BRUEL & KJAER 4128-C - 2.3 Ear Simulator Operating Characteristics

CHAPTER 2
Characteristics
27
CAUTION: If the mouth simulator is excited with pulses of more than 50 W, damage may
occur before the protection circuit reacts.
When operating at relatively high power levels, the output SPL of the mouth simulator changes
with time for a short period after the signal is applied due to heating of the loudspeaker coil.
Fig.2.5 illustrates the short-term effect for different input power levels. If the mouth simulator
is to be operated at consistently high power levels, then calibration should be performed at
these high power 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.
Fig.2.5
Short-term variation of
output sound pressure
level with input power.
Operation above the
broken curve triggers the
protection circuit
2.3 Ear Simulator Operating Characteristics
Listener Free-field Frequency Response
The frequency response from the undisturbed free-field to the eardrum of a manikin 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.2.6 shows a typical LFR for sound incident at 0

for Type 4128-C, with the ear simulator in
the 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 4128-C illustrates the measured LFR for sound incident at 0

for that
130
dB
SPL
120
110
100
10
9
8
7
65
4
32
1
0
990097
Time (s)
1 W (2 V)
2.5 W (3.16 V)
10 W (6.3 V)
25 W (10 V)
50 W (14.2 V)
100 W (20 V)
Measured at 500 Hz