Frye FONIX 7000 - Manual (Page 44)

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FONIX 7000 Hearing Aid Analyzer
In addition to the delays, the aid type will also have an effect on the way
some measurements are made. With a selection of ADAPTIVE, the analyzer
will throw away the first three measurements of all averaging (including refer-
ence test gain) and harmonic distortion measurements. This will increase the
amount of time it takes for the test, but it should increase the accuracy of the
test for adaptive AGC aids.
Note: The “Adaptive” aid type category was created for a specific type of adap-
tive-AGC aid, such as the model created by Telex.
2.3.1.4 Harmonic distortion
Harmonic distortion occurs when a hearing aid clips the peak of a pure-tone
input signal, resulting in artifacts at harmonics (integer multiples) of that input
signal. For example, if you present a 500 Hz tone to the hearing aid, distortion
artifacts could occur at 1000 Hz and 1500 Hz.
The harmonic distortion measurement is expressed as the percentage of the
power of these distortion artifacts to the power of the input signal. All hearing
aids will have some amount of distortion.
Usually, the strongest artifacts occur at the second and third harmonics of the
frequency. With the 7000 Test System, you can test the amount of distortion
available in the second harmonics, the third harmonics, or both harmonics
(considered “total harmonic distortion”).
Definitions
2nd Harmonic: Energy of the second harmonic or twice the presented frequency.
3rd Harmonic: Energy of the third harmonic or three times the presented fre-
quency.
TOTAL: Combined 2nd and 3rd harmonic distortion.
See Section 3.4.7 for instructions on performing the harmonic distortion mea-
surement.
2.3.1.5 Pure-tone Filter
Pure-tone signals are usually flat-weighted. That is, in a pure-tone sweep, each
tone usually has the same amplitude as every other tone in the sweep. In con-
trast, the wideband Composite and Digital Speech signals are usually speech-
weighted. That is, the lower frequencies of the wideband signals have higher
amplitudes than the higher frequencies, simulating the long-term average of
speech.
When viewing measurements in output (dB SPL), the weighting of the signal
is very important because the signal input is included with the measurement
results of the output. This could cause some problems when comparing pure-