A hearing aid is a small amplifier that amplifies sounds that are otherwise inaudible. Utilizing the residual hearing of the hearing-impaired person, the sound is transmitted to the auditory center of the brain, allowing them to perceive the sound. This brings great convenience to the hearing-impaired.
Sound is produced by the vibration of objects; the object emitting the sound is called the sound source. Sound is simply the movement of pressure waves through the air. These pressure waves vibrate the small bones in the inner ear (ossicles), and these vibrations are converted into tiny electronic brainwaves, which are the sound we perceive. The inner ear operates on the same principle as a microphone capturing sound waves or a speaker producing sound; it is the relationship between moving mechanical parts and air pressure waves. Naturally, when the sound waves are low-pitched, long-wavelength, and sufficiently loud, we can actually "feel" the air pressure waves vibrating our bodies.
