36 FONIX FP35 Hearing Aid Analyzer
Historically, the harmonic distortion measurement has been taken only at sig-
nal levels below 90 dB SPL. This is because it was assumed that most hearing
aids went into saturation at that loud of a level, resulting in very high harmonic
distortion measurements. For this reason, we introduced a “90 dB harmonic dis-
tortion” rule on our analyzers that automatically turned off harmonic distortion
measurements with signals that are 90 dB SPL or higher. On the FP35 analyzer,
this was implemented as the DIST 90 DB RULE setting in the Custom menu of
the Coupler Multicurve and real-ear measurement screens. (The USER LEVEL
must be set to ADVANCED. See Section 2.3.2.) In recent years, however, hear-
ing aids have become more sophisticated and better able to handle loud signals,
making harmonic distortion a more useful measurement at 90 dB SPL. The
DIST 90 dB RULE is still available, but we have turned it OFF by default.
Definitions
• 2NDHarmonic:Energyofthesecondharmonicortwicethepresentedfre-
quency.
• 3RDHarmonic:Energyofthethirdharmonicorthreetimesthepresented
frequency.
• TOTAL:Combined2NDand3RDharmonicdistortion.
2.5.1.3 Noise Reduction
Noise reduction is used in noisy testing environments. Pure-tone noise reduc-
tion takes several measurements at each frequency and averages those measure-
ments together. Larger noise reduction numbers lead to smoother curves but
increase the amount of time it takes to complete a pure-tone sweep.
For example, if you select “4” as the pure-tone noise reduction setting, 172 mea-
surements (43 x 4) will be taken with every normal pure-tone sweep
2.5.1.4 Warble Rates
When you take pure-tone measurements in a sound field environment, it is use-
ful to warble the tones to reduce standing wave effects and create more accurate
measurements. (Warbling is not usually necessary in a sound chamber testing
environment.)
The FP35 analyzer allows you to change the warbling rate of its pure-tone sig-
nals. You do this by altering the WARBLE SOURCE selection in the Custom
Menu of the Coupler Multicurve Screen and the Real-Ear Measurement Screens.
The USER LEVEL must be set to ADVANCED. See Section 2.3.2.
Some types of warbling can cause problems when used to test digital hearing
aids. This is because the digital processing delay of the aid conflicts with the
warbling timing. We have altered our SLOW warble signal to account for this
problem. This type of warbling is automatically selected during real-ear mea-
surements when the AID TYPE is set to ADAPTIVE in the local menu of the
measurement screen.
