Audio Engineering Society

Chicago Section

Meeting Review, December 2000

other meeting reports

12/6/00 Meeting Highlights
by Bob Zurek

Otoacoustic Emissions (OAEs)

On Wednesday December 6th, Dr. Laurel Christensen of Etymotic Research gave a presentation entitled “Otoacoustic Emissions (OAEs).” Laurel began her presentation with discussion of anatomy and physiology of the ear. Dr. Christensen began by explaining that the ear is divided into three regions, the outer ear, the middle ear, and the inner ear. She explained that the outer ear serves as an amplifier in the 2kHz to 5kHz region. She went on to explain that the middle ear’s purpose is that of an impedance matching device between the air in the outer ear and the fluid in the inner ear. Dr. Christensen stated that damage done to the middle ear is usually repairable. This damage can result from ear infections, head trauma, or the stapes ossifying into the window. She also mentioned that the middle ear serves another purpose, which is to attenuate high intensity sound over about 85dB. She stated that there is controversy as to whether this action is a hearing protection apparatus in the ear or not. Dr. Christensen then talked about the inner ear eventually leading to discussion of the Inner and Outer Hair Cells (IHC and OHC). The inner hair cells send the information to the brain whereas the outer hair cells amplify low level sounds. She then explained that the outer hair cells are typically the first to go, accounting for a large percentage of hearing loss cases. Dr. Christensen then showed the group audio-visual demonstrations of the workings of the ear using software developed by the University of Wisconsin’s Auditory Physiology Department. These demonstrations included pictures of cross sections of normal cochlea and damaged ones showing the damage done to hair cells at different levels of hearing loss. She explained that the high frequency OHCs of the cochlea are the first to experience damage due to their location. She also explained that human speech extends out to approximately 4kHz and that 90% of the speech signal is carried by the high frequency consonants. Thus this type of damage would lead to reduction in speech intelligibility. Dr. Christensen then presented the differences in losses due to different types of exposure. She specifically showed the distinction between asymmetric losses often found in musicians and the symmetric loss found in noise induced hearing loss. She also explained that while music may not be a steady state noise, long exposure to loud levels in music results in the same hearing loss. She also mentioned that impulse noise as seen in percussion may cause more damage than noise exposure, and that in all cases, the greatest damage is done in the first years of exposure.
Dr. Christensen then discussed the topic of Otoacoustic Emissions or OAEs. She described how the OAEs are produced by the contraction of the outer hair cells in the cochlea in response to sound applied to the ear and can be measured in the ear canal at levels between 5dB and 15dB. She explained the two methods used to measure the tones, the transient and distortion methods. She explained that the transient or “click” method allows for a measurement in the 500 Hz to 4kHz range, where the two-tone distortion method is useful in the 1.5kHz to 16kHz region. In the two-tone method, two pure tones are applied simultaneously to the sealed ear, and the Tartini tone generated by the hair cells is measured. Dr. Christensen next discussed some of the advantages to the measurement of OAEs. First, the test provides an objective clinical test of outer hair cell function, and second it often detects damage before the results of an audiogram does. Some of the other characteristics of OAE testing are that it is particularly good for newborns, since no response is required from the subject being tested, it is frequency specific, and it is a reliable form of measurement. One of the drawbacks of the method is that it is also sensitive to middle ear pathology as well as inner ear. In other words, an ear infection or middle ear problem will show a reduction in the OAEs. At this point Dr. Christensen discussed a few clinical examples of outer hair loss and the role OAE measurements played in diagnosis and treatment.
After the presentation, Dr. Christensen was kind enough to test the OAEs of attendees who were interested using Etymotic Research’s ERO*SCANtm OAE test instrument. The Chicago Section would like to thank Dr. Christensen and Etymotic Research for presenting and hosting the meeting.