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4.1. Calibrating the Speech Channel
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)
Frequency Range (Hz) Tolerance (dB)
125 to 250 +0/-10
250 to 4000 +3/-3
4000 to 6300 +5/-5
If necessary, adjustments should be made using the amplifier con-
trols or an additional graphic equalizer to achieve this response.
As an alternative to using an external speech source, the warble-
tone calibration method and controls (see Section 5 of this appen-
dix) may be used to achieve this response. Note that this will inva-
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