Frye FONIX 7000 - Manual (Page 48)

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FONIX 7000 Hearing Aid Analyzer
Figure 2.3.2.3—Comparison of the ICRA shape and the ANSI shape
2.3.2.4 Composite type
There are two types of the traditional Composite signal available on the 7000
test system. They are known as “Standard” and “Chirp.” The two signals differ
in the manner that their phase is generated, and this difference has an impact
on the crest factor of the signals. The crest factor of a waveform is the ratio of its
highest amplitude to its RMS amplitude. Human speech is often referred to as
having a 12 dB crest factor.
For the needs of most clinicians, the Standard and Chirp Composite signal are
virtually identical.
The Standard Composite signal is composed of 79 different individual frequen-
cies that are each generated with a random phase pattern that results in the
signal having a crest factor of 10 dB, close to the crest factor of human speech.
(If the phase components were not randomized or otherwise changed, and if
all the signal components were in phase, the crest factor of the signal would
increase to over 19 dB.) The Standard Composite signal is the standard signal
used on the 6500-CX test system, the predecessor to the 7000.
The Chirp Composite signal is another type of composite signal with a low
crest factor. The phase of the individual frequency components of the Chirp
Composite signal is determined in a way that makes the signal appear to be pre-
sented as a “chirp,” or very fast sweep. The Chirp has a crest factor of 6 dB. The