Frye FONIX 7000 - Manual (Page 84)

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FONIX 7000 Hearing Aid Analyzer
3.4.8 Intermodulation Distortion
The composite signal is helpful for identifying intermodulation distortion (IM),
a type of distortion that sometimes occurs in a hearing aid response.
IM distortion occurs when more than one frequency is present in the source
signal and those frequencies combine to create new frequencies not actually
present in the source. IM distortion is disturbing to the client, often more so
than harmonic distortion. When viewing a graph run with the composite signal,
look for points along the graph where the line “breaks up.” Such an appearance
indicates the presence of intermodulation distortion. As IM distortion increases,
the curve becomes more disconnected. See Figure 3.4.8 for an example of IM
distortion.
This type of distortion is only apparent when the composite signal source is
used because pure-tone source types do not contain more than one frequency at
the same time.
Figure 3.4.8—Measuring intermodulation distortion