Method for attaching an audio transducer to a string musical instrument
US5058173A
Combination inertia type audio transducer and loudspeaker
US4718098A
Multi-diaphragm artificial reverberation device
US4151379A
Electromagnetic speaker with bucking parallel high and low frequency coils drives sounding board and second diaphragm or external apparatus via magnetic coupling and having adjustable air gap and slot pole piece