Frye FONIX FP40D - Manual (Page 32)

24 FONIX FP40 Portable Hearing Aid Analyzer
Digital Speech ANSI uses the same speech spectrum as the Composite signal. This speech spectrum,
taken from the ANSI S3.42 standard, rolls off the high frequencies starting with 3 dB down at 900
Hz and continuing at a rate of 9 dB per octave. Digital Speech ICRA uses the ICRA speech spectrum
developed by the International Collegium of Rehabilitative Audiology. The ICRA spectrum is based
on the Long Term Average Speech Spectrum (LTASS) and rolls off the high frequencies more rapidly
than the ANSI spectrum. Figure 2.4.2.1 shows a comparison of the spectra.
Figure 2.4.2.1—Comparison of Digital Speech ICRA (CRV 1) and Digital Speech ANSI (CRV 2)
2.4.2.2 Noise Reduction
There is only one setting for the composite signals in the General Setup Menu: NOISE RED.
Composite noise reduction is a little different than pure-tone noise reduction, even though both are
used for noisy testing environments.
When a composite signal is running, the analyzer takes several different measurements a second
displays them on the screen. Composite noise reduction performs a “running average” of these com-
posite measurements. This means it averages together several of the previous measurements with
the current measurement to produce the next curve. If you select “2X” noise reduction, it will aver-
age the last two measurements together. A selection of “4X” averages the last four measurements
together.
Larger noise reduction numbers lead to smoother curves but increase the amount of time it takes
the analyzer to update its composite measurements.
2.4.2.3 Intermodulation Distortion
The composite signals are helpful for identifying intermodulation (IM) distortion. IM distortion
occurs when amplitudes at more than one frequency in a signal combine to create an amplitude at
a frequency not present in the original signal. When viewing a graph run with a composite signal,
look for points along the graph where the line “breaks up.” Such an appearance indicates the pres-
ence of IM distortion. See Figure 2.4.2.3 for an example of IM distortion.