External-Ear Replica for Acoustical Testing
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Abstract:
As part of the development of an acoustical manikin, an artificial ear was designed to simulate the mechanical and acoustical properties of the external ear, up to and including the impedance of the eardrum. The sensing element is a B&K 4132 electrostatic microphone terminating a simulated ear canal with an acoustical impedance-matching network that, combined with the microphone, furnishes the eardrum impedance. The canal proper has dimensions approximating those of the real ear and is placed inside a skull of polyester-impregnated fiberglass, provided with a plastisol pinna of realistic dimensions and texture. The head is mounted on a fiber torso. The new artificial ear is suitable for testing all types of receivers and ear enclosures under realistic conditions. The inner portion of the artificial ear is made of reproducible metallic components, making it suitable for consideration as an artificial-ear standard.Keywords:
Human ear
Acoustic impedance
Pinna
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A study was conducted to describe a probe microphone assembly to monitor sound pressures at the eardrum for hearing conservation. The probe microphone assembly permitted the uncertainty in probe-tip position to be eliminated and the transfer function to the eardrum to be estimated. The device was attached to an ear mold designed to self-locate within the concha and ear canal and position the probe tip reproducibly. The device consisted of a miniature probe microphone and a customized ear-mold. The ear mold was fabricated from an ear impression that was obtained for each subject following established audiological procedures. The ear impression provided the contour of the perimeter of the concha and cymba, which was required to produce the 'C'-structure used to position the ear mold external to the ear canal.
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Recent work has demonstrated that the geometry (size, shape) of the human ear canal leads to a sound field, at high frequencies, that varies within the canal in a complicated fashion. In particular, the eardrum terminates the ear canal at a rather sharp angle, and significant variations of sound pressure (over 15 dB) arise over the eardrum surface above 10 kHz. The implications of these observations, as they apply to several aspects of hearing research, will be discussed, utilizing a three-dimensional horn equation approach that has been developed. Knowledge of the sound pressure distribution within the ear canal is important in the extension of audiometry to high frequencies, and for guiding the development of hearing aids. Studies of auditory processes that use a reference microphone in the ear canal become increasingly sensitive to microphone location as higher frequencies are used; estimates of the errors of reproducibility are given. Present acoustic network models of the middle ear assume a uniform sound pressure at the eardrum; possible ways of handling a non-uniform pressure distribution are discussed.
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An artificial ear was designed to simulate the mechanical and acoustical properties of the external ear, up to and including the impedance of the eardrum. The sensing element is a B & K 4132 electrostatic microphone terminating a simulated earcanal with an acoustical impedance-matching network that, combined with the microphone, furnishes the eardrum impedance. The canal proper has dimensions approximating those of the real ear and is surrounded by a plastisol pinna of realistic dimensions and texture. The new artificial ear is suitable for testing all types of receivers and ear enclosures under realistic conditions. The inner portion of the artificial ear is made of reproducible metallic components, making it suitable for consideration as an artificial-ear standard. [Work supported by NASA-Manned Spacecraft Center, Houston, Texas.]
Human ear
Acoustic impedance
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This publication contains a review of several acoustic investigations in which the effects of probe location on real-ear gain were examined through theoretical models based on acoustic properties of the average human ear and ear simulator studies. The results of these investigations are used to demonstrate the effect of standing waves and eardrum impedance on probe measurements made in the ear canal. Investigations were also conducted in the sound field with a KEMAR manikin. A commercial probe microphone system was used to measure the SPL and real-ear gain at various locations within the KEMAR ear canal. The results emphasize the critical effect of probe location on absolute or relative ear canal measurements and indicate the necessity to establish clinical procedures for probe measurements based on relevant acoustic principles.
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Acoustic impedance
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Probe-microphone measurements of the real-ear response of an earphone are subject to inherent problems of interpretation. The cushion-enclosed sound field is homogeneous only in the frequency region below 1 kHz, and measurements outside this band can differ by more than 15 dB from one probe position to another. Probe measurements at the earphone diaphragm do not provide enough information on earphone response, because the earphone can create or modify resonances in the outer ear that affect the pressure delivered to the eardrum. Measurements at the canal entrance provide too much information, because they include pinna gain characteristics that are not functions of earphone performance. In the present experiments, the response of an earphone is derived by subtracting, from earphone-induced sound pressures at the eardrum, the sound pressures produced at the same point by a loudspeaker that the pinna faces. The speaker is used to establish a reference of outer ear acoustic gain before disturbance by the earphone, less the effects of head diffraction. The eardrum position was selected for the probe-tube measurements after a preliminary experiment showed that a supraaural earphone can affect ear canal gain by constricting the canal entrance and lowering the canal's resonant frequency. The derived response curves were in agreement with earphone frequency-response data derived from subjective judgments by the corresponding subjects.
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A two-microphone technique was used to determine the middle ear impedance of a live subject. the procedure involved the application of standing wave tube theory and the assumption that the ear canal behaves like an homogeneous cylinder with plane acoustic wave propagation up to a certain frequency-2 kHz for the current analysis. During experimentation the subject lay on a bench with his head braced against a wooden fixture. Acoustic pressures were recorded from the ear canal by the use of a spectrum analyser and probe microphones with flexible tips. Resultant impedance curves show middle ear natural frequencies at 831 Hz and 1, 970 Hz with high levels of damping. the reactive impedance curves show the influence of stiffness and ossicular mass on middle ear sound transmission. An advantage of the approach is that using features of the recorded data it is possible to calculate the effective probe tip to eardrum distance required for the calculation of middle ear impedance. the two-microphone technique appears to be a promising tool for assessing healthy and diseased middle ear function.
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Since 1978, when Kemp reported the existence of stimulated acoustic emissions from within the human auditory system [J. Acoust. Soc. Am. 64, 1386–1391], several studies have indicated that acoustic signals originating within the cochlea propagate through the middle ear into the ear canal. We have used nonlinear cochlear mechanical models to interpret distortion products observed both in the acoustic signal in the ear canal and in the response patterns of single cochlear nerve fibers. In addition to a nonlinear cochlear model, our modeling system includes a bidirectional middle ear model and an acoustic coupler model; these additional elements are needed in order to make quantitative comparisons between model results and the experimental observations, The mechanical middle ear model we have developed is based on experiments measuring the forward transmission characteristics of the auditory systems of cats [e.g., Guinan and Peake, J. Acoust. Soc. Am. 41, 1237–1261 (1967)], and is adequate for frequencies below 3 kHz. In order to extend the model to higher frequencies a more elaborate representation of the motion of the eardrum will have to be included. Our model results give indirect support for the adequacy of our middle ear model with respect to reverse transmission, but direct experimental measurements of reverse transmission properties of the middle ear are needed. [Supported by NIH grants NS07498, RR00396, NS00162, GM01827.]
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As part of the development of an acoustical manikin, an artificial ear was designed to simulate the mechanical and acoustical properties of the external ear, up to and including the impedance of the eardrum. The sensing element is a B&K 4132 electrostatic microphone terminating a simulated ear canal with an acoustical impedance-matching network that, combined with the microphone, furnishes the eardrum impedance. The canal proper has dimensions approximating those of the real ear and is placed inside a skull of polyester-impregnated fiberglass, provided with a plastisol pinna of realistic dimensions and texture. The head is mounted on a fiber torso. The new artificial ear is suitable for testing all types of receivers and ear enclosures under realistic conditions. The inner portion of the artificial ear is made of reproducible metallic components, making it suitable for consideration as an artificial-ear standard.
Human ear
Acoustic impedance
Pinna
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Standing waves in the ear canal can cause inaccurate quantification of the sound pressure level (SPL) entering the ear and therefore lead to unreliable results in clinical tests. Since it is impractical to directly measure the SPL at the eardrum position, in this study we proposed a new method to estimate the eardrum SPL by solely making measurement at the entry of the ear canal. To achieve this, the acoustic characteristics of the earphone were calculated using a calculation tube with variable lengths. Then the ear canal impedance was calculated according to the obtained source characteristics. Finally, the eardrum SPL was estimated by the ear-canal impedance and the SPL measured at the entry of the ear canal. The results showed that the eardrum SPL could be reliably estimated for all the five subjects participated in this study. The maximal estimation error was less than 3 dB for all frequencies from 0.5 to 10 kHz. These findings suggested that the proposed method could avoid the standing wave problem and therefore might be a great candidate for accurate calibration of sound pressure in various acoustic measurements.
Acoustic impedance
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A coupler for earphone calibration has been developed that mimics the acoustic properties of the outer ear and has a simple geometry. It consists of an outer portion corresponding to the auricle, a canal corresponding to the ear canal, and an acoustic network reproducing the acoustic impedance at the eardurm. The sound pressure is measured at the eardrum location by means of a 14-in. condenser microphone. According to comparative measurements, the acoustic impedance at the end of the coupler canal follows the average eardrum impedance with satisfactory accuracy within the frequency limits of 100 and 7000 Hz. The coupler also produces essentially the same pressure transformation between the microphone, midcanal, canal entrance, outer rim, and 1 cm outside the outer rim as the pressure transformation in an average ear between corresponding points. The coupler may be used for supraaural earphones as well as for insert and circumaural earphones. It can be manufactured according to rigorous specifications.
Acoustic impedance
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