Frye FONIX 7000 - Manual (Page 106)

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FONIX 7000 Hearing Aid Analyzer
Technical details
The digital processing delay measurement is taken by sending a short impulse
from the sound chamber speaker to the hearing aid. The 7000 Test System
microphone collects information from the hearing aid for 20 milliseconds from
the time the impulse is delivered. This information is a series of numbers of
varying amplitudes.
The 7000 Test System finds the maximum peak amplitude of the resulting
information. Since the impulse response of a hearing aid is not always simple,
the analyzer also checks for any peaks occurring before this maximum peak. If
a smaller peak exists, and it has an amplitude of at least 50% of the maximum
peak, the time of the smaller peak will be considered the processing delay point.
Otherwise, the time of the maximum peak will be considered the processing
delay point. The 7000 Test System delay is subtracted from this delay point in
order to form the actual aid processing delay time. (The 7000 Test System delay
is determined during the sound chamber leveling process.)
The data collected in the digital processing delay measurement is displayed in
graphical format as amplitude vs. time. A dotted vertical line is placed at the
calculated delay point. The numerical value is also displayed. A second dotted
vertical line shows the 7000 system delay for reference.
4.1.2 Signal Phase
The signal phase measurement is a test of the “pushing” and “pulling” of the
amplified sound of the hearing aid. Vibrations in the air create sound—these
vibrations can be thought of as air pushing and pulling against the ear. If sound
goes through a hearing aid, the hearing aid might cause a phase shift, turning a
“push” of the sound wave into a “pull.” This isn’t necessarily a bad thing; there
may be good reasons for a phase shift of the sound wave.
The components of a custom hearing aid are usually wired by hand. The
receiver is typically wired into the amplifier in such a way that it may or may
not cause a phase shift of the signal. If care is not taken, it’s entirely possible to
wire one hearing aid of a custom binaural set one way, and wire the other aid in
the opposite way. This could cause one of the hearing aids to be “pulling” while
the other aid is “pushing,” resulting in strange sound quality for the hearing aid
wearer.
We propose that it could be very important to check the signal phase of the
hearing aids when fitting a client with a binaural set in order to ensure that the
aids are working together as a team.
Technical Details
The signal phase measurement works by generating a 1 kHz cosine wave, turn-
ing it into a test signal, and delivering it to the aid. The cosine wave signal is
offset at the time of generation so that it starts at the baseline (the zero point).