摘要:

The pressure equalization vents of conventional microphones introduce bias errors in measurements of the sound intensity. Two error terms in the estimated active intensity are derived using a low‐frequency model of microphone. The first error term is associated with the lower‐limiting frequency of the microphones (1 to 2 Hz) and is proportional to the reactive sound intensity. It is shown here that the difference between limiting frequencies of the two microphones causes the second error term to be proportional to the mean‐square pressure and to be of comparable importance at low frequencies. The amplitudes of errors due to the vents are analyzed in a quasistanding wave. Unlike the first error, the second can be corrected at the same time as the phase error between measurement channels, and several correction techniques are examined. For the error proportional to the reactive intensity, a correction method in a standing wave tube is suggested, using the joint measurement of several other energetic quantities. Examples of the influence of these errors on typical parameter values for half‐inch microphones are presented in the case of measuring the acoustic impedance of materials using a two‐microphone probe.

ISSN:0001-4966    

DOI:10.1121/1.414625

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Federal and international standards recommend use of microphone placement either on or in the vicinity of the shoulder for dosimetry to minimize deviations from the undisturbed sound field. Probe microphone measurements from the ear canal were compared to shoulder and chest measures in order to investigate the validity of current dosimetry methodologies. Six subjects were monitored in an industrial setting. As expected, ear‐canal levels exceeded other measures for all subjects. Shoulder and chest measures showed very low intersubject variability whereas ear‐canal measures resulted in large intersubject variability. The ear‐canal methodology has the potential to identify individuals whose external ear gain exceed the mean, putting them at increased risk of noise‐induced permanent threshold shift (NIPTS). It is proposed that overall external ear pressure gain be used as an index to adjust exposure levels when predicting NIPTS using ISO 1999. A normative database of external ear pressure gain was constructed from 30 ears for this purpose.

ISSN:0001-4966    

DOI:10.1121/1.414626

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The subject of this paper is long enclosures, where one dimension is much greater than the other two, although the other two are still relatively large compared to the acoustic wavelength. A computer model is given for calculating the acoustic indices in long enclosures with multiple sources. Calculations in two long enclosures show that the reverberation at a receiver can be significantly increased by adding sources beyond a single source and could be decreased by adding sources between a single source and the receiver. The efficiency of reducing the reverberation of multiple sources by more absorption and directional sources is analyzed. It is also demonstrated that for a multiple loudspeaker public address system, depending on room and loudspeaker conditions, there is an optimal loudspeaker spacing for the speech transmission index (STI).

ISSN:0001-4966    

DOI:10.1121/1.414627

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Measurements of the sound absorption coefficients of blocks of occupied theatre chairs varied approximately linearly with the ratio of sample perimeter‐to‐area ratio. Extrapolations of such measurements were used to predict the absorption coefficients of occupied chairs in auditoria. Analyses of these and other published data show that the absorption coefficients of occupied chairs also vary with the absorbing properties of the chairs. A more general method for predicting the sound absorption of occupied chairs as a function of sample perimeter‐to‐area ratio is proposed based on classifying chairs as having low, average, or high absorption characteristics. The new method is simple to apply and is thought to be more accurate than currently accepted methods.

ISSN:0001-4966    

DOI:10.1121/1.414562

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A systematic study has been undertaken of the influence of diffraction on ultrasonic absorption coefficient and phase velocity measurements made using the through‐transmission substitution technique. A diffraction correction model dealing explicitly with the interaction of the sample with the plane‐ and edge‐wave components of the acoustic field of a piston transducer is introduced. The National Physical Laboratory ultrasonic materials characterization facility has been used to validate the model using a pulsed technique covering the frequency range 1–15 MHz. In the validation, absorption coefficient and phase velocity measurements are presented for test specimens of cross‐linked polystyrene of various thickness. Strong diffraction effects have been demonstrated in the measurements, particularly for the absorption coefficient where measured values can be typically a factor of 2–3 higher than plane‐wave values. Excellent agreement is demonstrated between theory and measurement for both absorption and phase velocity under a wide range of measurement conditions.

ISSN:0001-4966    

DOI:10.1121/1.414628

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The fine structures of 2f1−f2acoustic distortion products (ADP) were measured in four groups of human subjects: (1) young with normal hearing, (2) aged with normal or near‐normal hearing, (3) aged subjects with more severe hearing loss above 2000 Hz, and (4) young with hearing similar to those of the normal‐ or near‐normal‐hearing aged group. The purpose of this study was to investigate how ADP fine structure changes with age and hearing loss. Results show that the fine structure is observable whenever the ADP is measurable, and its sharpness, defined as the peak‐to‐peak frequency distance, is independent of age, hearing loss, and longitudinal amplitude change. There is a larger variance in ADP peak amplitudes in aged subjects with or without hearing loss as compared to young normal‐hearing subjects, the aged group generally having lower peak amplitudes than those of the young group. According to an ANOVA analysis with absolute threshold as covariate, the null hypothesis stating that there is no aging effect on the ADP peak responses cannot be ruled out, a conclusion similar to that made by Stover and Norton [J. Acoust. Soc. Am.94, 2670–2681 (1993)]. On the other hand, threshold changes cannot explain all the differences in ADP amplitude. It is clear that the relationship between ADP amplitudes and behavioral thresholds can differ between young and aged populations. At primary levels of 50 dB SPL, almost normal ADP magnitudes are often found in aged subjects even when associated with a 35‐dB hearing loss, whereas for young subjects ADPs typically were not measurable when associated with the pure‐tone thresholds of 20 dB HL or higher. This dichotomy may be explained by the different etiologies of the cochlear pathologies predominant in the two groups.

ISSN:0001-4966    

DOI:10.1121/1.414629

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A novel analysis approach has been developed to examine the latency of distortion product otoacoustic emissions (DPOAEs). DPOAEs were measured in ten normal‐hearing adults in a paradigm in whichf2was held constant andf1was varied. This paradigm was used with a wide range of primary levels. Latency was estimated in two ways. In the first, a phase‐slope delay measurement was used which showed a significant response latency increase as stimulus intensity was decreased. In the second approach, an inverse‐FFT procedure was used to provide a temporal analysis of the data. Results of this analysis reveal a complex latency structure with multiple peaks in the envelope of the time waveform. The latencies of individual peaks remain constant across level, however short latency peaks have the greatest amplitudes at higher levels, and longer latency peaks are largest at low levels. These results would be consistent with the idea that there are multiple intracochlear sources for distortion product generation; however, a simple model, in which generation is assigned to thef2and the 2 f1−f2place, does not adequately explain the number of envelope peaks that were present in many ears.

ISSN:0001-4966    

DOI:10.1121/1.414630

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Outer hair cells (OHCs) were subjected to three different hypo‐osmotic challenges which induced shape changes in OHCs. The longitudinal and circumferential strains, εzand εc, respectively, were extracted from a morphometric analysis of video images of the cells. A value of εz/εc=−0.72 was obtained and was found to be independent of the applied osmotic gradient. Using an elastic isotropic membrane model for the OHC lateral wall, the ratio of the elastic shear modulus μ to the elastic area expansion modulusKwas calculated yielding a value of 0.054. The contribution of this ratio to OHC mechanics is discussed.

ISSN:0001-4966    

DOI:10.1121/1.414631

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

From previous studies it appears that at least two factors limit the upper frequency at which auditory‐nerve (AN) fibers can entrain to the envelope of a sinusoidally amplitude‐modulated (AM) tone. Cochlear mechanical filtering insures that, in the local motion driving a fiber tuned to the carrier, sidetone amplitudes decrease as sidetone displacement envelopes separate with modulation frequency (fm). Only if at least one sidetone’s amplitude is large enough, relative to carrier’s, will there be modulation of basilar motion at the point tuned to the carrier. In addition, processes within haircell and fiber limit the upper frequency at which they follow variations in amplitude. To assess change, along the cochlea, in the two factors’ relative importance, AN modulation transfer functions (MTFs) [Joris and Yin, J. Acoust. Soc. Am.91, 215–232 (1992)] were replotted versus distance in mm between sidetones and carrier, using an empirical frequency‐place function [Greenwood, J. Acoust. Soc. Am.87, 2592–2605 (1990)]. MTF bandwidths, converted to mm, changed little over the apical 40% of cochlea but decreased basally. Expressed in Hz, they increased from apex to base and reached an upper limit at a characteristic frequency (CF) of 20 kHz. This is consistent with the idea that at high CFs phase‐locking constraints limit envelope‐following before mechanical filtering does, while in apical regions spatial filtering reduces envelope amplitude variation, hence envelope following, before limits on phase locking do. Consistently, MTF bandwidths parallel tuning curve bandwidths in the apical cochlea, but are smaller near the base, where tuning curve bandwidths and spatial filtering appear constant.

ISSN:0001-4966    

DOI:10.1121/1.414632

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Driven responses of cat auditory‐nerve fibers to long‐duration characteristic‐frequency (CF) tones could decrease substantially over time periods ranging from seconds to minutes. In extreme cases, the discharge rate could fall below the pre‐stimulation spontaneous rate (SR). Reductions in response were characterized by two processes, each of which followed a decaying exponential function. Long‐term adaptation affects the discharge rate in the first several seconds following stimulus onset. The average amount in high‐SR fibers was 42.5% for tones at 20–40 dB SL, and the mean time constant was 3.64 s. Long‐term adaptation increased significantly with sensation level (SL, or level above threshold), decreased with SR, and was not significantly correlated with CF or fiber response threshold. Time constants did not depend on CF, SR, or SL. Very‐long‐term adaptation refers to further, smaller reductions in the discharge rate that accumulate over a period of minutes. Fiber responses formed two groups. The larger group adapted with a mean time constant of 45.22 s for CF tones at 20–40 dB SL, and the smaller group did not adapt over very long terms.Considerable variability in amounts of long‐term and very‐long‐term adaptation was observed in similarly tuned fibers from the same ear. This suggests that long‐term effects do not arise from cochlear mechanics or middle ear muscle activity. No long‐term effects were observed in responses of fibers directly stimulated by high‐intensity electrical pulses presented at rates up to 500/s through a cochlear implant. This suggests that the effects do not arise from fundamental differences in spike‐generating properties of spiral ganglion cells. The data suggest that long‐term adaptation may occur either when neurotransmitter utilization at inner hair cell synapses exceeds the rate at which it is replenished from global stores or uptake mechanisms, or when metabolic resources influencing neurotransmitter release become depleted. The neural data are related to perceptual findings in human listeners, in which unusually large amounts of tone decay may be observed at high frequencies, and they indicate that the perceptual effects originate peripherally.

ISSN:0001-4966    

DOI:10.1121/1.414633

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP