a microphone
/ MY-kruh-fohn /
A microphone is a transducer for sound — it turns the tiny pressure waves wiggling through the air into a matching electrical signal. Sound is just air being pushed and pulled very fast; a microphone has a thin diaphragm that gets pushed and pulled along with it, and some clever trick converts that motion into a voltage that rises and falls in step with the sound. Speak into it and your voice becomes a wiggling voltage a circuit can record, amplify, or analyze.
There are two common types you will meet. An electret (or MEMS) microphone, the kind in phones and laptops, has a charged plastic film whose motion changes a capacitance; it contains a built-in tiny transistor (JFET) and therefore needs a small DC supply through a pull-up resistor, the output riding as an AC wiggle on a DC level. A dynamic microphone has a coil attached to the diaphragm moving in a magnet, generating a voltage directly by induction — no power needed, but a tiny output. Either way the signal is small (often a few millivolts), so a microphone is always followed by a preamplifier and usually a series capacitor to strip off the DC and pass only the audio wiggle.
Why this matters: microphones are the input to every voice assistant, phone call, hearing aid, and recording. They also stand in for any acoustic sensor — ultrasonic distance sensing, machine vibration monitoring, leak detection. The honest realities: the raw signal is small and easily swamped by hum and noise, so low-noise gain, shielding, and good grounding are essential; an electret needs its bias resistor sized right or it distorts; and microphones have their own frequency response and self-noise, so the sensor, not the converter, sets how clean your recording can be.
An electret microphone is powered through a 2.2 kΩ resistor from 3.3 V, so its output sits at a DC level of around 2 V with the audio riding on top as a few-millivolt wiggle. A series 1 uF capacitor blocks the 2 V DC and passes only the audio into a preamp with a gain of 100, lifting a 5 mV whisper to 0.5 V.
Sound pressure becomes a small AC voltage riding on a DC bias.
An electret microphone is not just a passive transducer — it has a built-in JFET and needs DC bias through a pull-up resistor to work. Connect it like a dynamic mic with no bias and you get nothing; a series capacitor afterward removes that DC before the audio is amplified.