audioscan RM500SL User Manual page 123

The speech-reception threshold (SRT) is attained
when the LTASS is at threshold (approximately - de-
pending on test material and the individual). These
scenarios are shown in Figures 10 - 12 which follow.
It should be noted that analysis methods which use
shorter time periods produce higher peak levels and
significantly different speech envelopes. In order to
produce results that can be directly compared to mea-
sures of threshold (and UCL), the analysis time period
needs to approximate the integration time of the ear.
Although this varies with frequency and individuals,
a value between 100 - 200 ms is likely. The Verifit and
RM500SL use a 128 ms analysis time period as an ap-
proximation because it also has considerable historic
support.
Because the spectrum of a speech signal varies with
time, it is necessary to average measurements over
Figure 9: Example of speech envelope and LTASS
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several seconds to obtain a stable, repeatable result.
However, the effects of adjustments to hearing aid pa-
rameters need to be seen quickly in order to be useful
for optimization. To resolve these conflicting needs,
speech signals are first presented in a repeating loop
with a sound-field equalization test prior to the start
of each passage. A running calculation of the short-
term speech envelope and average is performed on
2 - 3 seconds worth of data and is displayed in real
time so that the effects of adjustments to hearing aid
parameters are easily viewed. Pressing the Continue
key causes the passage to restart and run in its entire-
ty. The envelope and average are calculated over the
full passage to provide stable and repeatable data.
When using live speech as a test signal, a "Freeze
curve" function is available to capture the short-term
spectra for examination and counseling purposes.
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