Volume 57, Issue 2, February 1975
Index of content:
57(1975); http://dx.doi.org/10.1121/1.380457View Description Hide Description
Aircraft flyover noise was recorded and played back with a peak detector to detect the intensity fluctuations. The intensity fluctuations were digitized and plotted. Jet noise fluctuations appear more jagged, while prop noise fluctuations are smoother. Spectral analysis of fluctuations shows most of the power is at low frequencies and decreased by 80 dB between 0 and 5 Hz.
Subject Classification: 28.45; 50.50, 50.55.
57(1975); http://dx.doi.org/10.1121/1.380458View Description Hide Description
Use of the generalized WKB procedure in place of the WKB procedure to obtain solutions to the separated wave equation leads to modified ray theory. The quantitative consequences of this frequency−dependent theory are investigated and compared with ordinary ray theory and normal−mode theory for both bounded and unbounded media under conditions for the formation of a shadow region in ordinary ray theory. The behavior of the modified ray theory field, which always contains a caustic in the types of problems which are examined, depends on whether the ray theory shadow boundary is a limiting ray or a caustic. In the former case the modified ray theory field, suitably extended to take the caustic into account, approaches the normal−mode field far into the shadow region. In the latter case the modified ray theory field in the shadow region agrees with the ordinary ray theory field, also suitably extended.
Subject Classification: 30.20.
57(1975); http://dx.doi.org/10.1121/1.380459View Description Hide Description
An approximate method is presented for solving the equations of motion that describe the scattering of an acoustic wave by an elastic structure immersed in an infinite fluid medium. The mathematical model that is developed uses the finite−element method to calculate the vibrational response of the elastic body and to calculate the acoustic pressure field of a finite volume of the fluid medium which closely surrounds the elastic body. Analytical methods are used to obtain the boundary conditions for this mathematical model. Results are presented for the total scattering cross section, axial pressure, and scatteringpatterns for an acoustic plane wave at normal incidence on an aluminum disk.
Subject Classification: 30.30, 30.40.
57(1975); http://dx.doi.org/10.1121/1.380460View Description Hide Description
A volume reverberation model which utilizes fish distribution data has been developed to permit quantitative predictions of depth−dependent scattering strengths. The model was evaluated by using coincident acoustic volume scatteringmeasurements and biological trawl data collected in the Mediterranean Sea. Biological specimens were collected at discrete depths with an Isaacs−Kidd midwater trawl. Each fish collected was identified and sized, and its acoustic cross section calculated by utilizing regression equations relating fish species, length, and swimbladder volume at frequencies near swimbladder resonance, or equations relating fish length to acoustic cross section at frequencies far above resonance. Scattering strengths for each species were then determined, indicating dominant species at each trawl depth for 1/3−octave frequency bands between 2.5 and 20 kHz. The acoustic data were obtained with surface−vented explosive sources and a shallow onmidirectional hydrophone. These data were processed to yield integrated column strength versus depth profiles, which were then numerically differentiated to produce scattering strength versus depth profiles for each frequency band. Comparison of the model predictions to the scattering strength profiles showed good agreement at the higher frequencies, with the agreement decreasing with frequency below 6.3 kHz.
Subject Classification: 30.30, 30.40.
57(1975); http://dx.doi.org/10.1121/1.380461View Description Hide Description
The parabolic approximation to the acoustic waveequation is examined within the framework of normal−mode theory. For the case where sOund speed and water de pth are independent of range, three types of errors are found to occur. Discrete modes are shown to propagate with the correct amplitude and mode shape but with errors in phase and group velocities. In addition to these errors, which are inherent in the method, two other sources of error are present. The effect of these other errors may be made negligible in one case by limiting the sound−speed gradients and range considered and, in the other case, by introducing a highly absorptive layer deep in the ocean bottom.
Subject Classification: 30.20,30.50;20.40,20.15.
57(1975); http://dx.doi.org/10.1121/1.380462View Description Hide Description
The speed of sound in standard seawater (diluted with pure water and evaporated) has been measured relative to pure water with a Nusonics velocimeter as a function of temperature (0 to 40°C, at 5° intervals) and salinity (5 to 40 0/00, at 5 0/00 intervals) at a atm. The results have been fitted to an equation of the form c = c 0 + A S (0/00) + B S (0/00)3/2 + C S (0/00)2, where c 0 is the speed of sound in pure water,S (0/00) is the salinity in parts per thousand, and A, B, and C are temperature−dependent parameters. The sound speeds fit this equation to a standard deviation of 0.04 m sec−1 over the entire temperature and salinity range. Over the oceanographic range our results, on the average, agree with the work of Wilson to ±0.5 m sec−1 (max 1.08 m sec−1) and with the work of Del Grosso and Mader to ±0.05 m sec−1 (max 0.14 m sec−1). In the low−salinity range, (5−25 0/00 salinity) our results, on the average, agree with the work of Wilson to ±0.3 m sec−1 (max 1.1 m sec−1) and with the work of Del Grosso and Mader to within ±0.1 m sec−1 (max 0.20 m sec−1). From these comparisons it is clear that the 1−atm sound speeds of Del Grosso and Mader are more reliable than those determined by Wilson over the oceanographic range as well as at lower salinities (which is outside of the range of Del Grosso and Mader’s measurements). The adiabatic and isothermal compressibilities of seawater solutions reliable to ±0.003×10−6 bar−1 have been determined from the sound speeds. The isothermal compressibilities are in excellent agreement (avg. dev. ±0.02×10−6 bar) with the direct measurements of Lepple and Millero. The specific heat of seawater solutions at constant volume has also been determined from the sound−derived isothermal compressibilities.
Subject Classification: 30.25.
57(1975); http://dx.doi.org/10.1121/1.380463View Description Hide Description
Sound propagation in a deep−ocean sound channel is investigated theoretically. A normal−mode series solution which includes effects due to curvature of the earth is obtained as the residues in a pole series arising from the Watson transformation. Analytic expressions are found for the pressure amplitudes due to a continuous wave source using an Epstein sound velocity profile. Attenuation−versus−range curves are given for both phased and random modal sums. The continuous wave solutions are then used for the pulsed source problem. Dispersion curves are obtained. The higher−order modes are shown to arrive earlier, in agreement with experiment. An analytic expression for the pressure of a pulse as a function of time is obtained. The shape of the pulse at a range of several hundred miles is shown and agrees qualitatively with experimentally obtained pulse shapes. Such pulses begin at a low amplitude, increase to a relatively large amplitude peak, then cut off abruptly.
Subject Classification: 30.20.
57(1975); http://dx.doi.org/10.1121/1.380443View Description Hide Description
A new acoustic resonator with nonplanar quartztransducers for liquids (sample volume about 20 ml) in the 0.2− to 2−MHz range is described. The application of static overpressure onto the liquid cavity causes concave bending of both quartztransducers. This reduces loss contributions from diffraction and ’’side−wall effects’’ considerably, resulting in higher Q values for this resonator; moreover, air inclusions in the cavity are suppressed.
Subject Classification: 35.24, 35.80.
57(1975); http://dx.doi.org/10.1121/1.380444View Description Hide Description
To get high−resolution reconstructed images under the conditions of a restricted scanning size and a long wavelength of ultrasonic waves, a method of superresolutionimaging is proposed. The basic idea is to multiply computationally the phase of the detected signals and to eliminate the extra deteriorating terms also by means of computational manipulations, and to reconstruct the image using the resulting signals. The principle and some results by computer simulation showing the effectiveness of the method are shown, though the technique is useful only for a limited number of a few point targets at most, and it also requires a p r i o r i knowledge of the target.
Subject Classification :35.65.
57(1975); http://dx.doi.org/10.1121/1.380445View Description Hide Description
The excitation of Rayleigh surface waves by an interdigital transducer(IDT) on a piezoelectric layered structure has been studied in this paper. The surface wavevelocity, optimum finger width, and number of fingers for maximum bandwidth for the IDT have been calculated for several values of the product (k s ) of wavenumber and layer thickness. The frequency response and equivalent circuit of the IDT have been determined. Numerical examples are also presented and discussed.
Subject Classification: 20.15.
57(1975); http://dx.doi.org/10.1121/1.380446View Description Hide Description
A laser beam probe is used to study ultrasonic phenomena at a liquid−liquid interface. In particular, the technique has been used to detect and quantify evanescent waves generated at such an interface when the critical angle of incidence is exceeded. A mathematical description of the properties of evanescent waves is also given.
57(1975); http://dx.doi.org/10.1121/1.380447View Description Hide Description
An improved differential phase−comparison method for the measurement of small changes in ultrasonic velocity is described. Velocity dispersion measurements can be carried out in the frequency range from 3.3 to 70.2 MHz; the relative accuracy is one part in 105; only 12 cm3 of liquid are required. Results are presented for aqueous and organic solutions, which are in good agreement with known ultrasonic−absorption data.
Subject Classification: 35.24, 35.35.
57(1975); http://dx.doi.org/10.1121/1.380448View Description Hide Description
This paper is concerned with the steady−state radial−shear vibrations of an axially polarized piezoelectricceramic tube of infinite length, whose cylindrical surfaces are either traction−free or subjected to a relative displacement. Open− and short−circuit resonant frequency equations are formulated and the deviation between frequency pairs examined numerically. Fundamental resonant frequency curves are given for traction−free PZT−4 cylinders having arbitrary geometry, which approach the infinite plate solutions for large radius/thickness ratios. The theory is applied to the vibrations of annular accelerometers operating in the radial−shear mode.
Subject Classification: 40.26.
57(1975); http://dx.doi.org/10.1121/1.380449View Description Hide Description
The paper deals with the natural frequency distribution in beams, plates, and shallow spherical shells. Expressions are found for the distribution functions (number of modes not exceeding a given cutoff level), for these integral approximations, and for the frequency derivatives of the latter—the so−called smeared modal densities. Special attention is paid to the singularities in these densities (present in certain elements), which were noted by Bolotin and are referred to in literature as ’’condensation points.’’ It is shown that omission of the concept of lowest natural frequency leads to a physical paradox, and that the findings of most studies of modal densities should be revised. The method of integral approximations is applied to the problem of random vibrations of a shallow spherical viscoelastic panel. It is shown that the integral approximation of the averaged spectral density in the high−frequency range is equivalent to an infinite−system model, and, moreover that the physically inconsistent results reported in literature regarding infinite discontinuities in the spectral densities of damped shells are erroneous and due to application of asymptotic expressions, valid only at high frequencies, to the low−frequency range.
Subject Classification: 40.22, 40.24, 40.26, 40.35.
57(1975); http://dx.doi.org/10.1121/1.380450View Description Hide Description
A formalism for handling problems of wave propagation and forced vibration in ribbed plates is presented. A general solution is obtained for the forced vibration of an infinite thin plate, periodically stiffened by identical, uniform ribs. The ribs are idealized as parallel line attachments capable of exerting line forces and line moments upon the plate, and the magnitudes of these forces and moments are related to the motion of the plate through the impedances of the ribs and plate. The assumption of an externally applied pressure excitation, which varies harmonically in time and in the plane of the plate, permits an explicit solution, and the principle of superposition is then used to construct the solution to an arbitrary excitation. By setting the amplitude of the harmonic excitation equal to zero, an equation for the free modes of the ribbed plate is derived.
Subject Classification: 20.15.
57(1975); http://dx.doi.org/10.1121/1.380451View Description Hide Description
Statistical Energy Analysis (SEA) predictions of the power flow between two multimodal dynamic systems, i.e., two rods vibrating in compression, are compared with an exact calculation. The exact calculation when averaged in sufficiently broad frequency bands is found to agree closely with the SEA predictions for both s t r o n g and w e a k coupling.
Subject Classification: 45.40.
57(1975); http://dx.doi.org/10.1121/1.380452View Description Hide Description
A reverberant chamber for use with both air and water media is described. The chamber is used to investigate the acoustic coupling of simple vibrating structures under conditions of light or of heavy fluid loading. Calibration was effected by measuring 60−dB reverberation times in 1/3−octave bands; decay times on the order of 2 sec in air and the order of 0.2 sec in water were observed. These results were cross checked in water by measuring the spectrum of acoustic pressure in the chamber generated by a calibrated hydrophone source. Sound−pressure levels measured in narrow (50−Hz) bands varied ±3 dB spatially in the air−filled chamber and ±5 dB spatially in the water−filled chamber.
Subject Classification: 55.65; 50.25; 45.10.
57(1975); http://dx.doi.org/10.1121/1.380453View Description Hide Description
Observers rated 500−msec bursts of wide−band Gaussian noise for presence or absence of a 100−msec, 500−Hz signal tone. The tone was present on half of the 400 trials and was centered in the noise bursts. Bandwidth and integration time estimates were found for each observer by correlating the observer ratings with the output of energy detectors of various bandwidths (20, 40, 100, 250 Hz) and integration times (50, 100, 300, 500 msec) to find the best correlating energy detector. The results, computed separately for signal (SN) and no signal (N) trials, indicate a bandwidth of 40 Hz and an integration time of 100 or 300 msec fit best on both SN and N trials. Energy variations in 25 50−Hz by 100−msec segments of the noise were correlated with the observer ratings and showed negative correlations in frequency and time intervals immediately surrounding the signal region on SN trials, but not on N trials. The results suggest observer monitor detectors sensitive to temporal and spectral changes in the energy of the noise bursts, not just the absolute level of the output of a simple filter−integrator energy detector. The N trial results are accounted for by assuming that the observer is looking for the same pattern of temporal and spectral changes as on SN trials, but is uncertain as to the exact location of the signal tone.
Subject Classification: 65.58, 65.35.
57(1975); http://dx.doi.org/10.1121/1.380454View Description Hide Description
Some of the strategies which an observer might use in detecting a tone presented against a background of noise are discussed. In Experiment I thresholds for tones in noise were measured under a number of different conditions, using flat−spectrum and notch−filtered noises, in a two−interval forced−choice task. Threshold did not correspond to a constant difference in critical−band levels on the two halves of the trial. Performance at the highest frequency used (6.1 kHz) was worse than at lower frequencies (1055 and 4080 Hz), and, in contrast to them, was not independent of overall level. This is contrary to the classical data, and indicates that the critical ratio is not a constant fraction of the critical band. In Experiment II the intensity discrimination of 1/3−octave bands of noise was studied. The noises were presented alone or in a wide−band noise background. For noise bands presented alone, performance is roughly independent of overall level, and improves slightly with increasing center frequency. For noise bands presented against a wide−band noise background, performance is independent of overall level for frequencies up to 4 kHz. A change in level of about 2−3 dB is necessary for 75% correct detection. For frequencies above this, performance worsens with increasing overall level. These results may be explicable in terms of a saturation of neurons at high intensities. The saturation effects are not observed at low frequencies, possibly because the primary cue for detection is a change in the temporal pattern of neural firing, rather than a change in amount of neural firing.
Subject Classification: 65.58, 65,35.
Intensity discrimination for noise bursts in the presence of a continuous, bandstop background: effects of level, width of the bandstop, and duration57(1975); http://dx.doi.org/10.1121/1.380455View Description Hide Description
Weber functions for broad−band noise bursts were determined using a 2IFC procedure. Noise bursts were presented either alone or in the presence of a continuous, bandstop−filtered background whose spectrum level in the passband was 10 dB above that of the standard burst. The width of the bandstop was varied from 100 to 3160 Hz in half−log−unit steps. The center frequency of the bandstop was always 1800 Hz. Burst duration was either 10 or 250 msec, and levels ranged from 31 to 81 dB SPL. Weber fractions tended to increase with increasing level for all conditions using a bandstop background, but the effects were small. The form of the Weber function was similar for the long and the short duration bursts. Performance improved with increasing width of the bandstop, but the magnitude of the effect (on average, 2.5 dB per decade of bandwidth) was about half what would be predicted by stimulus−oriented models of discrimination (5 dB per decade).
Subject Classification: 65.35, 65.50.