Sound refers to longitudinal waves of compression and expansion that travel through air or other physical medium. Any vibrating object can be a source of sound. Musical instruments create vibrations and vocal cords set an air column into vibration. A loudspeaker is simply a vibrating diaphragm that is electrically driven. The human ear is generally sensitive to frequencies between 20 Hz and 20,000 Hz.
Lower frequencies are subsonic or infrasonic and higher frequencies are ultrasonic.
The human ear is an incredibly sensitive receiver of sound, capable of detecting sound for which the fluctuations of density of the air are less than one ten-millionth of 1 percent. A loudspeaker converts electrical energy into mechanical energy in the form of vibrations that are transmitted to the surrounding air, producing sound. The most common loudspeaker used today is the dynamic loudspeaker, a type of direct-radiator loudspeaker. Electrical current flows through a voice coil suspended in a uniform magnetic field, producing forward and backward movement of the voice coil. A rigid conical diaphragm attached to the voice coil transmits motion to the air.