General Operation 43
Default Settings
When IM DIST is saved as the COUPLER or REAL-EAR RESET SRC in the
Advanced Default Settings Menu, the IM FREQ DIFF, DISTORTION SWEEP, and
DIST SWEEP END settings in the local menu will also be saved as default set-
tings, even though these settings are not available in any of the Default Settings
Menus.
2.5.2.3 Noise Reduction
Composite noise reduction is a little different than pure-tone noise reduction
(discussed in Section 2.5.1.3), even though both are used for noisy testing envi-
ronments.
Noise reduction for a composite signal averages together several of the previous
measurements with the current measurement to produce the next curve. When
you start measuring with a composite signal, the noise reduction displayed on
the screen will show the number of curves averaged so far in the measurement.
Thus, when the composite signal is first turned on and only a few measurements
have been taken, the noise reduction will begin as a low number. This number
will increase as more measurements are taken, eventually reaching the target
noise reduction selected in the menu.
Larger noise reduction numbers lead to smoother curves but increase the amount
of time it takes the analyzer to update the composite display.
2.5.3 Measuring RMS
RMS (“root mean square”) is a measurement of the average energy of a response
curve. It comes from a mathematical formula in which the response at each fre-
quency is squared. The RMS is the square-root of the sum of all these squares.
RMS is normally used in all Composite and Digital Speech measurements for
both coupler and real-ear measurements. It is also used in pure-tone measure-
ments greater than 90 dB SPL in coupler mode, or greater than 85 dB SPL in real-
ear mode.
On the FP35 analyzer, the RMS analysis is normally done using an estimated
RMS (ERMS). In ERMS, the FP35 compensates for system response errors that,
while minimal, can amount to several dB SPL in some situations. Compensation
for these errors is done by passing the signal through an FFT (fast Fourier trans-
form), correcting the response, then converting the data back to RMS. This allows
for greater RMS accuracy as long as there is not a lot of noise in the signal.
A problem with ERMS is that, in order for the measurement to be optimal, the
signal measured must be synchronized to the measurement system. When using
a source generated by the FP35, this will be the case. However, if the FP35 is in
Spectrum Mode, or if there is a lot of noise in the signal, part of the measurement
data may not be fully analyzed by the FFT, particularly if there is high frequency
noise that is outside the range of the FFT. Normally, the errors associated with
the ERMS analysis method are less than the errors with the True RMS (TRMS)
method, but in specific cases, TRMS might be slightly more accurate.
