Skip to content

MEMS Microphone Cross Section

Run the MEMS Microphone Cross Section MicroSim Fullscreen

You can include this MicroSim on your website using the following iframe:

1
2
<iframe src="https://dmccreary.github.io/fft-benchmarking/sims/mems-microphone-cross-section/main.html"
        height="482px" width="100%" scrolling="no"></iframe>

About This MicroSim

The INMP441 you wire to the Pico is about the size of a grain of rice, and it does something that used to take three separate components: it senses sound, converts it to a number, and speaks a digital protocol — all inside one package.

This cut-away shows the five things inside and the four wires coming out. Click any part to read what it contributes.

The chain is short:

  1. Air pressure enters through the sound port.
  2. The diaphragm flexes. It is the only part that physically moves.
  3. Its distance from the fixed back plate changes, so the capacitance between them changes.
  4. The ASIC measures that capacitance and digitizes it.
  5. The result leaves on SD, timed by SCK and WS.

The part worth pausing on is the ASIC. On an older analog microphone you would wire the output to a separate ADC chip and configure that yourself. Here the conversion already happened inside the package, which is why the Pico reads bits rather than a voltage.

How to Use

  1. Click each part in order, following the arrows from sound to pins.
  2. For each part, state in one sentence what it does before reading the panel, then compare.
  3. Pay attention to the back plate perforations. Ask yourself why a plate that is supposed to be a capacitor electrode has holes in it.
  4. Click all four pins. Only one of them carries audio — identify which, and work out what the other three are for.

The Four Pins

Pin Name Carries
SD Serial Data The audio bits, one per bit-clock pulse
WS Word Select Which channel these bits belong to
SCK Bit Clock The tick that paces every bit
L/R Channel Select Which half of the WS cycle this mic speaks in

Lesson Plan

Grade Level

Undergraduate (college junior/senior)

Duration

8-10 minutes

Prerequisites

  • Sound is a pressure wave in air
  • A capacitor's value depends on the distance between its plates

Learning Objective

Students will be able to identify the internal parts of a MEMS microphone package and explain, in one sentence per part, what each does to turn sound into a digital bitstream.

Activities

  1. Trace the chain (4 min): Students click through the parts in signal order and write the five-step chain in their own words.
  2. Why no ADC? (3 min): Students explain what the ASIC replaces and why this is called a digital microphone.
  3. Pin sort (3 min): Given the four pin names, students classify each as data, timing, or configuration.

Assessment

Ask: "Your code reads bits from the SD pin. At what point in the chain did the sound stop being an analog quantity, and what component performed that conversion?"

References

  1. INMP441 datasheet — the specific microphone used in this course.
  2. Microelectromechanical systems — the fabrication technology behind the moving diaphragm.
  3. Condenser microphone — the capacitive sensing principle in its original, larger form.