Frye FONIX FP40D - Manual (Page 33)

General Operation 25
This type of distortion is only apparent when a composite signal source is used because pure-tone
sweeps do not present more than one frequency at a time.
Figure 2.4.2.3—IM distortion
2.4.2.4 Composite source levels
When you adjust the amplitude of a composite signal, you are adjusting the root-mean-square
(RMS) of the signal, not the amplitude of the individual frequency components. None of the ampli-
tudes individual frequency components will be as high as the amplitude of the overall signal.
This is particularly noticeable when you view the test signal in dB SPL because this allows you to
see the actual power of each frequency component. See Figure 2.4.2.4 as an example of this phe-
nomenon. In this figure, the displayed curve has a 70 dB SPL RMS amplitude. Notice that the level
at each frequency varies from –10 dB to –30 dB from the overall amplitude.
The amplitude differences in the individual components of the composite signal will be reflected in
the frequency response of a hearing aid. Keep this in mind when viewing results in dB SPL.
Figure 2.4.2.4—Composite signal with an amplitude of 70 dB SPL RMS