摘要:

An analytic solution for the acoustic radiation from a rib‐reinforced infinite elastic plate which is excited by a point force located on the rib was obtained. The solution exhibits a new coincidence angle, where the radiated pressure peaks, which depends on the relative stiffness and mass of the attached beam. An expression for the radiated power was also obtained which shows the general reduction of radiated power explicitly as a function of the mass and stiffness of the attached beam. Approximate formulas for the complex interaction between the beam and the plate were developed for frequencies above and below the coincidence frequency. Approximate expressions for the radiated power were obtained from simplified physical models.

ISSN:0001-4966    

DOI:10.1121/1.381506

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

An application of percentile measurements is presented as a method for estimating background ambient noise levels from spectral density functions. The advantage of a percentile method over an average method is that it allows an accurate measure of background noise containing a sine wave, that is essentially independent of the sine‐wave amplitude. This feature makes the percentile method very useful in the measurement of cw propagation loss, where it is frequently necessary to subtract an estimate of the background noise from a signal‐plus‐noise measurement, including the case when the noise contains sine waves close in frequency to the cw signal. A percentile‐level estimate may be easily used to calculate an average level estimate for chi‐square data and it is found that a percentile estimator is just as accurate and essentially just as stable as an average estimator. Theory based on chi‐square statistics and experimental data from the ambient sound field in the deep ocean are shown to be in good agreement.

ISSN:0001-4966    

DOI:10.1121/1.381478

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A calibrator for use with long line hydrophones (seismic streamers) has been designed, constructed, and tested. Comparison calibrations may be obtained for individual elements or small groups of interconnected elements of a line hydrophone array in the frequency range of 20 to 3000 Hz. Absolute calibrations may be obtained in the frequency range of 20 to 100 Hz. The calibrator may be operated in either of two configurations. The first configuration has the calibrator submerged in a water trough. In the second configuration the calibrator, with the hydrophone inserted, forms a closed chamber through which water is continuously circulated during operation. A comparison between results obtained by using the calibrator, and independent measurements is presented. The theory of operation of the device, and of the sonic resistors used to reduce standing waves within the calibrator, is presented in the appendixes.

ISSN:0001-4966    

DOI:10.1121/1.381477

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A new ultrasonic imaging system and apparatus are proposed which use a moving random phase mask placed just behind a coherently illuminated object, and a stationary‐point receiver located in the farfield. The power spectrum of the received signal represents the shadow of an object on the receiver plane and hence gives the image of the object under suitable conditions. The second‐order correlation function of the received signal gives the exact image of the object provided a reference‐point source is located nearby. The system constructed includes a minicomputer for signal processing and real‐time imaging. Experimental results for one‐dimensional objects are presented. It is also demonstrated experimentally that the system has advantages over conventional coherent ultrasonic imaging systems when propagating through a turbulent medium.

ISSN:0001-4966    

DOI:10.1121/1.381490

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

In this paper, a simple model is presented which predicts the ultrasonic frequency spectra for a broad class of layered composite materials having a finite number of laminas. This model predicts spectra for arrays of glass plates in water and these spectra are experimentally verified. Precisely regular spectra are predicted for single plates, while irregular spectra are predicted for all of the arrays studied. Results relating to nondestructive testing which have emerged from this investigation include methods for predicting spectra for layered composite materials and techniques for mapping small changes in the modulus and thickness of composite materials. Also discussed is the existence of forbidden frequency bands for which ultrasound transmission is strongly attenuated in thick layered composites.

ISSN:0001-4966    

DOI:10.1121/1.381491

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A shallow liquid surface‐wave analogy to turbulence‐induced sound was utilized to investigate the mechanisms of noise generation and propagation in jets. Theoretical analysis was used to determine the wave‐power dependence on Mach number and to derive the relationship between wave and turbulence spectra. The experimental investigation utilized a 24 ft2mercury ripple tank. The waves were produced by the turbulence in a mercury jet exhausting into the tank. The results substantiate the validity of the surface‐wave analogy as a jet‐noise simulator. Measurement of mean and turbulent velocity profiles proved that the jet development was similar to the theoretical plane‐free jet. Farfield measurements of wave power, directivity, and spectra compared favorably with the acoustic case. However, the wave spectra showed little of the higher‐frequency content found with the turbulence spectra. This difference was attributed to the vertical spatial integration of the turbulence‐source terms, which is not found in jet noise.

ISSN:0001-4966    

DOI:10.1121/1.381492

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This paper considers the joint estimation of the bearing and strength parameters of a noise source, by a uniformly spaced array of sensors in the presence of self‐noise, assumed to be independent between any pair of sensors. Contrary to the analogue systems of processing (i.e., correlation and beamforming), the present scheme, which is based on the maximum‐likelihood (ML) principle, operates on a set of time samples representing the bandlimited output of the array’s elements. The system, as a result, does not require the assumption of long observation time normally used in other schemes and is easily implemented on a digital computer. The resulting ML estimator is not in theory a sufficient one. Nevertheless, when the estimator’s variance is compared with the Cramer–Rao lower bound, the estimator virtually attains its asymptotic sufficiency as the number of array elements exceeds a ’’threshold’’ which is a decreasing function of the signal‐to‐noise ratio (SNR) and the length of the observation time. Except when the SNR is very poor and the observation time is quite short, the ’’threshold’’ is found to be surprisingly small. It is also demonstrated, at least when the error in the bearing estimate is small, that the ML estimator is unbiased.

ISSN:0001-4966    

DOI:10.1121/1.381493

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The tuning of single auditory nerve fibers in the rat in response to broadband noise was studied in a large sound intensity range. Pseudorandom noise was used as stimuli and the cross spectrum between a period histogram of the noise and a period of the noise was used as an approximation of the transfer function from sound to the modulation of the neural discharge rate. In units with a CF higher than 1000 Hz, the width of the cross spectra (at 10‐dB points) invariably increased as the stimulus intensity increased and the center frequency decreased. The few units studied with CF below 1000 Hz seemed to undergo somewhat less of a change in width. The cross correlograms of all fibers with a characteristic frequency (CF) below 5 kHz showed a damped oscillation the duration of which decreased as stimulus intensity was increased. The similarity between the nonlinearity shown here and that shown in measurements of basilar membrane motion using the Mössbauer effect is discussed.

ISSN:0001-4966    

DOI:10.1121/1.381495

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Postnatal development of masked auditory thresholds in the house mouseMusmusculuswere behaviorally measured from the 10th to the 24th day. An unconditioned stop reflex and an unconditioned pinna reflex to tones were used for threshold determinations. Masked threshold levels first decrease from day 10 to day 12, then increase or remain constant until day 14, and finally decrease until day 16–18, where the adult levels are reached. The results are in agreement with anatomical and electrophysiological data in the literature. It was concluded that in the developing acoustical system of the mouse two effects superimpose: peripheral completion and central maturation. The data can be interpreted such that the peripheral development which lasts until day 14 dominates between the 12th and 14th postnatal day, whereas development in the central nervous system influences thresholds between day 10 and day 16–18. The widths of critical masking bands are assumed to be constant during development and determined by peripheral mechanisms.

ISSN:0001-4966    

DOI:10.1121/1.381496

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The potential usefulness of multidimensional scaling techniques in the perceptual analysis of ’’real world’’ underwater sounds was demonstrated. Nineteen observers (nine with musical training, ten without) used a five‐point scale to judge the similarity of all possible pairs of eight passive sonar recordings. The eight s signals were selected to represent a range of common natural and man‐made underwater sounds. These data were analyzed using the INDSCAL multidimensonal scaling model, revealing an interpretable two‐dimensional psychological space. One of the psychological dimensions was interpreted as reflecting the overall shape of the 1/3‐octave spectra of the eight signals, while the second was seen to reflect the prominence of a low‐frequency periodicity present in some signals. Individual observer analysis revealed substantial differences between the musically trained and musically inexperienced observers in the relative importance or salience of the two dimensions. The relation of these findings to earlier work was discussed.

ISSN:0001-4966    

DOI:10.1121/1.381497

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP