Cancellations in normal room, how does it work?

[CableChannel]

I was recently listening to a talk, and the guy's portable beamer made a constant high pitched sine tone. Since this was really distracting me, I tried to move my head around since I noticed this influences the perceived volume of the tone. I was even able to find a position where the tone completely vanished. It was very hard to keep though, since you can't really keep an uncomfortable position without moving.

How does this work?

Are the reflections of the walls interfering so they cancel out at the spot where my ear was?

Or could it be that at my left ear the tone has the exact opposite phase as on my right ear and the brain sums this up to zero?

This is a serious question, sorry if it is dumb.

[BertKoor]

How does this work?

Are the reflections of the walls interfering so they cancel out at the spot where my ear was?

Indeed. But you have TWO ears. So the sweet spot is where BOTH your ears are in such a valley. Hard to find, but you managed anyway.
Read http://en.wikipedia.org/wiki/Interferen ... agation%29 for more on the subject. A reflection acts as just another source of waves.

Or could it be that at my left ear the tone has the exact opposite phase as on my right ear and the brain sums this up to zero?

No, that won't work. Your brain doesn't do mono summing or mid-side encoding. If the distance between your ears is equal to the wavelength, then any sound of that frequency coming from dead left or right wouldn't register. And that's not the case.

[CableChannel]

Ah ok, thanks for the link.

The animated gif is cool. Would be nice to have a full blown applet where you can watch the waves and reflections and test for dead spots of cancellation...

[WOK]

This problem is also known as modal wave in room acoustics.
Google for modal waves room acoustic
There are calculators for the frequencies. That's why reflective parallel walls should be avoided in studio rooms. These interferences happen especially in the lower frequency range.