General Operation 37
Here is an explanation of the available warble selections:
• FAST is usually used in real-ear measurements. In all screens except the
Audiogram Entry Screen, it warbles at a rate of 33 Hz. In the Audiogram
Entry Screen, it warbles at a rate of 20 Hz, ±5% deviation from central
frequency.
• SLOWisusedfortestingdigitalhearingaidsandforaudiometricmea-
surements. It warbles at a rate of 6.25 Hz, ±5% deviation.
• OFFturnsoffallpure-tonewarblinginallmeasurements.
• AUTOmakesthefollowingwarblingselections:
- OFF for all sound chamber measurements and real-ear measurements
made with NORMAL or SHORT pure-tone sweeps.
- FAST for real-ear measurements made with a FAST pure-tone sweep or
SINGLE pure-tone with an AID TYPE of STANDARD or AGC.
- SLOW for real-ear measurements made with a FAST pure-tone sweep or
SINGLE pure-tone with an AID TYPE of ADAPTIVE.
2.5.1.5 Adjusting the Signal Skew
The SIGNAL SKEW selection, available in the Custom Menu of several mea-
surement screens, is the number of samples from when the source signal leaves
the speaker and when it is measured by the microphone. (The USER LEVEL
must be set to ADVANCED. See Section 2.3.2.)
The amount of time it takes for a sound signal to travel from the speaker,
through the hearing aid (or device being measured), and to the ear or coupler,
where it is measured by the FP35’s microphone, varies naturally depending
upon the distance of the FP35 microphone from the speaker, and the amount of
digital processing delay inherent in the hearing aid. This time delay is normally
so small that it does not affect the quality of the hearing aid measurements.
However, some digital hearing aids have a signal processing delay significant
enough to make this amount of time more meaningful.
A warbling pure-tone signal, in particular, is dependent upon the FP35’s abil-
ity to synchonize the source signal and the measurement processing. When the
hearing aid adds additional delay through its internal signal processing, it can
throw the FP35’s internal synchonization off. The SIGNAL SKEW menu item
lets you adjust for this delay by specifying the number of samples between the
source emitting the signal and the FP35 processing it. There are 39 uS (micro-
seconds) in each sample for 100 Hz-interval frequency. The SIGNAL SKEW
selection is applied to all source types, but it should only have an effect on
pure-tone measurements.
