摘要:

A local active noise control system is described which uses a virtual microphone arrangement. This arrangement is based on the assumption that the primary pressure at the physical and at the virtual microphone locations are similar. The implication of this assumption on the acoustic performance of a local system in a diffracted primary sound field is theoretically studied. The results show that the error at the virtual microphone position is lower when the virtual microphone arrangement is in the vicinity of a diffracting surface. A practical local active noise control system in a headrest has been built and used to measure the zone of quiet produced by a single and a dual channel system when the total pressure is canceled at one or two virtual microphone positions. It is shown that this type of arrangement is capable of projecting the zones of quiet further away from the secondary source than the position of the physical microphone. The measured zones of quiet produced by a single-channel system have been compared with the results produced with a theoretical model which includes the diffraction between two spheres, one representing the secondary source and the other the listener’s head. It has been found that, in the frequency range of applicability of a local active noise control system, the two-sphere model predicts the experimental results well. The effect that the diffracting head has on the performance of an adaptive feedforward controller using a physical and a virtual microphone is also explored and the results show that this effect is small provided the initial system identification is performed in the presence of the head.

ISSN:0001-4966    

DOI:10.1121/1.418357

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An efficient modal approach to characterize the diffuseness of the sound field in a rectangular room is presented. Using two simple descriptors, the correlation function and the spatial uniformity of the pressure field, a practical and convenient tool is proposed to study the diffuse field in the room. A precise criterion has been given in terms of the least permissible number of room modes to achieve an adequate diffusion. It has been shown that the criterion is in great accordance with the well-known “Schroeder frequency” limit for the diffuse field. Detailed calculations of the correlation function are presented to show the importance of theΔk/kcorrection terms to the well-knownsin(kR)/kRprediction. A new closed form for the correlation function is thus derived. A discussion about the importance of having more than one descriptors is also presented. It is shown that a diffuse field can be established in a room with strong modal behavior under certain assumptions.

ISSN:0001-4966    

DOI:10.1121/1.418313

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper describes a general method to localize multiple broadband sources in a range- and azimuth-dependent environment when properties of the environment are not well known. Both the source locations and the unknown environmental parameters are included as variables in a simulated annealing inversion that searches for the best match between the measured and modeled fields. The example used to illustrate the method involves localizing an unknown number of ice-ridge building events in an Arctic environment where the bathymetry is poorly known. A number of aspects of this problem are examined in a synthetic study, including the effect of errors in the bathymetry on localization, the importance of simultaneous localization, and determining the number of sources present. The inversion accurately localizes multiple synthetic sources, particularly in cases where the sources are spatially separated and the spectral shape is known. The inversion is also applied to a set of ambient noise measurements recorded in the Lincoln Sea. The results indicate that a small number of ice-ridging sources is sufficient to accurately model the measured fields. Estimates of the number of sources and the source bearings are unambiguous and are reasonably consistent between samples; however, source ranges could not be estimated unambiguously.

ISSN:0001-4966    

DOI:10.1121/1.418382

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Interference from noise and uncertainties in the environmental parameters are arguably the two most serious limitations in matched-field processing (MFP). Among the techniques that have been developed for handling these difficulties are the noise-canceling processor [M. D. Collins, N. C. Makris, and L. T. Fialkowski, “Noise cancellation and source localization,” J. Acoust. Soc. Am.96, 1773–1776 (1994)] and focalization [M. D. Collins and W. A. Kuperman, “Focalization: Environmental focusing and source localization,” J. Acoust. Soc. Am.90, 1410–1422 (1991)]. The noise-canceling processor is a generalization of the Bartlett processor that is based on matching the covariance matrix of the data with replica covariance matrices of the signal and the noise. Simulations are presented to illustrate the performance of the noise-canceling processor when there are errors in the noise replica. Focalization is a generalization of MFP in which environmental parameters are included along with source parameters in the search space. An implementation of this approach that is suitable for applications is developed and tested. The noise-canceling processor and focalization are used to simulate the localization of a source buried in noise in an uncertain environment.

ISSN:0001-4966    

DOI:10.1121/1.418314

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The CBA mouse shows little evidence of hearing loss until late in life, whereas the C57BL/6 strain develops a severe and progressive, high-frequency sensorineural hearing loss beginning around 3–6 months of age. These functional differences have been linked to genetic differences in the amount of hair cell loss as a function of age; however, a precise, quantitative description of the sensory cell loss is unavailable. The present study provides mean values of inner hair cell (IHC) and outer hair cell (OHC) loss for CBA and C57BL/6 mice at 1, 3, 8, 18, and 26 months of age. CBA mice showed little evidence of hair cell loss until 18 months of age. At 26 months of age, OHC losses in the apex and base of the cochlea were approximately 65% and 50%, respectively, and IHC losses were approximately 25% and 35%. By contrast, C57BL/6 mice showed approximately a 75% OHC and a 55% IHC loss in the base of the cochlea at 3 months of age. OHC and IHC losses increased rapidly with age along a base-to-apex gradient. By 26 months of age, more than 80% of the OHCs were missing throughout the entire cochlea; however, IHC losses ranged from 100% near the base of the cochlea to approximately 20% in the apex.

ISSN:0001-4966    

DOI:10.1121/1.418315

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The fine structure of the2 f1−f2acoustic distortion product (ADP) was measured in humans with different primary level(L1/L2)and frequency(f2/f1,f2>f1)ratios. The(L1/L2)ratio was varied under two conditions. In the first conditionL1was fixed at 50 dB SPL whileL2was varied from 30 to 75 dB SPL in 5-dB steps. An upward frequency shift was observed in the ADP fine structure asL2was increased. In the second condition,L2was fixed at 50 dB SPL andL1varied, and a downward frequency shift was observed. These opposing frequency shifts are predicted by a vector-sum model [Sun et al., J. Acoust., Soc. Am.96, 2166–2174, 2175–2183 (1994)] and support the hypothesis that the ADP fine structure largely reflects place features of the area of overlap of the primary traveling waves. The mechanisms underlying the shifts in fine structure were further investigated by using three primaryf2/f1ratios: 1.11, 1.2, and 1.33. An orderly difference in the rate of fine-structure shift with level was observed as a function off2/f1ratio, with the largest rate of shift associated with the smallest frequency ratio. This observation, along with the fact that downward frequency shift (withL1varied) is always at a larger rate than the upward shift (withL2varied), suggests that ADP levels and fine structure are strongly influenced by the nonlinear compression present in the mechanics of the basilar membrane in the region of overlap between the primary traveling waves.

ISSN:0001-4966    

DOI:10.1121/1.418316

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Transiently evoked otoacoustic emissions (TEOAE) have been reported in several studies as absent in a small minority of normal ears. Other studies have reported TEOAEs in all normal ears. Differences between studies may arise directly from criteria for TEOAE identification, criteria for selection of normals, or statistically due to limited sample sizes. In order to understand and model cochlear processes involved in TEOAE generation, it needs to be known whether the presence of normal hearing leads automatically to generation of TEOAEs. The present study set out to establish in a large sample if any ears could be found that lacked TEOAEs despite normal hearing threshold levels (HTL). A total of 397 ears from highly cooperative adult subjects were examined under laboratory conditions. Using cross correlation between replicate nonlinear waveforms as the criterion, TEOAEs were present in 99.2% of the sample (lower CI 98.1%). However, careful visual assessment of the recorded waveforms for the remaining ears did not unequivocally show absence of TEOAE characteristics in any ear with normal HTLs. While TEOAE strength varies widely among ears, no clear evidence was found to show that TEOAEs can be absent when HTLs are normal.

ISSN:0001-4966    

DOI:10.1121/1.418317

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

To test if click-evoked otoacoustic emissions (CEOAEs) have frequency specificity, continuous changes in CEOAEs (especially frequency components of the CEOAE power spectrum) after pure-tone exposure in guinea pigs were examined. Pure-tone stimuli (0.5 kHz, 120 dB SPL; 2 kHz, 115 dB SPL; 4 kHz, 110 dB SPL) were given in a closed system for 3 min. After exposure, the frequency components in the CEOAE power spectrum decreased maximally at one-half octave or more above the overstimulation frequency. They partially recovered 2 h after exposure. The time course of compound action potential (CAP) thresholds after exposure was similar to that of the frequency components of the CEOAE power spectrum. It was concluded that some local damage caused by outer hair-cell dysfunction in the guinea pig cochlea can be detected by measuring shifts in frequency components in the CEOAE power spectrum.

ISSN:0001-4966    

DOI:10.1121/1.418318

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The purpose of this investigation is to present evidence from experimental as well as model results on temporal variations of the frequency of oscillation in the basilar membrane’s impulse response. Stimuli were either clicks leading to adirectestimate of the impulse response, or bands of pseudo-random noise (one or two octaves wide) which lead to anindirectestimate of the impulse response via a cross-correlation procedure. The noise bands were centered at the best frequency of the BM location under observation. Responses were obtained from the basal turn of the guinea-pig cochlea, from a location with a best frequency (for the weakest stimuli) between 17.0 and 18.5 kHz. Data acquisition was done with a sample frequency of 208 kHz. Input–output cross-correlation functions were found to share with impulse responses the property that the initial oscillations have a noticeably lower frequency than the later ones. During the impulse response the frequency of oscillation increases gradually. This increase occurs and continues to beyond the time that the oscillations reach the largest amplitude. This frequency variation is called a “glide.” Using the “analytic signal” method the frequency of oscillation is found to increase continually throughout the duration of the main lobe of oscillation, even at the lowest tested stimulus intensities (about 20 dB SPL). At high stimulus intensity both the direct and indirect impulse response change their appearance drastically but the glide retains its basic form. In the case of the direct impulse response estimate the glide can be attributed to temporal variation of the degree of nonlinearity. For the indirect impulse response this is not true, because with a constant level noise stimulus there is no regular temporal variation of nonlinearity. In this case the glide should be interpreted as an intrinsic property of the cochlear system. From our and others’ data the glide was found to exist over a topographic frequency range of best frequencies of at least from 1.76 to 18 kHz. Two examples of present-day models of the cochlea are discussed of which one is found to demonstrate the glide phenomenon in its response, and the other one does not.

ISSN:0001-4966    

DOI:10.1121/1.418319

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Current hypotheses regarding mechanotransduction rely upon motion of the stereocilia relative to the apical surface of the hair cell. The viscosity of the surrounding endolymphatic fluid will, however, attenuate stereocilia motion at higher frequencies of excitation. To investigate stereocilia motion for physiologically reasonable deflections and frequencies of excitation, the fluid–structure interaction of the stereocilia bundle is considered analytically. Solutions in the frequency domain are determined for stereocilia bundle dimensions at several locations along the cochlear duct of the chinchilla. Results indicate that motion of the stereocilia is analogous to that of a low-pass filter. Comparison of these solutions with Greenwood’s frequency-place map demonstrates that motion of the stereocilia bundle exists without substantial attenuation at least up to frequencies appropriate for the location of the corresponding hair cell along the cochlear duct. The variation in stereocilia morphology within the mammalian cochlea thus appears to provide a collection of low-pass mechanoreceptors, arranged in order of increasing corner frequency across the auditory spectrum.

ISSN:0001-4966    

DOI:10.1121/1.418320

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP