Microphone

Several types of microphone are used today, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal.

[4] 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.

[5] In 1856, Italian inventor Antonio Meucci developed a dynamic microphone based on the generation of electric current by moving a coil of wire to various depths in a magnetic field.

Better results were achieved in 1876 with the "liquid transmitter" design in early telephones from Alexander Graham Bell and Elisha Gray – the diaphragm was attached to a conductive rod in an acid solution.

Although Edison was awarded the first patent in mid-1877 (after a long legal dispute), Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention.

The absence of a high bias voltage permits the use of a diaphragm with looser tension, which may be used to achieve wider frequency response due to higher compliance.

A covert, remotely energized application of the same physical principle called the Thing was devised by Soviet Russian inventor Leon Theremin and used to bug the US Ambassador's residence in Moscow between 1945 and 1952.

The classic RCA Type 77-DX microphone has several externally adjustable positions of the internal baffle, allowing the selection of several response patterns ranging from figure-eight to unidirectional.

Carbon microphones were once commonly used in telephones; they have extremely low-quality sound reproduction and a very limited frequency response range but are very robust devices.

Carbon microphones found use as early telephone repeaters, making long-distance phone calls possible in the era before vacuum tubes.

They were difficult to match to early transistor equipment and were quickly supplanted by dynamic microphones for a time, and later small electret condenser devices.

They have proven especially useful in medical applications, such as allowing radiologists, staff and patients within the powerful and noisy magnetic field to converse normally, inside the MRI suites as well as in remote control rooms.

In a more robust and expensive implementation, the returned light is split and fed to an interferometer, which detects movement of the surface by changes in the optical path length of the reflected beam.

On August 25, 2009, U.S. patent 7,580,533 issued for a Particulate Flow Detection Microphone based on a laser-photocell pair with a moving stream of smoke or vapor in the laser beam's path.

Bell's liquid transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid added.

Other minor variations and improvements were made to the liquid microphone by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version was patented by Reginald Fessenden in 1903.

Major manufacturers producing MEMS silicon microphones are Wolfson Microelectronics (WM7xxx) now Cirrus Logic,[36] InvenSense (product line sold by Analog Devices[37]), Akustica (AKU200x), Infineon (SMM310 product), Knowles Electronics, Memstech (MSMx), NXP Semiconductors (division bought by Knowles[38]), Sonion MEMS, Vesper, AAC Acoustic Technologies,[39] and Omron.

Reciprocity applies, so the resulting microphone has the same impairments as a single-driver loudspeaker: limited low- and high-end frequency response, poorly controlled directivity, and low sensitivity.

The body of the microphone is not infinitely small and, as a consequence, it tends to get in its own way with respect to sounds arriving from the rear, causing a slight flattening of the polar response.

Common shapes include: Three such cardioid microphones/hydrophones could be orthogonally oriented as a collocated triad to improve the gain and also create a steerable beam pattern.

Sound engineers Ed Long and Ron Wickersham developed the concept of placing the diaphragm parallel to and facing the boundary.

Typical uses of this microphone, which has unusually focused front sensitivity and can pick up sounds from many meters away, include nature recording, outdoor sporting events, eavesdropping, law enforcement, and even espionage.

Other noise-canceling designs use one diaphragm that is affected by ports open to the sides and rear of the microphone, with the sum being a 16 dB rejection of sounds that are farther away.

One noise-canceling headset design by Crown using a single diaphragm has been used prominently by vocal artists such as Garth Brooks, Britney Spears and Janet Jackson.

To reduce the number of amplifiers in telephone lines, power loss needed to be minimal so source and load impedances were matched.

A downside to impedance matching is the 6 dB loss in signal that occurs as only half the voltage level appears at the pre-amplifier's input.

Measurement microphones are generally scalar sensors of pressure; they exhibit an omnidirectional response, limited only by the scattering profile of their physical dimensions.

There are many applications: Typically, an array is made up of omnidirectional microphones distributed about the perimeter of a space, linked to a computer that records and interprets the results into a coherent form.

While foams and wire meshes can be partly or wholly self-supporting, soft fabrics and gauzes require stretching on frames or laminating with coarser structural elements.

If not matted by wind and rain, the fur fibers are very transparent acoustically, but the woven or knitted backing can give significant attenuation.