Design fabrication and characterization of biologically inspired MEMS directional microphone

A microphone is a device that has been used by mankind since time memorable. It accumulates acoustic signals around it and transmits it further for signal processing. Depending on the type of microphone, it is in a position to accumulate the acoustic signal from sources in all directions (Omni directional microphone) or from one particular direction (unidirectional microphone). The earliest known device that could amplify the sound to a larger audience dates back to 600 BC [1], where the sound was captured by a mask that had an opening for the mouth. In 1665, an English physicist Robert Hooke [2] experimented and succeeded in sending an acoustic signal in a medium other than air. He made a device where two cups were attached to the two ends of a stretched wire. The signal travelled through the wire and the two cups acted as a transmitter / receiver interchangeably. This design was further modified by Johann Philipp Reis a German inventor, where he attached a vibrating membrane to a metallic strip. This metallic strip would generate intermittent current proportional to the vibration of the membrane. Alexander Graham Bell invented a telephone in 1876 in which the diaphragm was attached to a conductive rod immersed in an acid solution. The demerit of this system was the poor sound quality. In mid 1877 Thomas Alva Edison was awarded the patent for the first device which was successful in transmitting a voice signal. This formed the foundation of the present day telephony. The device consisted of loosely packed granules of carbon. These granules were subjected to varying pressure by the movement of the diaphragm and this caused a proportional change in resistance of the carbon granules. This transduction principle of the pressure being converted to a proportional electrical signal came into existence with this invention and it was Hughes who coined the word Microphone. The use of carbon in the microphone was the first stepping stone in building the modern day telephone. In 1923 the first practical moving coil microphone called the magnetophon was developed by Captain H.J. Round. It was the most commonly used microphone by BBC studios in London. The ribbon microphones were invented by Harry F. Olson in the year 1930. It also used the same principles of a Magnetophon. During the second half of the 20th century, microphone development advanced quickly with the Shure Brothers bringing out the Shure Microphone models SM57 and SM58. Digital microphones were pioneered by Milab in 1999, with the DM-1001.The latest developments include the use of fiber optics, lasers and interferometer in microphone / sound detection.