How do mems electric microphones work? By Hester Verweij
MEMS in microphones
The challenge right now is to explain in my own words how the MEMS in the plates in a microphone work.
Short explanation of my finding about MEMS so far: MEMS are microelectromechanical systems, this systems are like microsensors that interact with the surroundings. When sound waves arrive in a microphone the plates in the microphone converting these waves into a current that is pushed into the wire.
What I would like to learn more is: The membrane from the plate is receiving the sound wave, what is happening in a micro parts of the plate.
When an object is forced into resonance vibrations at one of its natural frequencies, it vibrates in a manner such that a standing wave is formed within the object[1].
MEMS chips convert the digital sound source into the sound waves again on the other side of the speakers, for example in a telephone.
Figure 1.
But what happens in these chips?
There are mainly two types of MEMS microphones – Analog which convert sound into corresponding voltage output and Digital which gives a digital output typically pulse density modulation [PDM].
MEMS microphone basically is an acoustic transducer.
• Transduction principle is the coupled capacity change between a fixed plate (back-plate) and a movable plate (membrane)
• The capacitive change is caused by the sound, passing through the acoustic holes, that moves the membrane modulating the air gap comprised between the two conductive plates
• The back-chamber is the acoustic resonator
• The Ventilation hole allows the air compressed in the back chamber to flow out and consequently allowing the membrane to move back
Figure 2.
Little deeper information:
What is a MEMS?
MEMS, microelectromechanical systems. Microelectromechanical systems (MEMS) (also written as micro-electro-mechanical, MicroElectroMechanical or microelectronic and microelectromechanical systems and the related micromechatronics) is the technology of very small devices. MEMS are made up of components between 1 to 100 micrometres in size. They usually consist of a central unit that processes data (the microprocessor) and several components that interact with the surroundings such as microsensors.
Figure 3.
Cymatics is the study of visible sound co vibration, a subset of modal phenomena. Typically the surface of a plate, diaphragm, or membrane is vibrated, and regions of maximum and minimum displacement are made visible in a thin coating of particles, paste, or liquid. Different patterns emerge in the excitatory medium depending on the geometry of the plate and the driving frequency[2]. And that is the effect that accurse when the sounds waves, shown in figure 1, arrive to the front plate.
In air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions, hence, a number of different types of sound waves are possible. Waves can be characterized in space by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. The propagation of waves is often described in terms of what are called “wave modes.”[3] Sound waves are longitudinal waves through the air[4], see figure 2.
Figure 4.