American Diagnostic Corporation (ADC) 657 Electronic Stethoscope - Manual (Page 6)

3. Name and function of each part
Piezopolymer film contact microphone beneath diaphragm
Soft touch volume control button to increase/decrease volume
Power ON/OFF and mode selection
LED: Indication of selected mode and low battery warning
B - Bell
D - Diaphragm (also battery indication light)
W - Wide (extended range)