<P> Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production, live and recorded audio engineering, sound recording, two - way radios, megaphones, radio and television broadcasting, and in computers for recording voice, speech recognition, VoIP, and for non-acoustic purposes such as ultrasonic sensors or knock sensors . </P> <P> Several different types of microphone are in use, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal . The most common are the dynamic microphone, which uses a coil of wire suspended in a magnetic field; the condenser microphone, which uses the vibrating diaphragm as a capacitor plate, and the piezoelectric microphone, which uses a crystal of piezoelectric material . Microphones typically need to be connected to a preamplifier before the signal can be recorded or reproduced . </P> <P> In order to speak to larger groups of people, a need arose to increase the volume of the human voice . The earliest devices used to achieve this were acoustic megaphones . Some of the first examples, from fifth century BC Greece, were theater masks with horn - shaped mouth openings that acoustically amplified the voice of actors in amphitheatres . In 1665, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of the "lovers' telephone" made of stretched wire with a cup attached at each end . </P> <P> German inventor Johann Philipp Reis designed an early sound transmitter that used a metallic strip attached to a vibrating membrane that would produce intermittent current . Better results were achieved with the "liquid transmitter" design in Scottish - American Alexander Graham Bell's telephone of 1876--the diaphragm was attached to a conductive rod in an acid solution . These systems, however, gave a very poor sound quality . </P>

Different types of microphones used in embedded systems