Headphone test: left, right, bass and treble

What each test tells you

Left and Right play a short burst of noise in one ear only. If you hear it in the other ear, the sides are swapped.
Both should sound as if it comes from straight ahead, inside your head. If it leans to one side, the balance is off.
Bass sweep rises from 20 Hz to 200 Hz. The first seconds may be silent: small speakers and many headphones play little at the very bottom. A buzz or rattle on the way up is a loose part, or earcups that don't seal around your ears.
Full sweep rises from 20 Hz to 20 kHz over 20 seconds. Where it fades out at the top is mostly your hearing, which drops with age.

For the mic side of a headset, use the mic test: it records your voice and can play it back to you.

If something sounds wrong

The sound comes from the wrong side. What now?

Check that the headphones are on the right way round (L and R are printed on the band or inside the cups). If they are, the swap is in software: on Windows open Settings → System → Sound → your output → Properties and check the left and right balance, and turn off any surround or spatial sound app while you test.

One side is silent.

Unplug and firmly re-seat the plug, then try another port or device. If one side stays silent everywhere, the cable or that driver is at fault; a headset with a detachable cable can often be fixed with a new cable.

Centre sounds like it comes from one side.

The balance is off. Set left and right to the same level in your sound settings, and check that mono audio (an accessibility setting) is off.

Is a surround sound test the same thing?

No. Gaming headsets with virtual surround (Windows Sonic, Dolby Atmos, DTS) only create surround from a game's own multichannel audio. A browser plays stereo, so this test checks the two sides, which is what every headset has.