Positioning The System; Background Noise - Brüel & Kjær 3638 Manuel De L'utilisateur

18 Exhaust Noise Inspector Type 3638 – User Manual

Positioning the System

Under normal operation, the controller and sound level meter remain in the carrying case.
Power supply, output and RS – 232 connections between the controller and sound level meter
interfaces are integrated in the case's interior. During measurement, only the microphone, con-
nected to its extension cable, is taken out and setup near the vehicle's exterior exhaust system.
Fig.3.1 The carrying case with the controller and sound level meter in their separate
compartments during measurement. The microphone is placed near the vehicle's
exhaust system
To measure, position the microphone near the exhaust system at a distance required by legisla-
tion. Place the Type 3638 A/B carrying case, with the all other instrumentation, near the test
operator for easy management of the controller display and LED indicators. Switch on the
controller, which starts-up the entire system including the sound level meter and internal acous-
tic RPM detector. You are ready to measure.
Note: The use of an external tachometer, for example, Tachometer Type 2979 (optional acces-
sory), requires the placement of the tachometer within the engine.

Background Noise

According to most national regulations, measurement of motor vehicle exhaust noise levels may
only take place in an environment where background noise is at least 10 dB below the actual
exhaust noise to be measured. It is, therefore, necessary to find out what the level of back-
ground noise is prior to making any measurements. Type 3638 A/B can be set up to automati-
cally measure background noise levels at start-up. Alternatively, you can take spot checks
manually using the entire system or just the sound level meter as a stand-alone instrument. In
order to ensure accurate results at each exhaust noise level measurement, it is recommended
that you set up Type 3638 A/B to measure background noise automatically. To do this, see
"Background Noise Measurement at Startup" on page 14. This means that every time you
switch on the system, the display will prompt a background noise measurement: