January 22, 2008 Feature

Wideband Measures of Middle-Ear Functioning

Wideband (WB) reflectance and WB absorbance are names for an emerging type of acoustic ear-canal measurement that provides new dimensions for middle-ear assessment across a wide range of frequencies.

Traditional types of aural immittance measurements are usually based on a single-frequency measurement of acoustic admittance—for example, at frequencies of 226 Hz or 1000 Hz—and do not provide a measure of middle-ear functioning over the full frequency range important for speech or music perception. Reflectance measurements are performed over a frequency range from 0.22 kHz up to approximately 8 kHz using a WB stimulus such as a click. Reflectance may be measured at ambient ear-canal pressure using a probe microphone and sound source similar to those used in recording otoacoustic emissions (OAE), or a WB tympanogram may be measured by also varying static pressure within the ear canal.

When sound is incident in the ear canal, some of the sound energy is reflected at the tympanic membrane and some is absorbed. The absorbed energy may then either be partially absorbed within the middle ear or transmitted to the cochlea, where neural transduction takes place. The acoustic reflectance is the ratio of the reflected to the incident pressure in the ear canal.

A key property is that the energy reflectance, which is the squared magnitude of the acoustic pressure reflectance, is insensitive to the location of the probe microphone, which simplifies interpretation at higher frequencies. The energy absorbance is the fraction of incident energy that is not reflected at the tympanic membrane, and is a measure (from zero to one) of the ability of the middle ear to collect sound energy. Energy absorbance has a maximum value at frequencies in the range from 2-4 kHz in normal-functioning adult ears, with reduced values at lower and higher frequencies when middle-ear transmission is less efficient.

These WB tests have potential diagnostic use in children suspected of having otitis media with effusion (OME). Based on surgical confirmation of fluid following a myringotomy, WB absorbance is accurate at classifying whether middle-ear effusion is present or not, and is also accurate at predicting the presence of a conductive hearing loss in children with OME.

Furthermore, these WB tests can detect the presence of transient middle-ear dysfunction in infants referred by a newborn hearing screening (NHS) exam based on a distortion-product OAE (DPOAE) test, the so-called false positives in NHS. Such testing also included a WB test of middle-ear muscle reflex threshold. Any WB test, including WB reflex, was more accurate than 1000-Hz tympanometry in predicting whether an infant passed or referred the DPOAE test, while WB tympanometry was the most accurate test.

The ability to assess middle-ear functioning over the full bandwidth of hearing that is critical to speech perception is an advantage of WB testing. Hearing screening and diagnostic devices are becoming available to support this new type of test.

An interdisciplinary group of investigators have contributed substantially to this project, including physicians, audiologists, engineers, and hearing scientists. These individuals include Michael Gorga, Edward Cohn, John Ellison, Sandy Estee, Denis Fitzpatrick, Michele Gortemaker, Dawna Lewis, Yi-Wen Liu, Ryan McCreery, and Chris Sanford. This project was supported by NIH grant DC06607 with core support from DC04662. 

Douglas Keefe, a hearing scientist at Boys Town National Research Hospital in Omaha, Nebraska, performs research on middle-ear and cochlear mechanics. His technology transfer activities on wideband reflectance occur through Sonicom, Inc. He presented a technology update at the 2007 ASHA Convention. Contact him at keefe@boystown.org.

cite as: Keefe, D. (2008, January 22). Wideband Measures of Middle-Ear Functioning. The ASHA Leader.

  

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