History lesson
If we go back 100 years or more, every telephone on the planet transmitted speech via a carbon microphone. This was a small container of carbon granules polarised by a low voltage and having a diaphragm at the front. As the diaphragm vibrated, the granules were subjected to varying pressure causing small changes in the voltage. This signal passed through a transformer and went off down the telephone line to be picked up by the receiving phone. Carbon microphones continued to be used in telephones all way up to the 1980s. Millions upon millions of carbon microphones were in daily use right across the world! The audio quality was a bit shit to say the least, with plenty of crackling and unwanted noise, BUT……. speech was intelligible, they were dirt cheap to manufacture AND they were utterly reliable over many years of use.
End of history lesson.
Scroll forward to the present day.
Exit the carbon mic, enter the MEMS mic. Today, mobile phones, laptops, tablets and other portable communication devices are everywhere. Most of these devices contain at least one MEMS microphone. Recent sales figures from US manufacturer Knowles Acoustics show sales for their SiSonic silicon based MEMS microphones in excess of 1 billion units!! 1 Billion units!! That is a lot of undercover microphones.
So What Is a MEMS microphone?
A MEMS microphone is manufactured using micro-electromechanical systems processing techniques. These microphones are etched into a semi-conductive silicon wafer. A pressure-sensitive moveable membrane (diaphragm) is etched behind a stationary perforated backplate. The perforated stationary plate and the diaphragm act together to form a capacitor (similar in design to a condenser microphone). The MEMS mic is designed to be mounted on a circuit board along with preamp and other application specific components.
Sketch of a MEMS microphone motor (not to scale). The diaphragm has a thickness of 1 µm, the gap between the backplate and the diaphragm is 4 µm, the diameter of the perforations in the backplate is 10 µm, and the thickness of the backplate is 2 µm. The distance across the motor from support post to support post is 590 µm. Sketch courtesy of Knowles Electronics.
Just like the carbon mics of the last century these MEMS microphones are extremely cheap to produce and are highly reliable. However, what is very different is the audio quality. The Knowles SiSonic MEMS microphone pictured above has a near flat frequency response from 100Hz to 10kHz which is pretty good for such a tiny, inexpensive, microphone. (<£1)
But what is really mind blowing is the Ultrasonic response!
The graph continues on up to 80kHz !!! (and beyond)
This brings me to my current project Ultrasonic Recording,
This MEMS microphone is perfect for recording the activity of bats, amphibians, crickets and other wildlife whose calls are in the ultrasonic range high above our rather limited human hearing.
AudioMoth https://www.openacousticdevices.info/ is a miniature recording device currently being developed to record and collect data on the activities of a variety of wildlife. The latest version of AudioMoth uses the Knowles SPU0410LR5H-QB MEMS microphone featured above.
So what does it sound like?
CLICK HERE to hear ‘BATS with Guitar’ (Yes, you heard right! )
N.B. The BATS have been slowed down to bring them within the range of human hearing
Conclusion
Hidden away out of sight on a circuit board inside a mobile phone or a laptop or some other electronic device the MEMS microphone is never going to gain any awards for stylish good looks. Even though its audio quality is amazing (considering its price) it isn’t about to replace the Neumann U87 in the recording studio! Nevertheless it is an extremely versatile tool that will be with us for many years to come and is without doubt the biggest selling microphone ever.
Martin Mitchell's Microphones 