values of K and G calculated from infrared data are given.

Ultrasonic Waveguide Effects in Inhomogeneous Rods of Radial Symmetry

Emmanuel P. Papadakis

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 729-731 (1971); (3 pages)

Online Publication Date: 03 Aug 2005

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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.

Measurements of the Refraction and Diffraction of a Short N Wave by a Gas‐Filled Soap Bubble

Bruce A. Davy and David T. Blackstock

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 732-737 (1971); (6 pages) | Cited 8 times

Online Publication Date: 03 Aug 2005

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The pressure signatures of sonic booms measured in the field often show considerable variations from the ideal N waveshape. Peaked and rounded versions of the N are 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. N waves 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 the N wave, 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.

Farfield of Pulsed Rectangular Acoustic Radiator

Albert Freedman

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 738-748 (1971); (11 pages)

Online Publication Date: 03 Aug 2005

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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.

Speed of Propagation of Acceleration Waves in a Binary Nonreacting Mixture

J. R. Leininger and R. Ray Nachlinger

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 749-752 (1971); (4 pages)

Online Publication Date: 03 Aug 2005

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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.

Dynamic Green's Function Technique Applied to Shells Loaded by Dynamic Moments

U. Bolleter and W. Soedel

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 753-758 (1971); (6 pages)

Online Publication Date: 03 Aug 2005

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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.

Vibrations of Cylindrical Shells in a Semiinfinite Acoustic Medium

Kurng Y. Chang and Frank L. DiMaggio

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 759-767 (1971); (9 pages)

Online Publication Date: 03 Aug 2005

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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.

Cube‐Resonance Method to Determine the Elastic Constants of Solids

Harold H. Demarest, Jr.

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 768-775 (1971); (8 pages) | Cited 50 times

Online Publication Date: 03 Aug 2005

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The Rayleigh‐Ritz method of eigenvalue approximation is used to obtain a solution for the free vibrations of an elastic solid. The resonant frequencies of an isotropic cube, calculated by this method, agree with the frequencies of the Lamé modes, for which an exact solution is possible, up to eight significant digits. Experimental measurements of the 13 lowest‐frequency modes of a cube of fused quartz are all within 0.5% of the results computed by using the elastic constants determined by another method. These constants differ from the elastic constants determined directly from the experimental frequencies by less than 0.2% for the shear modulus and by less than 0.003 for the value of Poisson's ratio. The cube‐resonance method of elastic constant determination is also applicable to substances of more general crystallographic symmetry.

Formalism of Geometrical Optics for Flexure Waves

O. A. Germogenova

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 776-780 (1971); (5 pages)

Online Publication Date: 03 Aug 2005

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The equations of geometrical optics governing the ray shape and the energy transport along the rays are obtained for flexure waves propagating on an inhomogeneous finite weight plate. Krasil'nikov's results for the energy flux vector and the amplitude relations along the rays are generalized both in the absence and in the presence of liquid loading.

Application of Conformal Mapping and Variational Method to the Study of Natural Frequencies of Polygonal Plates

James C. M. Yu

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 781-785 (1971); (5 pages)

Online Publication Date: 03 Aug 2005

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The stationary value problem of the dynamic action for the determination of the natural frequencies of the polygonal plates is transformed by a holomorphic function into an equivalent problem of another plate with a circular boundary. The equivalent problem is then solved by the Rayleigh‐Ritz method. Since the selected coordinate functions are independent of the actual shape of a plate, the formulation is valid for one plate and for all others with boundaries which are the conformal images of a unit circle. The numerical results for the fundamental natural frequencies of plates of various shapes are calculated and compared with the available data in good agreement.

Attenuation of Low‐Frequency Sound Waves in Sediments

Rockne S. Anderson and Abner Blackman

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 786-791 (1971); (6 pages) | Cited 1 time

Online Publication Date: 03 Aug 2005

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Magnetic tape recordings of seismic profiler signals were made across an acoustically transparent wedge of sediments varying from 80 to 500 m in thickness on the flanks of Bermuda. The data were processed using ⅓‐oct bandwidth filters. The average attenuation coefficient of the sediment, α, was determined from the equation d (y)/d (z) =α, where y is the ratio expressed in decibels of the peak pressure amplitude of the bottom to the peak pressure amplitude of the subbottom returns, and z is twice the sediment thickness. Possible errors resulting from spreading losses and changes in subbottom reflectivity due to increasing velocity and density with depth were considered and appropriate corrections made to the computed attenuation coefficients. For filters centered at 200, 250, and 315 Hz the average attenuation coefficients for the sediment were found to be 0.0031, 0.0069, and 0.0126 dB/m, respectively. For a filter centered at 400 Hz, the minimum value of the attenuation coefficient is 0.0110 dB/m. Outside this frequency range, the attenuation is too high or too low to be determined by this method. With attenuation expressed as being proportional to the frequency raised to the power n, the value of n is close to 3 for the frequency range 200–400 Hz. This high value of n is attributed to the effect of intermediate reflecting layers.

Acoustical Detection of Single Cavitation Events in a Focused Field in Water at 1 MHz

W. T. Coakley

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 792-801 (1971); (10 pages) | Cited 1 time

Online Publication Date: 03 Aug 2005

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Acoustic intensities up to 1400 W/cm2 have been generated at the focus of a ceramic bowl. The rf voltage applied to the transducer has been rectified and filtered and the signals produced from cavitation events have been examined. A directional hydrophone simultaneously monitoring the focal point aided interpretation of the signals detected by the driven bowl. Low‐amplitude low‐frequency signals appeared on the driven bowl synchronously with the onset of first subharmonic emission in the water at an intensity of 13 W/cm2. The subharmonic increased rapidly with increasing sound intensity and then leveled off. Just before this occurred, the scattering properties of the focal region increased markedly and a cyclic signal, interpreted as indicating the production of clouds of microbubbles when surface oscillations set in on the cavitation bubbles, was detected on the filter circuit. Strongly radiating single cavitation events lasting for the order of milliseconds were also detected. The amplitude, duration, and rate of occurrence of these events were all one to two orders of magnitude greater in tap than in degassed water.

Observation of Wind‐Wave‐Generated Doppler Shifts in Surface Reverberation

Yoshiya Igarashi and Richard Stern

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 802-809 (1971); (8 pages) | Cited 1 time

Online Publication Date: 03 Aug 2005

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Frequency shifts in surface reverberation were consistently observed when a directional transducer was pointed in the direction of ocean wave propagation. The magnitude of the Doppler shift systematically decreased to zero as the transducer was rotated 90° to point toward the cross‐wave direction, and the sign of the shift reversed as the transducer was horizontally rotated from the up‐wave to down‐wave direction. Doppler‐shift data were obtained mainly for 110‐msec tone transmission at 15 and 60 kHz. The measured shifts were generally in accord with first‐order resonance theory and may be attributed to ultragravity waves. Higher‐order shifts corresponding to large gravity waves were not observed. The spectral width of surface reverberation was essentially independent of wind and wave directions and roughly three times that of the transmitted pulse for the 60‐kHz data, and less wide in the case of 15‐kHz transmission. It is hypothesized that orbital motions of the water particles forming the larger‐scale ocean wave can impart a radial to‐and‐fro motion of the ultragravity wavetrain riding on it. Rapid up and down Dopplers induced by this motion are believed to contribute to the observed spectral broadening.

Low‐Frequency Acoustic Attenuation in the South Pacific Ocean

A. C. Kibblewhite and R. N. Denham

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 810-815 (1971); (6 pages)

Online Publication Date: 03 Aug 2005

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The first known measurement of the attenuation of sound in deep water at frequencies below 1 kHz in the South Pacific Ocean is described. The receiver was situated off the east coast of the North Island of New Zealand. It was found that the attenuations are in most cases comparable with the values of attenuation measured in the North Pacific Ocean and the Atlantic Ocean between Bermuda and South America. However, on one of the paths over which the attenuation was measured, there was an abrupt change in the quality of sound transmission, which is thought to be a result of the path crossing the Subtropical Convergence; south of this discontinuity the attenuations were found to be comparable with those obtained previously in the Southern Ocean.

Dorsal‐Aspect Target Strength of an Individual Fish

Richard H. Love

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 816-823 (1971); (8 pages) | Cited 5 times

Online Publication Date: 03 Aug 2005

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Experiments are described in which the dorsal‐aspect target strengths of a number of individual teleostean fishes of eight species were measured at various frequencies. The results of these experiments indicate that the variations of target strength with frequency are different for fishes in two major teleostean groups, the malacopterygians and the acanthopterygians. These results are combined with results from eight other sources and an empirical equation approximating the dorsal‐aspect target strength of an individual fish determined for 0.7⩽L/λ⩽90, where L is the fish length and λ is the incident acoustic wavelength. The combined results are compared to similar results for the maximum side‐aspect target strength of an individual fish, and curves showing the trend of dorsal‐aspect and maximum side‐aspect acoustic cross sections of an individual swimbladder‐bearing fish are presented for all L/λ⩽90.

Focusing and Refraction of Harmonic Sound and Transient Pulses in Stratified Media

D. A. Sachs and A. Silbiger

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 824-840 (1971); (17 pages) | Cited 3 times

Online Publication Date: 03 Aug 2005

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In media for which the speed of sound is position dependent, propagating sound will be refracted and, in some cases, focused. In the focusing regions, usually referred to as caustics or convergence zones, significant amplification of the pressure levels above those predicted by spherical spreading has been observed for continuous waves as well as for explosive pulses. In addition, the waveforms of explosive pulses undergo drastic distortion. In the present paper, an asymptotic theory of the refraction and focusing of sound originating from a point source in a stratified medium is presented. It is applicable to realistic velocity profiles and encompasses both transient pulses and harmonic waves. A comparison with Barash's and Goertner's recent experiment involving explosive pulses indicates that the theory gives reliable estimates of the peak pressure levels at caustics, but reproduces only qualitatively the shape of the focused pulse. The discrepancy is attributed mainly to the neglect of finite‐amplitude effects in the theory's formulation. The inaccuracies inherent in the high‐frequency asymptotic methods employed in the theory are discussed in some detail.

The Time‐Dependent Force and Radiation Impedance on a Piston in a Rigid Infinite Planar Baffle

P. R. Stepanishen

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 841-849 (1971); (9 pages) | Cited 16 times

Online Publication Date: 03 Aug 2005

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An approach is presented to compute the time‐dependent force acting on a piston in a rigid infinite planar baffle as a result of the specified velocity of the piston. The approach to computing the force is applicable to both sinusoidal and nonsinusoidal velocity pulses and is valid for all piston shapes. The approach, which is based on a Green's‐function solution to the time‐dependent boundary value problem, utilizes a transformation of coordinates to simplify the evaluation of the double surface integrals. An impulse response function is defined such that the time‐dependent force can be obtained by differentiating the convolution of the impulse response and piston velocity time functions. A closed‐form expression for the impulse response of a circular piston is derived and discussed. Numerical results for the impulse response and the forces on large square pistons resulting from sinusoidal piston velocities are then presented and discussed to compare the transient and steady‐state behavior of the forces. Finally, an approach is presented to compute the radiation impedance as a function of normalized frequency from the impulse response data, and the approach is used to obtain the normalized radiation resistance and reactance for square pistons.

A Portable Hydrophone Calibrator

W. James Trott

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 850-855 (1971); (6 pages)

Online Publication Date: 03 Aug 2005

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A portable hydrophone calibrator with associated electronic circuitry is described for calibration of a wide variety of hydrophones. The calibrator covers the frequency range from 1 Hz to 3.5 kHz and simulates operating depth by means of hydrostatic pressure to 1000 psi and temperature variation from 4°C to room temperature. The calibrator is a closed chamber in which a sinusoidal pressure is produced by a piezoceramic tube that forms half of the cylindrical wall. A second piezoceramic tube forms the other half and measures the pressure in the region of the hydrophone. A step pressure establishes the absolute calibration of the system.

Wave Propagation in a Liquid Layer Bounded between a Rigid Plane and a Semiinfinite Fluid Space

W. M. Wang

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 856-861 (1971); (6 pages)

Online Publication Date: 03 Aug 2005

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The acoustic field induced by a point source in a liquid layer, bounded between a rigid plane and a semiinfinite fluid space, is analyzed. The total field is decomposed into the field of normal modes and a lateral wave, and they are studied separately. The farfield solution is obtained for the sound field within the liquid layer. The solution is applicable to the case of mutual coupling between the array elements in an underwater transducer with a ρc‐rubber window, and some discussions are presented.

Wavenumber‐Frequency Spectra of Turbulent‐Boundary‐Layer Pressure Measured by Microphone Arrays

William K. Blake and David M. Chase

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 862-877 (1971); (16 pages) | Cited 2 times

Online Publication Date: 03 Aug 2005

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Measurements of frequency spectra of pressure along a wind‐tunnel wall have been made by single microphones and by a longitudinal array of four flush 0.8‐in. circular microphones connected with alternating and with common phase. The alternating‐phase array was designed to suppress by wavenumber filtering the background acoustic duct noise at frequencies near 3 kHz. Measured levels set upper limits on low‐wavenumber boundary‐layer pressure. Analysis indicates that the high‐wavenumber (convective) contribution in this frequency range was probably negligible, but it could not be definitely established whether background noise dominated the spectra or whether the upper bound set on low‐wavenumber boundary layer noise is a close one. On assumption of wavenumber independence in most of the pertinent low‐wavenumber domain, an upper bound is given for the wavenumber spectral density of boundary‐layer pressure, and its generalization by assumption of δ∗, independence is discussed. At lower frequencies, in identifiable domains where single‐microphone and array spectra are dominated by the convective‐wavenumber component of boundary‐layer pressure, satisfactory agreement is found with theoretical predictions based on current knowledge of the spectral density in the convective‐wavenumber domain and on a measured facial‐sensitivity distribution for the microphone. In general, salient features of the array spectra correlate well with expectation, and the array technique is demonstrated to be a useful one for the subject purposes.

Sound from Turbulent‐Boundary‐Layer‐Excited Panels

Huw G. Davies

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 878-889 (1971); (12 pages) | Cited 8 times

Online Publication Date: 03 Aug 2005

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The acoustic power radiated by thin flexible panels excited by turbulent‐boundary‐layer wall‐pressure fluctuations is estimated using a modal analysis, light fluid loading effects being included. Previous estimates of the modal radiation coefficients are improved. Radiated power is estimated in two ways: the first estimate is made by directly summing over resonant modes and the second is based on a statistical energy analysis (SEA). The estimates are compared with experimental results. It would seem that the additional computation required to sum directly does not lead to a prediction significantly better than that obtained by SEA.

Effects of Humidity on the Velocity of Sound in Air

Cyril M. Harris

J. Acoust. Soc. Am. Volume 49, Issue 3B, pp. 890-893 (1971); (4 pages) | Cited 1 time

Online Publication Date: 03 Aug 2005

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Measurements have been made of the effects of humidity on the velocity of sound in air at 20.00°C and normal atmospheric pressure in the audio frequency range. The velocity of sound was determined from measurements of the frequency of the decay of normal frequencies of a spherical chamber of very high Q. Contrary to the often‐made assumption that the addition of moisture increases the velocity of sound in air, these results show that as moisture is added to dry air the velocity of sound at first decreases. The velocity is minimum at a relative humidity (RH) of about 14%; then it rises. Above about 30% RH, the velocity increases linearly with increasing moisture content.
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