FONIX® 8000 - Frye Electronics

Real-Ear Measurements 185
15. Look at the difference between the “forward” (REAG 2) and the “reverse”
(REUG 1) frequency responses (Figure 8.6.1). The “forward” frequency
response should show more amplification than the reverse frequency
response. Take note of the differences in the RMS OUT of the two curves.
This will tell you the average amount of amplification advantage that the
directional microphones provide.
8.6.2 Determining the “null”
When performing a directional test, the position of the sound field speaker can
be used to determine the “null” of the directional microphones. The “null” is
the azimuth at which the directional hearing aid provides the least amount of
amplification.
To determine the null, follow Steps 1-8 in Section 8.6.1 to measure the reverse
curve. While performing this measurement, move the sound field speaker
around the patient (this is easier using a speaker on a swing arm than a floor
stand). Try to keep the speaker equidistant from the patient while moving the
speaker in a semi-circle behind him.
Look at the frequency response as the speaker is moved. If the aid is directional,
its frequency response will decrease as the speaker is moved from the
front of the patient to the back of the patient. The angle at which the frequency
response is at its lowest is the “null” of the directional hearing aid.
8.7 Open Fit Hearing Aids
Open fit hearing aids require additional consideration during real-ear measurements
because of the special technology used by these aids to eliminate feedback.
This feedback suppression technology can interfere with the sound field
measured by the reference microphone outside the ear. Therefore, in order to
get an accurate real-ear measurement of an open-fit hearing aid, the reference
microphone of the 8000 Hearing Aid Test System should be disabled.
to set up the analyzer for open fit real-ear testing:
1. Enter the Real-ear Insertion Gain or Real-ear SPL screen.
2. Press [MENU] to open the local menu.
3. Select Reference Mic under Measurements with [, ].
4. Choose Off with [].
5. Press [EXIT] to close the local menu.
6. Level the sound field speaker as usual (even though the reference microphone
is disabled for measurements, it will still be used for leveling).
7. Test as usual. With the reference microphone disabled, it is even more critical
than usual that the patient not move after leveling and that the sound
field be undisturbed during testing.