FONIX® 8000 - Frye Electronics

216 FONIX 8000 Hearing Aid Test System
a good hearing aid fitting. It also felt that the approach taken in the use of the
MZ coupler has been well accepted by professionals throughout the world and
that the new CIC coupler should use a similar approach, with response corrections
modified to take the smaller CIC volume into account.
The Basic Problem
The ear is not a simple structure. It is a biological coupling device that converts
sound energy to nerve impulses. It also has a pinna that helps to direct higher
frequency sounds into the external canal. The part of the structure we are concerned
with is the external ear canal, or cavity, which is terminated by the TM.
The ear canal can be considered to be fairly rigid when it is compared to the
TM. In the lower frequencies below 2000 to 3000 Hz, the frequency-related
changes in impedance that we see in an ear are thought to be mostly caused by
the TM. When we reduce the volume of the cavity between the hearing aid and
the TM by moving the aid closer to it, we should expect to see the TM play a
more important part in determining the response of the aid.
For more shallow standard earmolds, the volume of the central cavity of the ear
reduces the effect of the TM’s frequency impedance changes. This is because
the volume of the cavity is added to the equivalent volume of the TM. If the
cavity volume is large and does not change with frequency, then the large
changes in impedance of the TM are swamped by the large volume of the ear
canal. If, on the other hand, the TM is working into a very small volume, then it
would affect a large change in impedance across the frequency range.
CIC Hearing Aid—Gain and Frequency Response Changes
From the above discussion we see that we can expect that the frequency
response of the CIC hearing aid will be greatly influenced by the frequency
dependent impedance changes of the TM. What is the magnitude of these
changes? A fairly typical ear fitted with a standard hearing aid and earmold
should have characteristics that would normally be predicted by a KEMAR
manikin and standardized ear simulator. When that ear is fitted with a CIC aid,
what is the volume between the hearing aid and the TM? Because of the tilt of
the TM, most professionals probably don’t fit the aid right next to the TM. A
reasonable figure may be 0.25 cubic centimeters. It should be realized that this
number could be higher or lower, depending on circumstances. 0.2 to 0.4 cc
may be a reasonable range.
Now, how much response variation will be introduced because of the smaller
volume of 0.25 cc? This variation is what is used in the frequency response
correction table used with the CIC coupler. One assumption that we make in
calculations of volumes is that the simulator is small as compared to the wavelength
of sound at the frequency we are examining. In the case of the standard
ear simulator, the length of the cavity begins to affect its response to sound at
frequencies above about 3000 Hz.