In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be captured as another form of energy (or the reverse).
In a loudspeaker, a diaphragm (generally, but not exclusively cone shaped) is the thin, semi-rigid membrane attached to the voice coil, which moves in a magnetic gap, vibrating the diaphragm, and producing sound. Diaphragms are also found in headphones, and microphones.
Similarly, the eardrum uses this same principle, using a diaphragm to stimulate nerves to transmit a neural "image" of sound to the brain. In loudspeakers, cellulose fiber (paper) has historically been the most common material used for the diaphragms, based on its low mass, and controllable acoustic properties. Synthetic fibres and binders may be added to provide specific properties. Other materials used for diaphragms include: polypropylene (PP), polycarbonate (PC), Mylar (PET), silk, glassfibre, carbon fibre, titanium, aluminium, aluminium-magnesium alloy, nickel, and beryllium.
In a phonograph reproducer, the diaphragm is a flat disk of typically mica isinglass that converts the mechanical vibration imparted on the needle from the recorded groove into sound. In the case of acoustic recording the reproducer converts the sound into the motion of the needle that scribes the groove on the recording media.