Iranian professor claims invention of world's tiniest microphone

Professor Behram Azizollah Gazi of the Noshirvani University of Babol in Iran claims to have developed a capacitance microphone measuring only 0.5 X 0.5 mm, which would make it the world's smallest microphone. Previous tiny microphones, like the ones made by the Danish company Sonion, measure 2.6 mm X 1.6 mm. Dr. Gazi has published several papers on fabricating very small capacitance microphones by the MEMS ( Micro Electromechanical Systems) process. He also holds the patent for a high sensitivity single chip MEMS microphone using a slotted membrane, awarded in 2007.

World's tiniest microphone

Microphone technology

A microphone is a transducer that converts sound energy into an electrical signal. Around 2 billion small microphones are used around the world each year. About half of these are for low fidelity applications such as in toys. Higher performance microphones are being built into mobile phones, tablets and laptop computers where both size and power requirements are important. In addition, these chip microphones need to be capable of being soldered on a printed circuit board, which means that they have to withstand temperatures up to 260 degrees C, and subsequent cleaning with solvents and water jets. These microphones need to engineered to pick up signals only from the voice source and be immune to other noises in the device.

High performance microphones are also built into hearing aids and used in medical equipment to monitor blood flow, lung function, in fetal heartbeat monitoring and other applications. Microphones are also used in defense and security surveillance, for noise and vibration measurement, and in marine sonar and industrial ultrasonic equipment.

While several types of microphones exist, the capacitive microphone is the most widely used due to its small size, ease of manufacture and long term stability. The capacitance microphone is made of a fixed back plane and a silicon membrane with an electret layer in-between. The sound waves cause the membrane to move, causing the capacitance to change. This signal is picked up and amplified in an ASIC chip built into or alongside the microphone. The power supply for the microphone and ASIC is from the same leads that carry the signals out for external processing.

Dr Gazi's invention

Small capacitive microphones are made by micromachining of a silicon substrate to form a cavity. A silicon membrane is stretched over the cavity and and the gap is filled with electret material. When the microphone is miniaturized, the membrane's ability to vibrate with sound decreases. Dr. Gazi came up with the idea that if slots are machined on the membrane surface, it allows the trapped air to escape and makes the microphone more responsive to sound waves. He has written several research papers on this topic and the invention of the tiny microphone is the outcome of his research into this subject over several years.

Noshirvani University is one of Iran's oldest technical universities, set up in Babol, a town 200 km north of Teheran. Dr Gazi, is 48 years old and has teaching and research experience both in Iran and Malaysia. The capacitive microphone for which he received the patent was developed during his PhD studies in Malaysia, between 2005 and 2007.

This invention is sure to lead to further miniturization in the design of hearing aids and other microphone related appliances.

Via: Tech2