44 FONIX FP35 Hearing Aid Analyzer
True RMS (TRMS) does not compensate for system response errors. It can be
more accurate for measurements using external signal sources or measurements
in an extremely noisy test environment. It is also a much faster measurement
because it doesn’t use an FFT – using TRMS can speed up the measurement
by nearly 50%. TRMS does contain a frequency response correction at 1 kHz,
regardless of the selected frequency.
TRMS cannot be used with the probe microphone. This is because the probe
microphone of the FP35 analyzer does not have a flat response – to achieve an
accurate measurement, the analyzer must compensate for the response of the
microphone. As such, all RMS measurements made with the probe microphone
are always done as ERMS.
TRMS can also not be used when the input speech filter is enabled.
Measurements made with the Composite and Digital Speech signals are usually
speech weighted. ERMS must be used for those measurements.
Corrected True RMS (CRMS) is True RMS with a correction in the frequency
response at the current selected frequency. CRMS can provide slightly more
accurate measurements when using the FP35 signal source, but potentially can
be less accurate if the dominant signal is not the source signal or there is no
source signal.
The accuracy difference between the different RMS analysis methods is nor-
mally +/- 3 dB, and typically less than 2 dB. By allowing the RMS method to be
selected, the user can improve the accuracy for a particular measurement. For
best accuracy, the microphones should be custom calibrated.
The RMS analysis method can be set in the Custom menu of the real-ear mea-
surement screens and the Coupler Multicurve, Coupler Target, and Coupler
EarSim screens. The USER LEVEL must be set to ADVANCED to change these
settings. See Section 2.3.2 for details.
To change the method:
1. Press [MENU] in one of the screens described above to enter a local menu.
2. Press [NEXT] twice to enter the Custom menu.
3. Use [
∨, ∧] to select RMS ANALYSIS.
4. Use [<, >] to choose the selection. The choices are: ERMS, TRMS, NONE,
AUTO.
The RMS analysis choices are described here:
AUTO or ERMS:
• RMSOUTwillbemeasuredasERMSwithCompositeorDigitalSpeech
• ERMSwillbemeasuredwithreal-earpure-tonesatorabove85dBSPL.
• ERMSwillbemeasuredwithcouplerpure-tonesatorabove90dBSPL.
