Robert L. Martin, PhD, has been in private practice in the San Diego area for more than 20 years. He holds a doctorate in audiology from the University of Tennessee and was formerly on the faculty of San Diego State University.
Correspondence to Dr. Martin at 1109 Third Avenue, Chula Vista, CA 91911.
When a hearing aid is placed in the ear it occludes the canal, causing an elevation in the sound of the wearer's voice in the lower frequencies. For some patients, this creates a big problem.
In the old days, venting was used to reduce the problem. But, with the new circuitry and multi-microphone technology, you sometimes get stuck between a rock and a hard place. The patient wants a solution to the occlusion, but you can't vent because it would cause feedback or a loss of microphone efficiency. So, this month I would like to discuss voice problems and ways to reduce the occlusion effect by the use of jaw blocks and increased canal lengths.
I once took a very helpful class on the occlusion effect. The teacher had us put our fingers in our ears and say “eeee.” This exercise was invaluable for me because it taught me that the hearing aids I was ordering at the time (ITEs with short-to-medium canal lengths and small vents) were the ones most likely to create unacceptable voice problems due to the occlusion effect.
If you have never tried the fingers-in-your-ear exercise, do it now. This lesson has lots of clinical application and will instantly teach you how to deal with problems created by the occlusion effect. (Those of you with long, beautiful fingernails will have to ask someone with short fingernails to help you with this exercise.)
If I place my fingers loosely in my ear canals, my voice sounds very loud. The real-ear probe reading of my voice is close to 100 dB SPL. Try this exercise. Put your fingers in your ears and say “eeee.” Find the position where your voice sounds the loudest. Note how the insertion depth is midway into the canal, not shallow, not deep. You will also find that your voice level changes with the tightness of your fingers in your ears. You hear yourself most loudly when your fingers are placed loosely in your ear canals. Remember this situation and avoid it in fitting hearing aids.
If I break the seal created by my fingers, my voice drops dramatically—by about 60 dB. I can adjust the position of my fingers in my ears, pushing up, down, front, or back, to simulate a wide range of hearing aids and venting conditions.
Venting and reducing the canal length have always been the easiest ways to eliminate the problems associated with the occlusion effect.
Sometimes, though, venting is not an option. For such cases, this same exercise also teaches me how to control (reduce) the occlusion effect with deeply fitting hearing aids. When I say “eeee” and very slowly and carefully insert my fingers deeply, tightly into my ears, the intensity of my voice decreases about 30 dB or 40 dB as measured with my real-ear system. In this exercise, I am carefully pushing my fingers into my ear canals deeply enough to be pressing on my skull; my fingers are at (against) the entrance to the bony portion of the ear canal, sealing the canal.
If you have a real-ear system, place the probe tube into your ear canal, and try the exercise I described above. You will probably be surprised by how different the measurements are depending upon where your fingers are in the canal.
THE SNUG FIT APPROACH TO OCCLUSION
The occlusion effect is caused by sound radiating into the ear canal through the soft tissue of the ear, not through the bony portion. So, in order to make an impression for a hearing aid that will reduce the occlusion effect, you need a snug fit that extends into the bony part of the ear canal. A moderately viscous impression material is required to get this snugness.
You also need to use jaw blocks for patients who have considerable TMJ movement in their ear canals. Otherwise, when the patient opens his mouth to speak, the aid changes from a snug fit to a loose fit.
Remember, the transmission path of the patient's voice is through the soft tissue into the ear canal. Therefore, a medium-to-short canal instrument allows the patient's voice to radiate into the canal, while a deeply fitting aid does not.
The fingers-in-the-ear-while-saying “eeee” exercise has taught me that there are only two types of hearing aid fittings that control the occlusion effect:
1. a hearing aid with a large vent and/or a shortened canal length, and
2. a hearing aid made from an impression that fits snugly and deeply in the soft portion of the ear canal, sealing off the sound of the incoming voice.
Venting and reducing the canal length are the easiest ways to control the occlusion effect. However, these “tricks” are often counter-productive. The best alternative is a snug, deep, yet comfortable canal that seals off the soft portion of the patient's ear canal. The end of this aid needs to be tapered or ground and sanded to avoid pressure problems (sensitivity issues) in the bony part of the canal where the skin is extra sensitive.