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3.2.3
Harmonic Distortion
Fig.3.4 shows typical second-harmonic and third-harmonic distortion of the mouth simulator.
3.2.4
Overload Protection
The mouth simulator of Type 5128 has a built-in overload protection circuit, which will trigger
automatically if the drive signal applied to the loudspeaker is too great, or remains on for too
long. It allows continuous operation up to 0.7 V
1.5 V
for 2 seconds. If these limits are exceeded, the circuit disables the mouth simulator
rms
output. The protection circuit resets approximately 30 s after the drive signal is disconnected.
If the mouth simulator is excited with pulses of more than 0.7 V
CAUTION:
before the protection circuit reacts.
When operating at relatively high voltage levels, the output sound power level (SPL) of the mouth
simulator changes with time for a short period after the signal is applied due to heating of the
loudspeaker coil. If the mouth simulator is to be operated at consistently high voltage levels, then
calibration should be performed at these high voltage levels. Calibration performed at low levels
would lead to inaccuracies due to the heating effects.
Variation of performance with variation of ambient temperature and pressure is negligible within
the specified operating temperature and pressure of the mouth simulator.
3.3
Ear Simulator Operating Characteristics
3.3.1
Listener Free-field Frequency Response
The frequency response from the undisturbed free-field to the eardrum of a mannequin has a
variety of names. In this manual we refer to it as the listener free-field frequency response (LFR).
Names which may be found in other publications include listener orthotelephonic (reference)
sensitivity frequency response (particularly in ITU literature), mannequin frequency response
(MFR) (particularly in connection with hearing aids), and free-field frequency response.
Fig.3.5 shows a typical LFR for sound incident at 0 for Type 5128-C, with the ear simulator in the
left and right ear. The LFR is the overall frequency response from the free-field to the eardrum and
includes the effects of the head, torso, pinna, ear-canal and ear simulator. The calibration chart
supplied with each Type 5128 illustrates the measured LFR for sound incident at 0 for that
particular model, and this measurement can also be found in file
on the data disk. A similar measurement for the left ear can be found in file
LFR_L-5128-C-111-xxxxxxx.csv
and pulsed operation up to a maximum of
rms
.
CHAPTER 3
Characteristics
, damage may occur
rms
LFR_R-5128-C-111-xxxxxxx.csv
23
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