idea
The missing fundamental
Play a note through a filter that removes its lowest harmonic and keeps harmonics 2 to 6. You might expect the pitch to jump up, but it does not: you still hear the original low note. The brain recognises the pattern of the harmonics, 2, 3, 4, 5 and 6 times the same frequency, and hears that frequency as the pitch, even though nothing is sounding there. This is why a small phone speaker, which cannot produce low frequencies, still lets you follow a bass line.