I've previously only used one with the diaphragm replaced with a length of polythene tube to discover where squeels or squeaks were coming from in mechanical equipment. This time a bass speaker that was making rattling/scratchy noises when driven with sub-100Hz feed. Nothing wrong with cone/gluing/skirt/spider etc and fingering the driven cone (wearing ear defenders) made no real difference to the random rattle. Could only summon it to order by placing the cab on its side, otherwise in band use it would appear intermittently. Decided to listen with the stethoscope , with diaphragm attached , while driving with 60Hz, not deafening as presumably because not tuned for sub-100Hz. Rattley/scratchy noise was then noticeably prominent in one area of the speaker frame mounting. Removing again from the cab , there was a length of unglued and uncompressed foam, about 4mm at widest, around the mounting flange that must have been resonating and flapping against the frame.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on