Question
Can you review the effect of venting on directivity?
Answer
Directivity decreases mainly in the low frequencies with increasing vent size. This is one reason we get less directivity in open canal fittings. As vent size increases, you will get less gain and low frequency sound will also leak back in to the ear canal affecting directivity. But even with a lot of gain in the low frequencies, you'll still end up with less directivity as the vent size increases.
Directivity index (DI) is a measure of how well a microphone or microphone system attenuates signals coming from directions other than the front, and ranges from -2 dB to a theoretical maximum of +6 dB when a two microphone (or two port) system is used. One study (Ricketts, 2000) electroacoustically evaluated DI in three groups of hearing aids across four different venting configurations (no vent, 1 mm, 2mm and open). All measurements were made for instruments in directional and omnidirectional modes. In the closed configuration, average DI was approximately +4 dB DI and decreased as venting increased, especially in the low frequencies, with an average DI of approximately +2.4 dB measured in the open configuration.
Reference
Ricketts, T. (2000). Directivity quantification in hearing aids: Fitting and measurement effects. Ear & Hearing, 21(1), 45-58.
This Ask the Expert was based on information from Dr. Rickett's course, Open Canal Fittings: Considerations Regarding Prescriptive Methods and Function of Special Hearing Aid Features
Todd A. Ricketts, Ph.D, CCC-A, FAAA, is an associate professor at the Vanderbilt Bill Wilkerson center for Otolaryngology and Communication Sciences and Director of the Dan Maddox Hearing Aid Research Laboratory. Todd has published more than sixty scholarly articles and book chapters. To date he has presented over 150 scholarly papers/poster presentations, short courses, mini-seminars, and workshops to professional and scholarly conferences both nationally and internationally. He continues to pursue a federally and industry funded program studying various aspects of hearing, hearing aids and cochlear implants. He was named a Fellow of the American Speech Language and Hearing Association in 2006 and his article "Directional benefit in simulated classroom environments" received the Editors award from the American Journal of Audiology at the 2008 AHSA convention. He also is a past editor-in-chief of the quarterly journal Trends in Amplification and is the current chair of the Vanderbilt University Institutional Review Board Behavioral Sciences Committee.
