Built-in microphone

How it tests

Records the built-in mic at 16 kHz and measures the room, including its own level at each note's frequency. Then it plays five soft notes through the speakers, D5 E5 C5 C4 G4, and looks for each in the recording with a Goertzel filter; then a silent control round as long as a note. If a note can't play or isn't found, or the control round hears one, it asks for 2 to 4 claps and counts them, one retry. Headphones have to come out first.

How it decides

Supported: every note found and the control round silent (automatic), or the claps counted right (challenge). A note is found in 8 or more 50 ms windows at 10 dB over the frequencies around it and over the room's level there, above −75 dBFS, and only where it's the loudest of the five. Only C4 (262 Hz) may go unheard: laptop speakers are weak under about 300 Hz. Partial: neither, but the level rose 6 dB over the room and above −60 dBFS. Failed: no capture device, an error, digital silence, or no rise and the tester says they clapped.

How sure it is

The mic has to hear the notes the test played, and a silent round catches a detector that always says yes, so a key click or a bump can't pass.

On a real Mac

• On a MacBook Air (13-inch, Mid 2012): three notes and the silent round.

What could mislead it

• Could pass when it shouldn'tA codec that mixes its own output into the capture path would hand the notes to a dead mic. Macs' Cirrus codecs have no such path, and the capture mixer is set to the mic.
• Could fail when it shouldn'tClaps have to be 15 dB over the room's peaks: a soft clapper in a noisy room, after the notes were missed, gets partial.