Just like [Optical illusion][1] happens to our eyes. can this happen to our ears? Consider a well tuned Piano. can we make hearing illusion with music? because of my horrible English and lack of knowledge about music theory i show example with steps: 1- play a note (or specially chord) on high octaves 2- play a note or chord on lower octaves than previous one (in general) 3- a second chord be heard like it have more pitch than previous chord. (but in general first chord was on higher octaves.) when i said in general means not all notes have to be lower or higher than other chord. also two chords should not be too far from each other.i think only 1 or 2 octaves at most. i feel like i heard it before like in `Rachmaninoff Prelude Op. 23. No. 5` or `Beethoven piano sonata no 32 first movement` but im not sure if thats true or not. Note that this kind of illusion happen to me not because of just playing two chords. i think the music makes the meaning and changes everything and it depends on sentences that was played moment ago. i think that's how it feels. sorry i dont know much about music theory terms. so bear with me. [1]: https://en.wikipedia.org/wiki/Optical_illusion