Standing Waves and Otoacoustic Emissions

Question

Answer

During conventional audiometric testing, there are opposite surfaces in the ear canal, the transducer (conventional earphone, insert earphone, otoacoustic emissions probe, etc.) at one end, and the eardrum at the other, a distance of approximately one inch in an adult. A standing wave in this situation is an apparently stationary waveform caused by reflections between these opposite surfaces. At certain points along the standing wave, the direct and reflected waves can partially cancel resulting in a lower level of the test signal, or even completely cancel resulting in no signal at all.

This phenomenon is a factor when the wavelength (the distance between peaks of the signal) is near the physical dimensions of the structure. The wavelength for a 1000 Hz tone is a little more than 12 inches which is much longer than the one inch length of the ear canal, so standing waves are not a factor at this frequency. As frequency gets higher, wavelength gets shorter such that at 2000 Hz, the wavelength is about 6 inches, at 4000 Hz, about 3 inches, and at 8000 Hz, about an inch and a half - approaching the one inch length of the ear canal. Given this progression, standing waves can be an issue for most audiometric testing above 8 kHz. One solution is to reposition the transducer (to slightly alter the distance between opposite surfaces). A second solution is to change the wavelength (select other frequencies as suggested).

However, there is a third approach. All otoacoustic emission measurement devices use a microphone in the probe that measures and readjusts the signal before every measurement. So, if a standing wave causes a partial cancellation of the measurement signal during an otoacoustic emission measure, the device automatically adjusts the level to compensate. Note however, that this compensation only controls for the level of the measurement signal. The same standing wave phenomena can occur for the emission itself.

In summary, standing waves are a consideration for all audiometric measures when using higher frequency signals, except when measuring otoacoustic emissions which uses a technique to correct for them. All higher frequency audiometric testing would be improved if conventional audiometers used the same compensating system that otoacoustic emissions devices use.

Gerald Popelka holds a PhD degree from the University of Wisconsin in Madison with an emphasis in neuroscience, and a two year post doctoral research fellowship in otolaryngology from UCLA. Prior to these he earned a masters degree in audiology from Kent State University. He was a faculty member for 24 years at Washington University in St. Louis and joined the faculty at Stanford in 2004.

Dr. Popelka's research has been funded continuously with grants from a wide variety of agencies. He has initiated and completed successful collaborative research projects among diverse academic divisions including medicine and engineering.

Dr. Popelka is a co-inventor of the world's first all digital hearing aid. The resulting patent formed the basis of all current programmable and digital hearing aids or ~90% of all hearing aids currently sold. In 1996 he conceived and lead the development of JARO, the Journal of the Association for Research in Otolaryngology, now recognized as a premier, international auditory scientific research journal that was launched in 2000.

With over 80 research articles, many research presentations, two college textbooks, and various achievement awards, he has developed an international reputation for creating and using leading-edge technology that addresses both basic science and clinical applications. He remains in the forefront of developing innovative technology for biomedical applications, currently focusing on noninvasive measures of neonatal auditory function and hyperbilirubinemia and real-time MRI imaging for speech and swallowing.