FONIX® 8000 - Frye Electronics

40 FONIX 8000 Hearing Aid Test System
For the needs of most clinicians, the Standard and Chirp Composite signals are
virtually identical.
The Standard Composite signal is composed of 79 different individual frequencies
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 7000 test system, the predecessor to the 8000.
The Chirp Composite signal is another type of composite signal, but with a
lower 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 presented as a “chirp,” or very fast sweep. The Chirp has a crest factor of 6
dB. The Chirp Composite signal is the standard composite signal used on the
FP40 and FP35 portable hearing aid test systems.
Although the Standard and Chirp Composite signals sound the same to most
people, an analysis tool such as Tempus3D will show the difference between
them. Both the Standard and the Chirp Composite signals are further speechweighted
to agree with the user-selected composite filter (described in Section
2.3.2.3.)
2.3.2.5 Composite source levels
The Composite signal (and its Digital Speech counterpart) is a complex signal
consisting of 79 different frequency components. When you set the source level
of a composite signal (50 dB, 60 dB, etc), you are actually setting its overall
energy, known as the RMS (root-mean-square) of the signal; you are NOT setting
the amplitude of the individual frequency components.
The actual amplitude of each of frequency of the Composite signal is less than
the overall RMS of the signal. See Figure 2.3.2.5 for an example of the SPL levels
of each frequency of the Composite signal with an RMS of 70 dB. Be aware
of this difference, especially when viewing frequency response results in overall
amplitude of dB SPL.