42 FONIX FP35 Hearing Aid Analyzer
Here’s the technical details of the frequencies examined for those of you who
are interested:
2ND: [F1 + F2] + [F1 – F2]_3RD: [(F1*2) + F2] + [(F1*2) – F2] + [(F2*2) – F1]
+ [(F2*2)+F1]_TOTAL: All frequencies excluding F1 and F2.
(F1 and F2 are the primary frequencies of the IM distortion sweep.)
Delays
Just like a pure-tone sweep, you can change the delays in the IM distortion sweep. The
PREDELAY time sets how long the first signal will be presented in a distortion sweep
before the measurement is made. The SHORT SETTLE time determines how long the
source is presented for the remaining sweep frequencies in the distortion sweep. See
Section 2.4 for more information. The USER LEVEL must be set to ADVANCED to
change these settings. See Section 2.3.2.
Distortion measurement noise
When measuring distortion, particularly IM distortion, it is important to under-
stand the contribution of noise in the measurement. The more distortion com-
ponents that are measured, the more that noise affects the results.
When distortion is being measured, it is usually desired to find out how much
the hearing aid distorts the original signal. However, room noise also distorts
the signal. For example, for total IM distortion, a noise floor 20 dB below the
source will result in a 1% distortion contribution of noise to the test results. For
this reason, it is recommended to perform the IM distortion test 30 dB above the
noise floor of the room. This will drastically reduce the influence of room noise
in the IM distortion test.
Conversely, it is also important to be careful not to measure IM distortion with
too loud of a source level because you could risk creating distortion caused by
overdriving the hearing aid or the hearing aid analyzer.
You can reduce the affect of noise on the IM distortion test results by reducing
the number of frequencies that are measured in the test. Do this by measuring
the 2nd or the 3rd IM distortion instead of the total IM distortion. See Section
4.2.6.3.
Impulse rejection
Impulse rejection can be used to reduce distortion caused by impulses in the
room noise. When noise reduction is used with IM distortion measurements,
several measurements are made to obtain the test results. When impulse rejec-
tion is on, the FP35 analyzer looks at the peak response of each measurement.
If the peak of the last measurement plus the impulse rejection amount is more
than the peak of the current measurement, the current measurement will be
rejected and taken again. This can reduce the effect of room noise in the overall
test results. Impulse rejection applies to all measurements, not just intermod-
ulation distortion.
