What is a tone generator used for?
A tone generator plays a steady sound at one exact frequency, which makes it the precision tool of speaker testing. Where the speaker test plays fixed reference tones, the generator lets you choose any pitch from 20 Hz at the bottom of human hearing to 20 kHz at the top, so you can probe exactly where a speaker behaves badly.
Common uses include finding the frequency where a speaker rattles or buzzes, checking how low or high a speaker can actually play, comparing left and right earbuds at the same pitch, and rough-checking your own hearing range with the high presets. It is also handy for testing subwoofers, car audio, and Bluetooth speakers, not just phones.
How to find a rattle with a frequency sweep
Speaker rattles usually live at specific frequencies: a loose grille, trapped debris, or a damaged diaphragm resonates at some pitches and stays silent at others. Start the tone around 100 Hz at moderate volume and slide upward slowly. When the buzz appears, you have found the resonant point.
A rattle in the low range, roughly 100-400 Hz on a phone, often means debris on the mesh, which the dust cleaner sweep and a soft brush may fix. A buzz that appears across many frequencies, or distortion that worsens over time, points at the driver itself.
Which waveform should I use?
Sine is the right default: it is a pure tone with no overtones, so anything extra you hear, buzz, crackle, rattle, is coming from the speaker rather than the signal. That purity is what makes sine tones the standard for audio testing.
Square, triangle, and sawtooth waves add harmonics above the base frequency, which makes them sound brighter and harsher. They are useful for hearing how a speaker handles harmonically rich content, and square waves make low frequencies audible on small speakers that cannot reproduce the fundamental. Keep the volume lower with these; harmonically rich waves sound louder at the same setting.
Why play a 20kHz tone?
20 kHz sits at the very top of the human hearing range, which makes it the classic reference point for a high-frequency sound generator. Most adults cannot hear a 20 kHz tone at all: the upper limit drifts down with age, from around 20 kHz in children to roughly 15-17 kHz for most adults. Playing the high presets in descending order, 20 kHz, then 17 kHz, then 15 kHz, gives you a rough, unscientific read on where your own hearing tops out.
It is also a quick tweeter and earbud check. A speaker that claims full-range response should produce something at the top of the range, and a totally silent 18-20 kHz band on headphones marketed as hi-fi tells you where the marketing ends. Keep expectations realistic with phone speakers, though: many small drivers genuinely roll off before 20 kHz, and some phones apply processing that attenuates extreme frequencies. If you hear nothing at 20 kHz, try 15 kHz before blaming the speaker, and blame your ears last.
Safe listening with test tones
Steady tones are more fatiguing than music at the same volume, because the energy sits at one frequency instead of moving around. Start every session at low volume and raise it only as needed, especially between 2 kHz and 5 kHz where human hearing is most sensitive.
Take the tone out of your ears before changing anything drastic: if you are testing headphones, set the volume with them off your head first. And there is no reason to run any test tone at maximum volume; every diagnostic described here works at moderate levels.