Real Ear Measurements
for Prescription of Hearing Aids?
by Larry Revit, hearing scientist
Can hearing-care professionals use probe measurements in the prescription process, to improve the chances for a successful fit?
Yes, we can.
We are living at a time when communications and technology are showing some real benefits for everyone. Our field is no exception. The word is out: individual ears are different from one another. And those differences can largely affect the benefit a hearing aid provides to the wearer.
Fortunately, we now have a fast and reliable way to measure real-ear differences. In a few short minutes, we can check the real-ear insertion gain of a hearing aid using probe equipment.
But, can't we use probe measurements before the fitting stage, to prescribe the hearing aid reponse that we know will work better when the aid is first placed in the client's ear?
The answer is most certainly "yes". Since the earcanal resonance (a natural peak) is lost when an aid is inserted in the ear, a hearing aid must compensate for the lost resonance before it can provide any benefit to the wearer. Conversely, if the hearing aid has a peak at a frequency that does not correspond to the peak of the earcanal resonance, the fitting could be far too strong at the frequency of the "misplaced" peak.
We can easily get useful information about the client's earcanal resonance before ordering a hearing aid. Measuring the client's unaided ear canal resonance with Fonix probe equipment is fast and easy. We don't need precise placement of the probe tube; the probe tip just has to be a few millimeters into the earcanal to measure the primary resonance peak (in the region of 3 Khz). An effective rule of thumb, then, is to specify that the primary peak of the ordered hearing aid should match the frequency of the measured earcanal resonance. (The above does not necessarily apply to IROS fittings.)
Beyond this rule of thumb, we can look forward to conversion factors that effectively translate target insertion gain into prescribed coupler gain. Several sets of conversion factors have been proposed, but the extent of their clinical effecitiveness has not yet been demonstrated. When there is a proven set of conversion factors available, Frye Electronics will put them into Fonix real-ear analyzers. And, only when manufacturers can match our prescribed coupler responses, can any prescription routine be effective. Even then, (and all researchers agree on this), every fit will need to be confirmed by real-ear measurements.
Fortunately, we now have the technology and the know-how to improve hearing aid prescriptions and fittings. And soon we will have the means to make the process even easier.