used then only the warble part of the calibration procedure may be carried
out.
However, if speech mode is required later (and a speech calibration is
performed) this will invalidate any previous warble calibration which would
then need to be repeated. If warble tones are to be used as a means of
equalising the frequency response in the speech calibration (see Section
A2.4.1.1) then this will invalidate any previous warble calibration which
would then need to be repeated when warble tone testing is required.
A2.4 Free-field Speech Calibration
This is carried out in two stages:
1) the speech channel, which contains two elements:
an optional equalisation phase
a level-setting phase
2) the competing noise channel, which may be omitted if competing
noise is not required
A2.4.1 Calibrating the Speech Channel
A2.4.1.1 Equalisation (Optional)
To perform equalisation, connect an external speech source to the
audiometer (e.g. CD or tape player). From the default (switch-on)
condition of the audiometer select SPEECH and FREEFIELD and then
play the test signal from the speech recording. This should either be:
pink noise used with a third-octave spectrum analyser and the SLM
third-octave noise bands used with the SLM
Use the SIGNAL control to set the output to 70dBHL, and adjust the
external amplifier to give a reading of 90dBSPL as measured by the SLM
at the reference point.
The response should then be checked to be within the following limits
(IEC 60645-2:1993 Section 10.1):
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