Functional Test - Bowers & Wilkins 704 S2 Series Service – Technical Manual

10. Functional Test (Stethoscope Examination of Speakers)
Aim of the Test
The stethoscope functional test can quickly reveal any air leaks or flaws following a repair
to a loudspeaker. The only sound coming from a loudspeaker enclosure should be from the
driver components themselves, not the surfaces or seams of the speaker cabinet.
We normally hear in stereo as a result of our ears being placed at two separate points in
space separated by several inches. When you use the stethoscope both of your ears hear
from the same single point at the position of the listening piece. You are now listening not
in stereo, but rather in mono mode which allows a greater accuracy in examining a
particular sound source.
Performing a Stethoscope Examination on a Speaker
For the following tests you should remove the chest piece from the end of a stethoscope
and listen via the tube only.
There are two test options for this speaker:
Test 1
Drive the speaker via an amplifier and signal generator set in the low bass region (20 to 60
Hz). Listen for air leaks at the enclosure seams, driver mounting edges, and jack plates. The
smallest air leak will be heard clearly.
Test 2
Start by playing your choice of music (frequency) through the speaker you wish to examine
and keep the volume moderate.
Place the earpieces of the stethoscope in your ears and place the listening tube around the
piece on the Speaker Cabinet that you wish to examine.
Place the listening tube several inches in front of each driver, in a multi-way speaker system
you can hear each individual driver to a great extent.
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