Occluded-ear simulator with variable acoustic properties

Abstract

Ear simulators were designed to replicate acoustical characteristics of the average adult ear. Due to variability of ear-canal geometry and eardrum impedance among individuals, the possibility of any one person exhibiting such "average" characteristics--especially if that person is a child and/or has a conductive pathology--is remote. Thus, ear simulators have been of only peripheral value when prescribing a hearing aid (a high output impedance device) to fit the acoustical requirements of a particular patient. Reported herein is development of a programmable artificial ear (PAE) that can account for individual differences in ear-canal geometry and eardrum impedance. It consists of a 2.0-cc coupler, microphone, amplifier, computer, PAE code, and a computer card and/or software for digitization and Fourier transformation. Required input data includes ear-canal dimensions, eardrum impedance, and output impedance of the hearing aid being tested. Sound-pressure recordings produced in the 2.0-cc coupler by the hearing aid are adjusted by the computer to what they would have been had the recordings been made at the eardrum of a particular patient wearing the same hearing aid. Good agreement was observed between experiment and theory for one test case involving a totally occluding miniature earphone.

Quantifying ear-canal geometry with multiple computer-assisted tomographic scans

“…Discussed in this article is a noninvasive procedure for remote determination of ear-canal geometry. The technique was first reported by Nelson (1989) and was later summarized by Egolf et al (1992). It involves the use of a computer-assisted tomographic (CAT) scanner to obtain radiographic images of cross sections of the ear canal in each of several parallel sagittal (i.e., parasagittal) planes from the tympanic membrane to the concha.…”

“…However, due to the difficulties and risks associated with making sound-pressure measurements at or near the eardrum (Egolf et al, 1992), investigators have resorted to various alternatives for obtaining such information. Most of these involve the microphone a)Also affiliated with The Parmly Hearing Institute, Loyola University of Chicago, 6525 N. Sheridan Road, Chicago, IL 60626. recording of sound-pressure data at another, moreaccessible location in the ear canal and/or in a mechanical coupler.…”

Real Ear to Coupler Differences in Children and The Effects of Hearing and Microphone Location

“…In addition, the coupler allows verification of the hearing aid response with a prescribed target prior to fitting. The coupler was not designed to simulate the acoustic resonating characteristics of the human ear, (Egolf, Kennedy, & Larson, 1992;Green, 1988;Killion & Monser, 1980) in particular a child's ear.…”

“…Unfortunately, large differences can be expected in the acoustic response of a child versus an adult ear (Nelson-Barlow, Auslander, Rines, & Stelmachowicz, 1988;Feigin, Kopun, Stelmachowicz, & Gorga;. Thus, utilizing available conversion formulas can introduce large errors into the hearing aid selection process for a pediatric patient (Egolf, Kennedy, & Larson, 1992).…”

“…In an attempt to avoid errors introduced through the use of standardized conversions, several researchers have recommended the use of individualized conversion values to predict real ear responses from 2 cc coupler responses (Egolf, Kennedy, & Larson, 1992;Moodie, Seewald, & Sinclair, 1994;Fikret-Pasa & Revit, 1992). Of the four characteristics described above, estimation of an individual's insertion loss is easily derived during the real ear measurement process (Kruger & Ruben, 1987).…”