What this proves
Left versus right tells you if a TRS cable, a Bluetooth profile, or a balance slider has dropped a channel. The sweep is a quick check for rattles and dead bands — not a calibrated RTA. Laptop speakers will sound thin at 120 Hz; that is physics, not a failed test.
Playback uses OscillatorNode and StereoPannerNode when the browser has it. Mono devices will not pan.
FAQ
No sound?
Unmute the tab (the speaker icon in Chrome), check the OS output device, and confirm the gain slider is not at zero.