Frye FONIX 7000 - Manual (Page 46)

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FONIX 7000 Hearing Aid Analyzer
A significant advantage of using a composite signal is that each frequency pres-
ent in the signal can be individually controlled in amplitude and phase. As
the analyzer goes through the leveling process, each component is adjusted to
produce a signal that is optimally accurate at the reference point where the lev-
eling microphone is located. The 7000 test system is able to equalize the ampli-
tudes to within 0.25 dB for coupler measurements made with the Composite
signal. The spectrum limits are broadened to within 2 dB for real-ear measure-
ments.
2.3.2.1 Noise reduction
Composite noise reduction is a little different than pure-tone noise reduction
(discussed in Section 2.3.1.2), even though both are used for noisy testing envi-
ronments.
When a composite signal is running, the analyzer takes several different mea-
surements a second and displays them on the screen. Composite noise reduc-
tion performs a “running average” of these composite measurements. This
means it averages together several of the previous measurements with the cur-
rent measurement to produce the next curve. If you select “2X” noise reduc-
tion, it will average the last two measurements together. A selection of “4X”
averages the last four measurements together. When performing composite mea-
surements, it will take several seconds for the actual noise reduction used to
reach the set value. This is due to the nature of the running average.
Larger noise reduction numbers lead to smoother curves but increase the
amount of time it takes the analyzer to update its composite measurements.
2.3.2.2 Intermodulation distortion
The composite signals are helpful for identifying intermodulation distortion
(IM). 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 2.3.2.2 for an example of IM distortion.
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.