Frye FONIX FP40D - Manual (Page 145)

The FONIX CIC Option 137
Background
Zwislocki built an ear simulator coupler years ago to better approximate the real ear’s impedance vari-
ation with frequency. The ear’s volume appears to get larger at lower frequencies. Mahlon Burkhard
at Industrial Research Products agreed with this approach, especially when they built the KEMAR,
and designed an ear simulator that had impedance changes that matched the Zwislocki figures. This
ear simulator was later standardized by the publication of American National Standards Association
standard, S3.25. Another ear simulator that has similar characteristics was introduced in Europe a few
years later by Bruel and Kjaer, and is characterized in the standard IEC 711.
Frye Electronics introduced a slightly different approach in the 1980’s when it came out with the
INSITU option (and later, the OES option) for its 5500-Z Hearing Aid Analyzer. Realizing that ear sim-
ulators which contain frequency sensitive elements are somewhat fragile and can be damaged as they
are handled in every day use, Frye made a coupler which it labeled the MZ (for Modified Zwislocki).
This coupler had a central volume very similar to the standardized Zwislocki, but had no frequency
sensitive elements. Instead, an analyzer program was used with the coupler to apply correction factors
to the measured curves from the hearing aid so that the output was very similar to that which would
be obtained if the aid were tested on a standardized ear simulator as built by Knowles or B&K.
These software corrections work well for most regions in the frequency response of the aid. In low fre-
quency areas up to about 1500 Hz, if the aid has a response peak that is influenced by the volume of
the cavity, the peak will be slightly higher in amplitude and slightly higher in frequency than that peak
would be if the aid were measured in a standardized ear simulator. The CIC hearing aid is not usually
affected by this problem.
The Need for a CIC Coupler
The introduction of the CIC hearing aids has made it desirable to be able to test them with a coupler
that more closely approximates the actual volume and frequency response characteristics of the real
ear. The CIC aid fits so close to the tympanic membrane (TM) of the ear that the volume of the cavity
is reduced greatly and the aid produces a significant amount more gain. Further, its response can be
expected to be substantially influenced by the frequency dependent impedance variations of the TM.
Frye Electronics felt that the use of CIC coupler with a proper response correction would give better
data to a hearing professional than the use of the standard 2cc coupler, or even a Zwislocki ear simu-
lator, when attempting to produce 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 can be thought to be mostly caused by the TM. When we reduce the volume
Appendix E: The FONIX CIC Option