What it is for
- Tuning by ear: play the note you want and tune to it until the beating disappears.
- Intonation practice: add a fifth and the octave below for a drone, and play or sing scales against it.
- Testing speakers and rooms: a slow sweep from 20 Hz to 20 kHz shows rattles, resonances and where your hearing tops out.
Protect your hearing: start quietly. High frequencies at high volume can be harmful even when they seem faint, and small speakers can be damaged by loud low tones.