Volume 108, Issue 3, September 2000
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Acoustic impedances at the oval window, and sound pressure transformation of the middle ear in Norwegian cattle108(2000); http://dx.doi.org/10.1121/1.1287027View Description Hide Description
In 15 cadaver ears from Norwegian cattle, sound pressure transfer functions have been measured (1) for sound input to the tympanic membrane, (2) for sound input to the oval window with the footplate in place, but with the ossicular chain removed, and (3) for sound input to the oval window with also the footplate removed. The output pressure was measured in an enclosure cemented to the round window. The data allow calculation of equivalent sound pressures at the input positions, as well as the acoustic input impedances at the oval window with intact footplate, and with the footplate removed, The difference is the acoustic impedance contribution of the footplate and annular ligament. is mainly determined by the stiffness of the annular ligament at low frequencies, and by the cochlear input impedance at higher frequencies. is predominately resistive, a minor reactive part at low frequencies is attributed to the stiffness of the round window membrane. and are equal in magnitude at about 0.4 kHz. Rather close RLC fits have been obtained for all the three impedances, and The fitted values for the resistive parts of and are 62.9 and 58.2 acoustic GΩ, respectively. The relatively small difference, 4.7 GΩ, is attributed to the resistance of the annular ligament. The fitted resistance of is somewhat larger, 8.6 GΩ, but is anyway of minor importance relative to the dynamic stiffness of the annular ligament. This stiffness depends on the static pressure difference across the footplate. Each of the averaged corresponds to minimum stiffness. The fitted acoustic compliance is The acoustic inertance plays a minor role. It is attributed to the mass of the footplate and the co-vibrating liquid in the inner ear, and has a fitted value of A sound pressure at the eardrum is equivalent to a larger pressure at the footplate, about 16 dB larger at frequencies below 100 Hz, increasing to about 30 dB at 10 kHz. In the vestibulum at the inner side of the footplate, the sound pressure at 20 Hz is about 20 dB below the equivalent pressure at the outer side. The two pressures approach toward higher frequencies, and above 1 kHz they are nearly equal.
108(2000); http://dx.doi.org/10.1121/1.1287026View Description Hide Description
An algorithm is described for objectively identifying and measuring spontaneous otoacoustic emissions (SOAEs) using the spectrum that results from transformation of the acoustic waveform measured in the outer ear canal. Prior to spectralanalysis, the rms level is calculated for successive short segments of the waveform and only the weakest 25% of the segments are retained for the spectralanalysis [the quietest 150 when using 16k-point fast Fourier transforms (FFTs)]. The resulting initialspectrum is scanned for peaks (potential SOAEs) which are then deleted from the spectrum. New values are estimated for the deleted values using linear extrapolations from frequency ranges on either side of the deleted values. The end result is a smoothedspectrum devoid of all local peaks. The initial spectrum is then compared peak-by-peak with the smoothed spectrum, and those peaks having differences that exceed an objectively determined decision criterion are identified as likely SOAEs. The effects of varying some of the important parameter values of the algorithm are described, and the sensitivity of the procedure is evaluated by measuring the detection rate for a Lorentzian peak of known amplitude added to a spectrum otherwise devoid of SOAEs.
On the spectral periodicity of transient-evoked otoacoustic emissions from normal and damaged cochleas108(2000); http://dx.doi.org/10.1121/1.1288936View Description Hide Description
The spectral quasi-periodicity of transient-evoked otoacoustic emissions (TEOAE) is well acknowledged since Zwicker described a preferred spacing of 0.4 bark between consecutive peaks in the spectrum of otoacoustic emissions from normal ears. While there is scarce evidence of any anatomical reason for this regularity, several functional models of the cochlea have predicted that the structure of emission spectra reflects important characteristics of cochlear filters. In an attempt to check such predictions, the average regularity of TEOAE spectra was studied in three groups of human subjects, normally hearing adults, healthy neonates, and adults suffering from noise-induced hearing loss. Significant differences in emission periodicities were found. Around 1 kHz, the preferred spacing was close to 130 Hz in normally hearing adult ears and neonates. In contrast, no clear periodicity was found in the group of damaged ears, even though they had clinically normal pure-tone audiometry below 2 kHz. Around 4 kHz, the preferred spacing was close to 240 Hz in normal adults and neonates, whereas TEOAEs were absent in many impaired ears. A phenomenological model assuming that TEOAEs stem from the responses of a slightly disarrayed bank of highly tuned filters predicts that the filter width would be the same in healthy young adults and neonates. In contrast, ears suffering from high-frequency hearing loss could exhibit early damaged filters. The proposed method might provide an objective assessment of parameters otherwise difficult to evaluate, especially in neonatal cochleas.
108(2000); http://dx.doi.org/10.1121/1.1287024View Description Hide Description
The acoustic admittance at the tympanic membrane (TM), describes the linear acousticproperties of the ear. Here, a noninvasive measurement procedure is developed for estimating in intact ears. The method consists of (1) measuring the admittance in the ear canal with a commercially available earphone-and-microphone system, and (2) estimating via a uniform-tube approximation of the space between the measurement point and the TM. The dimensions of this space are estimated from via an area-estimation algorithm [Keefe et al., J. Acoust. Soc. Am. 91, 470 (1992)] and measurements made with controlled static pressures in the canal. Measurements in artificial loads are used to test the accuracy of the measurement system and to determine sources of error. For accurate admittance measurements: (1) extension of the microphone tube medially beyond the earphone’s port is necessary for frequencies above 2 kHz; (2) the acoustic system must be calibrated in known loads with diameters within 15% of the canal diameter, because the source’s output characteristics vary with load diameter. The method is applied to intact ears of anesthetized domestic cats; for frequencies below 5 kHz, the estimated in four ears have features that are similar to those of previous measurements made at the cat TM. Sources of error include nonuniform waves generated at the earphone’s narrow port, inaccuracy in estimation of canal dimensions, irregular geometry of the canal, and earphone-microphone cross talk.
108(2000); http://dx.doi.org/10.1121/1.1287025View Description Hide Description
The accuracy of ear-canal admittance and reflectance as measures of the ear’s properties depends on the acoustic effects of the canal. Here, measurements of acoustic admittance at different canal locations in domestic cats are used to test three common assumptions. (1) Can a uniform-tube model of the canal represent spatial variations in admittance? Data from cats support this assumption for frequencies below 3 kHz, where the admittance inferred at the tympanic membrane (TM) based on a uniform-tube model differs by less than 3 dB in magnitude and 0.07 periods in angle from the admittance measured at the TM; for higher frequencies greater differences occur. (2) Do large static air pressures in the canal make the middle ear rigid without affecting the properties of the canal space? The measurements reported indicate that large negative static pressures reduce the low-frequency compliance of the cat middle ear to about 10% of the compliance of the canal air volume. Static displacements of the acoustic probe, TM, and canal walls with static pressure may affect estimates of the canal volume and middle-ear compliance by as much as 15% to 20%. (3) Is the acoustic-reflectance magnitude constant with position along the canal? Reflectance data from cat ear canals generally support this idea, except within a frequency region near 0.5 kHz for which there is evidence of energy loss. These results demonstrate that noninvasive measurements in the canal describe middle-ear acoustic properties to within tolerances that depend on the effects of the canal.