Frye FONIX FP40D - Manual (Page 31)

General Operation 23
In determining the length of time needed for the proper measurement, a good rule is to use twice
the published attack time of the hearing aid. If you are unsure of the attack time, you can experi-
ment with longer times and shorter times and see if there is any difference in the test results. Linear
aids can be tested very quickly, so a delay of 20 mS is usually fine. Other aids are quite variable.
2.4.1.4 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 distor-
tion 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
FP40 analyzer, you can test the amount of distortion available in the second harmonics, the third
harmonics, or both harmonics (considered “total harmonic distortion”). This is selectable in the
General Setup Menu.
2.4.2 Understanding Composite signals
There are three types of composite signals: Composite, Digital Speech ANSI, Digital Speech ICRA.
The Composite signal is a continuous broadband signal containing 79 different frequencies pre-
sented simultaneously. This makes it much faster than a pure-tone because there is no waiting for a
progression of tones to complete – instead, you get the entire frequency response instantly, updating
about twice a second.
The digital speech signals are interrupted versions of the Composite signal that are used for testing
high end digital hearing aids. Many high end digital aids (though not all) use a technology called
“speech enhancement” or “noise reduction.” These aids respond to any continuous signal as if it
were noise, and lower the gain at the offending frequencies. Unfortunately, these aids regard the
Composite signal or pure-tone sweeps as noise, making them difficult to test using traditional meth-
ods.
The Digital Speech signals were developed as a way to test these high end hearing aids. Instead of
presenting a continuous signal, they present an interrupted signal that the aid regards as speech
instead of noise. There are two varieties: Digital Speech ANSI and Digital Speech ICRA.
2.4.2.1 ICRA vs. ANSI
There are two types of Digital Speech signals: Digital Speech ICRA and Digital Speech ANSI. Both
are interrupted composite signals for testing digital hearing aids. They differ in the speech spectrum
they use.