摘要:

The acoustic scattering by two fluid-filled spherical elastic shells in close proximity to each other and insonified by plane waves at arbitrary angles of incidence is analyzed exactly in the frequency range that includes the midfrequency or coincidence enhancement region of the backscattered echoes. The incident and scattered wave fields are expanded in terms of the classical modal series and the addition theorem for the spherical wave functions is used to determine the exact expression for the sound fields scattered by each spherical elastic shell in the presence of the other, referred to coordinate systems at the centers of either spherical shell. The solution to the scattering problem is found by simultaneously solving the Helmholtz equations governing the wave motion in the fluid medium in which the two shells are submerged as well as in the fluid media contained in the shells, together with the two sets of equations of motion of the two elastic shells obtained from the complete three-dimensional elasticity theory after satisfying the boundary conditions at all fluid-shell interfaces as well as the far-field radiation condition. Again, the numerical computation of the scattered pressure wave involves the solution of a truncated ill-conditioned complex matrix system the size of which depends on how many terms of the modal series are required for convergence. This in turn depends on the value of the frequency, and on the proximity of the two spherical elastic shells. The ill-conditioned matrix equation is solved using the Gauss–Seidel iteration method. Backscattered and bistatic echoes from two identical spherical elastic shells are extensively calculated. The result also exhibits the large enhancement present in the backscattered echoes for the endfire situation after the midfrequency or coincidence enhancement has taken place. This can be attributed to the effects of focusing by the front elastic shell and to the reflection and refocusing by the back elastic shell of thea0Lamb wave reradiation in the observer’s direction.

ISSN:0001-4966    

DOI:10.1121/1.418507

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Sound generally reaches the shadow zone behind a noise barrier by diffraction, or scattering, from the edge at the top of the barrier. By redirecting the incident sound into the shadow zone, the edge acts as a line source. For the traditional straight-edge barrier, the line source is coherent. Since a crooked line source is less coherent, we propose to improve barrier performance by making the edge randomly jagged. Laboratory model experiments to compare insertion loss of straight- and jagged-edge barriers are reported here. A spark was used as a point source, the barriers were thin (compared to a wavelength), and ground and meteorological effects were not important. After preliminary measurements showed that a jagged edge can produce significantly more insertion loss at high frequency, a three level full factorial experiment was done. The results led to an empirical equation for insertion loss of a jagged-edge barrier. Improvement over the straight barrier was found to increase with jaggedness. An unexplained result was the poorer performance of the jagged-edge barrier at low frequency.

ISSN:0001-4966    

DOI:10.1121/1.418508

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In a laboratory experiment, the relationship between the annoyance caused by sounds of tracked and wheeled vehicles was investigated. Sounds of various vehicles, ranging from a main battle tank (MBT) to a passenger car, were recorded at several distances. Subjects were presented with these sounds and were asked to judge the annoyance if they were exposed to them at home on a regular basis. Overall, sounds emanated from military tracked vehicles were judged to be less annoying than civil passenger cars, provided that the indoor A-weighted sound-exposure levels (ASELs) were the same. It should be noted that receiver distance and operating conditions were not specifically matched for military and civil vehicles but were chosen to represent typical ranges of values found in practice. For steady-speed conditions, the sounds were judged to be equally annoying if the (indoor) ASELs of the tracked armored personnel carrier (APC) and the MBT exceeded that of a passenger car by 5 and 10 dB, respectively. In specific driving conditions such as alternate accelerations and decelerations, however, these differences were smaller. For ASELs below 55 dB, passenger cars were found to be more annoying than heavier wheeled vehicles, such as a bus or a truck with a trailer. The data indicate that the difference between the high-frequency part and the low-frequency part of the spectrum might play a role in the annoyance. With respect to noise-zoning procedures it is highly interesting that the outdoor ASEL could predict the annoyance, as rated indoors, better than could any indoor noise measure. The 5- and 10-dB bonuses to the indoor level of the APC and the MBT, respectively, were reduced to 1 and 3 dB if the dose was measured outdoors.

ISSN:0001-4966    

DOI:10.1121/1.418556

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Backward integration of a room impulse response has long been used in room acoustics for the estimation of reverberation time. However, the inevitable noise floor limits the minimum level of the measured impulse response, thereby leading to errors. This paper shows how the error is influenced by the selection of truncation time and evaluation range. A general guideline that emerges from this study is to truncate the measured impulse response at the knee where the main decay slope intersects the noise floor, then measure the slope of the backward integrated truncated impulse response down to a level about 5 dB above the noise floor.

ISSN:0001-4966    

DOI:10.1121/1.418557

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An investigation was conducted to explore potential improvements provided by alternate implementations of two impedance-measurement methods, known as the two-microphone and multipoint methods. This investigation is part of a continuing technology development to find more efficient and convenient methods for impedance measurements in harsh environments at high frequencies. The two alternate methods are compared with other methods to determine relative merits. As expected, the “best method” depends upon trade-offs between convenience and accuracy. The single-microphone method eliminates one of the two microphones typically used in the two-microphone method. The single-microphone method should be useful where mechanical constraints allow only one microphone to be placed into the test environment. It is found to be quite accurate with a single discrete frequency source, but the most inaccurate with a random noise source. An alternate implementation of the multipoint method, achieved by replacing the single-tone source with a pseudo-random noise source, requires significantly less time with minimal loss of accuracy. Typical agreement between this implementation and the accepted standard is within 0.1ρcunits. However, slightly larger deviations at frequencies above 2.5 kHz indicate challenges in the proposed extension to high-frequency (up to 25-kHz) applications.

ISSN:0001-4966    

DOI:10.1121/1.418558

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Neural representations of pinna-based spectral cues for sound localization were modeled by simulating auditory nerve discharge rates to noise bursts that had been shaped by filtering properties of the cat’s head-related transfer functions (HRTFs) at 179 locations in the frontal field. The auditory nerve model transformed spectral differences between HRTFs into simulated neural rate differences. Linear equations for this transformation were developed from actual auditory nerve responses to a limited subset of HRTF-filtered noise bursts [Rice et al., J. Acoust. Soc. Am.97, 1764–1776 (1995)]. Signal detection methods were used to investigate simulated neural responses to pairwise changes between HRTFs. The quality of neural representation for these changes, in terms ofd′values, declined when the reference HRTF was moved from a central location (0° AZ, 0° EL) to a large positive azimuth in the horizontal plane (75° AZ, 0° EL) or a high elevation in the median plane (0° AZ, 75° EL). Most simulated responses exhibited larged′values for comparisons of contralateral versus ipsilateral azimuths, or eccentric versus frontal elevations. This rate information resulted from directionally dependent changes in the overall gain of HRTFs. In addition, fibers with best frequency (BF: the frequency of greatest sensitivity for individual fibers) between 5 and 18 kHz showed larged′values for HRTF contrasts in the immediate frontal field because of the effects of spectral notches (i.e., sharp drops in gain over a narrow frequency range). Spectral notches also played a prominent role in simulations that required identification of HRTF location in the absence of a fixed reference stimulus. These modeling results correspond well with previously described patterns in the cat’s localization behaviors.

ISSN:0001-4966    

DOI:10.1121/1.418559

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The perstimulus compound action potential (PCAP), unlike the more familiar compound action potential (CAP), can be recorded in response toasynchronousas well as synchronous auditory nerve activity. When all neurons contribute equally to the PCAP, the area under the PCAP (the PCAP area) is proportional to the number of action potentials fired by auditory nerve neurons (the auditory nerve spike count). The auditory nerve spike count is one proposed code for stimulus intensity, and our goal is to use the PCAP to test this hypothesis. In this study, two independent tests were developed to measure the contributions of neurons to the PCAP as a function of their characteristic frequency (CF). The test results were verified using a model of the auditory periphery designed to calculate the auditory nerve spike count as a function of pure tone intensity and frequency. In nearly all experiments, neurons having CFs that span contiguous three or four octave bands contribute equally to the PCAP. For pure tones that stimulate only those neurons contributing equally to the PCAP, the PCAP area grows over intensity ranges frequently exceeding 80 dB, and in one case equaling 108 dB. These results demonstrate that the auditory nerve spike count, at least for pure tones, is capable of encoding changes in stimulus intensity over the entire dynamic range of the auditory system.

ISSN:0001-4966    

DOI:10.1121/1.418560

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

It is often asserted that the physiological correlate of loudness is the simple sum of the spike activity produced by all neurons in the auditory nerve (the auditory nerve spike count). We will refer to this hypothesis as thespike count hypothesis. The spike count hypothesis has been tested in the past using models of the auditory periphery and in almost all cases, the hypothesis has been supported. Our new technique for recording a compound potential from the chinchilla auditory nerve, the perstimulus compound action potential (PCAP), makes possible the measurement of the growth of the auditory nerve spike count, thus providing data that can be used to test the spike count hypothesis empirically. It was observed that the growth of the auditory nerve spike count in response to a 1-kHz pure tone (in dB/dB) is 33% shallower than the growth of loudness for a 1-kHz tone, and this discrepancy increases to 66% for an 8-kHz tone. In addition, “equal-count” contours were constructed in a manner analogous to equal-loudness contours. It was found that as reference intensity increases, equal-count contours become sharply curved upward at high frequencies whereas equal-loudness contours become increasingly flat. These differences are unlikely to be the result of the cross-species comparison, since the discrepancies are mostly attributable to the skewed pattern of spread of excitation along the basilar membrane, a property shared by humans and chinchillas. Therefore, we conclude that the simple sum of the spike activity in the auditory nerve cannot be the physiological correlate of loudness.

ISSN:0001-4966    

DOI:10.1121/1.418561

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The present investigation is part of an ongoing series of studies aimed at discerning the neural bases of presbycusis. Presbycusis is a sensory perceptual disorder involving loss of high-pitch hearing and reduced ability to process biologically relevant acoustic signals in noisy environments. The purpose of the present experiment was to delineate the efferent projections of a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adult CBA mice. The CBA strain’s progressive loss of hearing over its lifespan approximates many aspects of the mild-to-moderate hearing loss experienced by a significant number of humans suffering from presbycusis. Focal, iontophoretic injections of HRP were made in the 18–24 kHz region of dorsomedial IC of the CBA strain following physiological mapping experiments. Serial sections were reacted with a chromagen, counterstained and examined for anterogradely labeled fibers and boutons. Efferent projections were observedipsilaterallyin: medial and ventral divisions of the medial geniculate body (MGB); middle layers of the superior colliculus; central gray; and external nucleus (E), dorsal cortex (DC) and central nucleus of IC.Contralaterally, labeled fibers and boutons were seen in the IC at a location homologous to the injection site, as well as in E and DC. A small projection was noted in contralateral MGB. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mice and animals of other species of older ages and with varying degrees of hearing loss.

ISSN:0001-4966    

DOI:10.1121/1.418562

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The middle-ear pressure gain for the 50-Hz to 12-kHz range was determined from the ratio of sound pressures measured in the vestibule and the ear canal of four human-cadaver ears. The magnitude of the middle-ear pressure gain is 20 dB for frequencies between 500 Hz and 2 kHz. Above 4 kHz, the gain changes as a function of frequency at a rate of approximately −8 dB/octave and below 400 Hz at 4 dB/octave. The standard error of the mean magnitude across the four ears is typically less than 3 dB. The phase angle of the pressure gain also changes with frequency. Interruption of the ossicular chain decreases the vestibule pressure by at least 20 dB. It is shown that air bubbles in the inner ear can diminish the vestibule pressure; procedures are used to remove bubbles. From these pressure measurements and previous measurements of stapes motion, the frequency dependence of behavioral thresholds for tones was tested to discover whether it corresponds to the constancy of a physiological variable at the cochlear input. Among pressure, power, or stapes-motion measures, the vestibule pressure is most nearly constant with frequency at the behavioral “minimum audible pressure.”

ISSN:0001-4966    

DOI:10.1121/1.418563

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP