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
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.
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