Frye FONIX FP40D - Manual (Page 146)

138 FONIX FP40 Portable Hearing Aid Analyzer
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 hear-
ing 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 do not 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 rea-
sonable range.
Now, how much response variation will be introduced because of the smaller volume of 0.25 cc?
This variation is that which 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 stan-
dard ear simulator, the length of the cavity begins to affect its response to sound at frequencies
above about 3000 Hz.
Knowing the physical volume of the occluded ear canal and its frequency response variations, it is
possible to calculate the equivalent volume of the TM itself at each frequency and to apply this fig-
ure to the response of the 0.25 cc cavity between the hearing aid and the TM.
When the calculated volume variation of the TM is applied to the smaller volume of the CIC cou-
pler, the total response variation comes out to be from -8.6 dB at 200 Hz to +5.5 dB at 8000 Hz for
a total variation of 14.1dB.
Summary
Using the CIC coupler with its software option gives the dispenser an immediate idea of how much
gain that this new type of hearing instrument is going to give the hearing impaired individual. It
is nice to see that the CIC hearing aid can really produce significant amounts of gain in spite of its
apparently poor performance in the 2cc world of the ANSI standard test.
The user must remember that an actual ear may produce differences from the predicted values.
Acknowledgment
Mead Killion, Mahlon Burkhard and Elmer Carlson are to be thanked for helping assemble the data
from which the CIC corrections were derived.