The Electret Microphone

In the next installment of Black Inventors on Friday in February, I’m featuring Dr. James Edward Maceo West. The vast majority of microphones are electret condenser microphones, a technology that was co-invented by Dr. West in 1962. You can find the patent for the “electroacoustic transducer” here: https://patents.google.com/patent/US3118022A

For being so common, the electret microphone uses some esoteric stuff. The “electret” itself is made of a ferroelectric material. If you’ve never heard of that, don’t feel bad. I have a degree in physics, and I didn’t even know about it until a couple of years ago.

Similar to how a ferromagnetic material (also known as “a magnet”) is magnetically polarized even without a flowing current, a ferroelectric material (aka “an electret”) is electrically polarized even without a static charge. What does that mean? It means that moving the electret around can change the voltage on nearby wires. Here’s a circuit from the schematic.

That black line 10 represents the diaphragm. As sound waves come in, they move the diaphragm 10 back and forth between the conductors 15 and 16. The key here is that the diaphragm 10 is an electret, so when it moves between the conductors 15 and 16, its electric polarization causes a measurable voltage change. Basically, it drags electrons to and from the conductors, creating an electrical signal that matches the sound waves. You can then amplify and record that electrical signal, and what you have is a microphone!

So that’s great and all. But West’s name is also on over 250 other patents. Not only that, but he’s still actively pursuing his research. At 91 years old he is apparently making a device to detect pneumonia in baby lungs. His birthday was yesterday, in fact! Happy birthday, Dr. West!