A microphone, colloquially mic or mike (), is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. Electromagnetic transducers facilitate the conversion of acoustic signals into electrical signals.〔F. Rumsey & T. McCormick,2009, Sound and Recording, Elsevier〕 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, 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 checking or knock sensors.
Most microphones today use electromagnetic induction (dynamic microphones), capacitance change (condenser microphones) or piezoelectricity (piezoelectric microphones) to produce an electrical signal from air pressure variations. Microphones typically need to be connected to a preamplifier before the signal can be amplified with an audio power amplifier and a speaker or recorded.
In order to speak to larger groups of people, there was a desire to increase the volume of the spoken word. The earliest known device to achieve this dates to 600 BC with the invention of masks with specially designed mouth openings that acoustically augmented the voice in amphitheatres.〔Montgomery, Henry 1959 Amplification and High Fidelity in the Greek Theater. The Classic Journal〕 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.〔McVeigh, Daniel P 2000. An Early History of the Telephone: 1664–1866: Robert Hooke's Acoustic Experiments and Acoustic Inventions〕
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.〔MacLeod, Elizabeth 1999 Alexander Graham Bell: an inventive life. Kids Can Press, Toronto〕 These systems, however, gave a very poor sound quality.
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