General Operation 35
• DIG SPCH—A randomly interrupted composite signal designed to mimic
speech. This signal is for use with hearing aids with a “noise reduction”
or “speech enhancing” feature that lowers the gain of the aid in the pres-
ence of a continuous signal (such as the regular composite signal). See
Appendix F for more details.
• LONG—Afrequencyresponsecurvethatmeasures64differentfrequen-
cies, containing more information (and taking longer) than the standard
NORM sweep.
2.5.1 Understanding Pure-Tone Signals
The different adjustments and measurements that you can make with pure-tone
signals are described in this section, including a discussion of the delay set-
tings, harmonic distortion, noise reduction, and warble rates.
2.5.1.1 Delay Settings
You can refine your pure-tone measurements by adjusting their delay times.
These adjustments are provided because some hearing aid circuits take a longer
time than others to adjust to changes in amplitude or frequency. If the measure-
ment is made too quickly, an testing artifact can result. If the measurement takes
too long, the test is longer than necessary.
In determining the length of time needed for a 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 experiment 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 msec is usually fine. Other aids are quite variable. See
Section 2.4 for a description of the available delay settings.
2.5.1.2 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 FP35 analyzer, you can test the amount of distortion avail-
able in the second harmonics, the third harmonics, or both harmonics (consid-
ered “total harmonic distortion”).
In general, you should ignore the harmonic distortion measurement when the
amplitude of the response curve measurement at the second harmonic is more
than 12 dB greater than the amplitude at the first harmonic. This is called the
12 dB rule and is part of the ANSI S3.22 standard.
