BRUEL & KJAER 4185, User Manual

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Chapter 1 – Description
3 Ear Simulator for Telephonometry Type 4185
User Manual
1.1.1 The Acoustic Coupler
Type 4185 consists of an acoustic coupler that has a built-in miniature
sound source for seal checking, a microphone and a preamplifier.
The seal between Type 4185 and the handset under test must be
completely tight. The quality of this seal can be verified using the
miniature sound source located inside the acoustic coupler. This sound
source is a small transducer that can be excited with an external signal
to produce a sound pressure inside the main cavity. If the seal between
the coupler and handset is poor, the sound pressure measured by the
microphone will be considerably lower at low frequencies than if the
seal is tight.
Fig.1.3 Sectional view of the acoustic coupler used in Type 4185
As shown in Fig. 1.3, the acoustic coupler contains three volumes
acoustically connected in parallel by means of a narrow annular slit and
four parallel holes. The volumes are:
•
V
1
= 2.5 cm
3
•
V
2
= 1.8 cm
3
• V
3
= 7.5 cm
3
The acoustic resistances and inductances of the slit and parallel holes
are given in Chapter 4.
Soft Seal
Sound Source
Measuring Microphone
V
3
V
2
V
1
Black Collar
900339e
Acoustic
Coupler