Why 100 Hz is the standard bass check
100 Hz occupies a sweet spot for testing: true sub-bass frequencies (below ~60 Hz) are beyond many speakers entirely, but 100 Hz is reproducible by almost anything larger than a phone - so a weak or distorted 100 Hz tone tells you about the equipment, not physics. It is also the classic subwoofer crossover neighborhood: play it while switching between speakers and sub to hear whether the handoff is smooth or has a hole.
Musically, 100 Hz sits between G2 (98 Hz) and A2 (110 Hz) - the low end of a male bass voice, the fourth string of a bass guitar, the body of a kick drum. When mixing engineers talk about a track's "low-end weight," this octave is most of what they mean.
What to listen for
A clean 100 Hz sine should sound like a smooth, even hum with no buzz or rattle. Buzzing usually means something mechanical - a loose grille, an object on the speaker cabinet, a wall fixture joining in. If the tone gets dramatically louder or quieter as you move around the room, you are hearing room modes: standing waves that boost or cancel bass at specific spots. Predict where yours sit with the room mode calculator, then confirm with a slow sweep.
On headphones, 100 Hz is a fair test of seal: with in-ears, break the seal slightly and listen to the tone thin out - most "weak bass" complaints are exactly this effect at work.