How the tones are built
Press Play on a channel and the page creates an OscillatorNode (sine) connected through a GainNode into a StereoPannerNode, then into the AudioContext destination. MDN documents StereoPannerNode.pan as a value from -1 (full left) to +1 (full right), with 0 as center. Left-only uses -1, right-only uses +1, and the stereo button uses 0 so both speakers receive the same mono tone. Default gain stays low (about 0.15 on a 0 to 1 linear gain) so a sudden full-scale blast is less likely; you can still turn the OS volume down before the first press.
Tone presets are ordinary reference frequencies: 440 Hz (A4), 1 kHz, 250 Hz, and 4 kHz. They are alignment tones, not a substitute for a calibrated hearing exam or a manufacturer frequency-response plot. Each press runs for about 1.5 seconds and then stops; Stop ends a tone early.
Reading left, right, and stereo marks
After each play, mark Heard or Silent. When left, right, and stereo are all Heard, channel balance matches a normal stereo pair. Only left Heard points at a right-side cable, mute, or swap; only right Heard is the mirror case. Stereo Heard while both solos stay Silent often means the browser or OS is downmixing to mono, or one physical driver is carrying both channels. The wording is a checklist, not a repair order: re-run any unclear channel before reseating plugs.
Headphones reverse the same checks in a smaller space: left-only should sit in the left cup. Bluetooth devices sometimes collapse to mono in certain OS modes; if solos fail but stereo works, check the device stereo setting before replacing hardware.
What this sound test does not measure
It does not plot frequency response, THD, or loudness in dB SPL. It cannot see a hardware loopback, and it will not replace a clinic hearing test. Browsers also require a user gesture before AudioContext can start; if Play fails, allow sound for the tab and try again. For microphone input instead of speakers, this page is the wrong tool. For other desk hardware on this site, see the button mapping check and key matrix check.