VIBRATION

 Answer 

The mathematically correct octave is pair #4.  Most listeners, including those who are musically trained, are more likely to say #6, or thereabouts. This is because ear training is normally done with sounds that have a strong harmonic spectrum.

Sine waves, lacking this richness, are typically heard to sound "flat". The purpose of this experiment is to show that the timbre of the sound reinforces pitch relationships.