Nuts &
Bolts
Robert L. Martin
Introducing “open-ear” couplers
for testing open-fitted hearing aids
By Robert L. Martin
Sales of open-fitted hearing aids are exploding. Patients love how
natural their amplified voices sound and the big improvement in
word understanding that these instruments can provide. I believe
open-fitted hearing aids will soon dominate the new user market, and the number of units sold will skyrocket as news of their
success reaches consumers. When these hearing aids are fitted on
appropriate candidates, the patient benefits are very great and the
negatives are negligible.
However, until now, there has been no simple way to conduct
meaningful tests on these hearing aids without doing real-ear tests
on the patient. This article introduces two new devices that can
do this. The first is an ear simulator developed by Michael J.
Rensink, MD, an otolaryngologist; you use your real-ear system
to make measurements with this device. The second is a completely open coupler developed by Frye Electronics; measurements are done in the test box.
As you know, conventional test couplers are closed, airtight
cavities. It is possible to test an open-fitted hearing aid with a traditional closed coupler. However, the results become meaningless when the hearing aids are placed on the patient’s ears; the
frequency response and feedback characteristics are entirely different in a closed cavity from how they are in the ear, where the
open fitting allows almost all the low-frequency sound to escape.
Our industry needs a reliable way to simulate open fittings
without the patient’s being present. Clinicians need measurements
that tell them how much amplification the patient will receive.
The ear simulator and open coupler also give us a convenient way
to study feedback without irritating or alarming the patient.
THE RENSINK
EAR SIMULATOR
Dr. Rensink designed the ear
simulator shown in Figures 1 to
3. It is a 3-cc syringe inserted Figure 1.
into a rubber ear. Figure 1 shows the syringe
with the needle removed and discarded.
The rubber plunger on the injection shaft
of the syringe has been removed and reinserted back into the syringe to the 2-cc
line. The cavity (column of air) created in
the open end of the syringe matches the
diameter and length of the human ear
canal.
Figure 2 shows the syringe pushed into
a rubber ear. The rubber ear adds “air” to Figure 2.
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The Hearing Journal
the simulator to match the external ear. This simple, inexpensive, easy-to-make device reacts to amplification just as the human
ear does. Real-ear measurements from a human ear (mine) and
the real-ear measurement from the open-ear simulator are nearly
identical.
Figure 3 shows the Rensink ear simulator with the real-ear
measurement tube in the syringe and the reference microphone
in place.
Important note: When you make a real-ear measurement on
an open fitting, you place the reference microphone on the ear
(or beside the ear simulator) for sound field leveling, but turn it
off when making an aided measurement. Sound from the openfitted hearing instrument will be “heard” by the reference microphone and affect the measurement.
Here are some advantages of using the Rensink ear simulator:
❖ The total volume of air and the length of the air cavity in the
coupler are about the same as in the human ear canal. Therefore, the acoustical response measured in this cavity matches
that of the ear.
❖ The coupler is inexpensive and easy to make.
❖ The coupler enables clinicians to study open fittings when
the patients are not present.
Why use an ear simulator in the clinic?
I have been conducting research on open-fitted hearing aids for
the past 2 years. I’ve learned that these hearing aids can give
patients with mild high-frequency hearing loss wonderful
hearing and good word understanding in most listening conditions.
Unfortunately, despite what
you may have heard—even
from supposed experts—there
are many patients who will not
benefit from open-fitted hearing aids. For example, open fittings are not suitable for patients
Figure 3.
who need significant gain in the
lower frequencies or for people with severe-to-profound hearing
loss in the higher frequencies.
Real-ear tests tell us exactly how much amplification we are
giving the patient with an open fitting. However, if we want to
pre-fit the instruments or to study feedback when the patient is
absent, the only choice has been to fit the hearing aids on our-
Nuts & Bolts
August 2006 • Vol. 59 • No. 8
Figure 4.
Figure 5.
selves or members of our staff. If you’ve
run real-ear tests on yourself, you know
it is difficult, and you end up with sore
eardrums!
Making an open-ear simulator
It is easy to make an open-ear simulator
like that invented by Michael Rensink.
Here’s how.
Obtain a 3-cc syringe and dispose of
the needle properly. Pull the rubber end
(the plunger) off the injection shaft. Carefully push the rubber plunger into the
syringe to the “2-cc” line. Run a real-ear
test (unaided) on your ear and save the
curve. Run a real-ear test on the open end
of the syringe. Move the rubber plunger
in the syringe until the peaks of these two
curves match. Push the syringe into a rubber ear.
THE FRYE OPEN COUPLER
Figure 4 is a photograph of a prototype
of the new Frye open coupler, while Figure 5 shows a coupler and attached measurement microphone in the sound box.
This coupler allows the clinician to do
measurements in the sound box and
August 2006 • Vol. 59 • No. 8
roughly approximate the amplification
that will be delivered by an open-fitted
instrument. The open-fitted instrument
is inserted into the coupler and measurements are made.
Neither the Rensink nor the Frye
device in any way reduces our reliance on
or need for real-ear tests. Real-ear studies
always provide the most accurate measures of a hearing aid’s performance. However, using an ear simulator and/or an
“open” coupler offers several advantages.
❖ It is much easier to place a hearing aid
on a coupler than on a patient.
❖ You don’t need to worry about feedback bothering the patient.
❖ You can make all your pre-fit programming adjustments before the
patient comes to the office.
In short, these devices can make our
jobs easier.
IN CONCLUSION
Our profession is going through a learning curve regarding the use of open-fitted instruments. Some practitioners want
to fit them on people with substantial
hearing loss, e.g., a flat 65-dB hearing loss.
This is a mistake. Open fittings give you
little or no amplification in the lower frequencies. You quickly learn the limits of
open fittings when you adjust the programming and measure the result on an
open coupler.
The good news is that open-fitted
hearing aids perform fantastically when
fitted on appropriate patients (people
with mild-to-moderate hearing loss in
the higher frequencies). The potential
market growth for these instruments is
almost unlimited.
You can easily make yourself a Rensink
ear simulator using a common surgical
syringe, or you can purchase a new “open
coupler” from Frye.
I suggest you spend some time measuring the amplification produced by
open-fitted instruments so you learn their
limitations. Without measurements you
end up relying on marketing information
and feedback from patients, which is not
necessarily reliable. Measurements are part
of the science of our profession.
Robert L. Martin, PhD, has been a Dispensing Audiologist in private practice in the San Diego area for more than 30 years and has
been writing Nuts & Bolts since 1989. Readers may contact Dr. Martin at 7750 University Avenue, La Mesa, CA 91941.
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