How a keyboard sound app works

Updated October 25, 2026

A keyboard sound app asks macOS for a copy of every key event, picks a sound for the key that moved, varies it slightly so no two presses match, and plays it through your Mac's output. Here is each step as Teclo does it, from its source code.

1. Hearing the key

Once you allow it under Input Monitoring, Teclo creates an event tap: a request to macOS for a copy of certain events on your login session. Teclo asks for key presses, key releases and modifier keys, plus mouse buttons, scrolling and trackpad gestures for its other sounds.

The tap is listen-only. macOS hands Teclo a copy of each event and passes the original on unchanged, so Teclo cannot block, change or add a keystroke. What the permission allows is covered in Input Monitoring on a Mac, and what it means for privacy in is a keyboard sound app safe?.

Two things can stop the copies arriving. Secure Input, turned on by password fields and some terminals, holds back key events from every app (Secure Input on a Mac). And macOS switches off a tap whose app is too slow to answer; Teclo watches for that notice and switches its tap straight back on.

2. Choosing a sound

Each key event carries a key code, a number for the physical key. Teclo uses it three ways:

• Which sound. Space, Return and Delete have sounds of their own. Every other key gets one of five ordinary-key sounds, always the same one for the same key, so neighbouring keys differ but a key never changes.
• Press or release. Each pack has a separate sound for a key going down and coming back up, as a real switch does.
• Where on the board. Teclo has a map of where each key sits across a standard keyboard, from the left edge to the right, which it uses for stereo (stereo keyboard sounds).

A held key plays again on every key repeat. Modifier keys sound too, unless “Silence modifier keys” is ticked.

3. Making each press different

One file on every key gets old fast; as one Reddit user put it about another app, “all letter key sound the same. That will make it repetitive and boring real fast”. Teclo varies every press three ways:

The pitch range is kept small on purpose. When pitch variation was added to Thock, another Mac app, its maintainer settled on ±10 cents: “10 is honestly the max i can tolerate”.

4. Playing it

Teclo plays through Apple’s AVAudioEngine at 48 kHz in stereo. Four choices in its code keep a key press from waiting:

• Everything is in memory. All of the current pack’s sounds, and their pitched copies, are decoded before you type, so playing never reads the disk.
• Sixteen voices. Each new sound takes the next of 16 players, so a fast run of keys overlaps instead of cutting itself off.
• No per-voice effects. Pitch comes from copies made in advance, and level and stereo are applied to the samples directly; the code notes that the mixer’s own pan and volume would ramp in over the first 10 ms, which is where a key’s click is.
• A limiter tuned for keys. It lets the volume slider go past the Mac’s own level without distortion, with its attack set to 1 ms instead of a default the code notes would delay each key by 12 ms.

After a minute without a sound, Teclo lets the audio engine stop so the output device can sleep, and starts it again on the next key.

5. Reaching your ears

The sound goes to the Mac’s current output device. Built-in speakers and wired headphones add little; Bluetooth headphones add a delay no app can remove (why typing sounds lag over Bluetooth).

Where the sounds come from

That chain is the same whether an app plays recordings of a real keyboard or sounds it synthesized. Teclo’s are synthesized; Klack’s and Kiiwi’s are recordings. Synthesized or recorded keyboard sounds compares the two.