Posted on 09 February 2011 at 14:19h

The other day I got my students moving there heads around whilst I played the same note through the speakers of my laptop. If you try it you can hear a marked difference as you move from a quite loud spot to a quite one. It seems to due to the interference of the sound waves coming from the speakers like Young's slits for sound. I thought it would be nice to turn this into a class practical so set about playing with possibilities. I tied an equal length string to the position of each speaker and walked around my study listening for changes of intensity, I was hoping to measure the path difference at my ear using the string and then find the wavelength, repeat for different values of f and you've got a graph that can be used for assessment lah de dah.  I thought I could do this with a whole class by simply giving each student a pair of strings that were attached to my computer, I can imagine it could have been fun if I hadn't started to think. 

Lets take a typical frequency 660Hz, this gives a wavelength of 50cm, my speakers are about 25cm apart so the only place that there will be destructive interference is in line with the speakers so what's causing the loud and quiet regions that the students (and me) were hearing, were we simply imagining it because that's what we expected? Or was it due to interference between the sound and the reflection off the walls?