Play a steady tone at any frequency, or sweep from low to high. Useful for tuning by ear, finding rattles and room resonances, and checking what your speakers can reproduce.
Start with the volume low. Pure tones at high level can damage hearing and small speakers, especially high frequencies you can barely hear.
Tuning by ear: play A4 = 440 Hz and tune a string until the beating between the two disappears. Finding rattles: sweep slowly through the low end; a window, shelf or speaker grille that buzzes at one frequency will give itself away. Room resonances: in a small room, low notes that suddenly boom (or vanish) are room modes — note the frequencies before placing bass traps. Speaker range: sweep 20 Hz to 20 kHz and note where the sound fades out at each end.
A sine is a single frequency with no overtones — the cleanest test signal. A triangle adds faint odd harmonics and sounds soft, like a flute. A square has strong odd harmonics and sounds hollow, like a clarinet. A sawtooth has every harmonic and sounds bright and buzzy — the starting point of most synth leads. The slider is logarithmic, so every octave takes the same width.
Why can't I hear the highest frequencies?
Hearing above about 15–17 kHz fades with age, and many laptop and phone speakers stop well
before 20 kHz. Silence at the top end can be either.
Is it safe to use with headphones?
Keep the volume low and short. A steady tone is more tiring than music at the same level,
and high frequencies can be loud before they seem loud.