Basic Acoustic Considerations of Ear Canal
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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.Keywords:
Human ear
Acoustic impedance
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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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.
Human ear
Acoustic impedance
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A non-invasive method of measuring or estimating accurately the head-related transfer functions (HRTFs) and headphone transfer functions (HpTFs), i.e., the pressure at the eardrum rather than at the blocked ear canal entrance is called for. In this work, a miniature-sized acoustic pressure-velocity sensor is used to measure both pressure and velocity along the ear canals of human test subjects. The measurements are used to study the applicability of a recently proposed method of estimating the pressure at the eardrum from pressure-velocity measurements made at the ear canal entrance. The measurement results are compared to results from computational modeling with human ear canal parameters. In addition, the effect of the PU-sensor itself on the pressure at the eardrum is studied. It is shown that the estimation method is reliable and accurate for most human subjects. The diameter and the shape of the ear canal affect the results in such a way that the best results are obtained with wide and straight ear canals. It is concluded that the estimation method facilitates the obtaining of individual HRTFs and HpTFs at the eardrum using non-invasive measurements.
Human ear
Pressure measurement
Head-related transfer function
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A non-invasive method of measuring or estimating accurately the head-related transfer functions (HRTFs) and headphone transfer functions (HpTFs), i.e., the pressure at the eardrum rather than at the blocked ear canal entrance is called for. In this work, a miniature-sized acoustic pressure–velocity sensor is used to measure both pressure and velocity along the ear canals of human test subjects. The measurements are used to study the applicability of a recently proposed method of estimating the pressure at the eardrum from pressure–velocity measurements made at the ear canal entrance. The measurement results are compared to results from computational modeling with human ear canal parameters. In addition, the effect of the PU-sensor itself on the pressure at the eardrum is studied. It is shown that the estimation method is reliable and accurate for most human subjects. The diameter and the shape of the ear canal affect the results in such a way that the best results are obtained with wide and straight ear canals whereas less accurate results are obtained with narrow and curved ear canals. It is concluded that the estimation method facilitates the obtaining of individual HRTFs and HpTFs at the eardrum using non-invasive measurements.
Human ear
Pressure measurement
Head-related transfer function
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A consistent description of the human external ear must take into account a diversity of data concerning (a) the transformation of sound-pressure level from the free field to the eardrum, (b) the transfer functions from the ear-canal entrance and insert positions to the eardrum, (c) the pressure distributions in the ear canal and concha, (d) eardrum impedance, (e) insert earphone response. (f) ear-canal length and volume (directly measured), and (g) sex differences. A cylindrical canal 23 mm in length and 7.5 mm in diameter satisfies most of the data fairly well but places the primary resonance frequency of the complete external ear at a frequency 5%–7% higher than the observed value. The discrepancy is probably associated with the connection between the canal and the concha.
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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.]
Distortion (music)
Human ear
Auditory System
Sound transmission class
SIGNAL (programming language)
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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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The response of a hearing aid is affected by many factors which include the head and outer ear, the microphone, amplifier, and receiver used in the hearing aid, the properties of the ear canal and the eardrum, and acoustic feedback through the vent. This article presents a computer simulation of an in-the-ear (ITE) hearing aid that includes all of the above factors. The simulation predicts the pressure at the eardrum for a frontal free-field sound source. The computer model was then used to determine the effects on the hearing aid response due to variations in the size of the ear canal. The simulation indicates that, for an unvented hearing aid, changes in the size of the ear canal shift the overall sound-pressure level at the eardrum but have only small effects on the shape of the frequency response. The situation is more complicated when a vent is present, however, since changes in the size of the ear canal that cause apparently small perturbations in the acoustic feedback signal may, nonetheless, have large effects on the overall system response.
Hearing aid
Outer ear
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The geometry of the human ear canal differs considerably from its conventional description as a uniform cylinder with a plane perpendicular end. The differences can be expected to produce significant effects especially at frequencies greater than 10 kHz. Measurements of sound pressure distribution within an accurately scaled (×2.5) replica of a real ear canal confirm the effects of the tapered end observed previously in simple models [J. Acoust. Soc. Am. Suppl. 1 71, S88 (1982)] and show that the standing wave patterns in the main body of the canal are appreciably disturbed. In agreement with the earlier work, the variations in SPL across the human eardrum must be expected to exceed 15 dB at 15 kHz. An approximate theory has been developed to describe the sound field within ear canals of varying cross section and direction. The theory is an extension of Webster's horn equation, quantifying the canal geometry using effective cross-sectional areas defined along an appropriate curved axis. Agreement between theoretical and experimental pressure distributions is good.
Human ear
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