Actually, an
electric guitar (or
bass)
pickup works by measuring the changes in the
magnetic field around it by the
metal vibrating strings. This is the same principle that most
traffic light sensors use, except they use a large
loop of
wire buried under the
asphalt instead of a
pickup and vehicles instead of
metal strings.
The pickup would work just as well in an airless chamber. The string would produce no sound, but the pickup would convert the change in inductance into an electrical signal representing the sound waveform just the same.