Where the sound gets in

One small hole

The microphone is the small metal can in the middle of the board, 2.75 × 1.85 mm. Its sound port is the hole in the can's lid, 0.5 mm across, on the front. Everything the microphone hears comes in through it, from any direction: the part is omnidirectional.

Anything over that hole muffles the lot: tape, glue, a finger, a closed case. A board mounted in a box needs a hole in the box over the can.

What the datasheet forbids

Behind the port is the microphone's membrane, and the datasheet's handling rules protect it:

• Do not wash the board, and no ultrasonic cleaning.
• No compressed air into the port from closer than 5 cm.
• Never pull a vacuum on the port, such as a solder sucker held over it.

The loop a speaker makes

The live build in this book plays the microphone back through the TK103 as you speak. With a speaker a few centimetres away, the sound it plays reaches the port again, gets multiplied again, and comes out louder than it went in. Each trip is bigger than the last until the sketch clips and the loop feeds itself: a howl.

Headphones break the loop, which is why the live build uses them. The TK100 amplifier and its speaker suit a recording played back after you stop talking, not sound played while you talk.