Cancel the anechoic chamber
From the photo it looks like my class are all chatting to friends on their mobile phones, I wonder why people always wander about like this while speaking on the phone. Anyway they aren't doing that, I'd not allow it of course, they are trying to hear minima in sound intensity created by the interference between sound waves from the two large speakers on my new SMARTboard. Having told my class that it's very difficult to perform sound experiments in anything but an anechoic chamber I proceeded to prove myself wrong by conducting a demo that did just what you see/hear on the phet sound simulation. It worked so well that I decided to go quantitative and got the students to stand in the minima that runs down the centre of the classroom. I then took a string and measured the difference between the distance from each speaker and the minima. Pretty close to half a wavelength it was too. It was all such fun (and the students looked so silly) that I made a video. Unfortunately I pressed the wrong button on my phone and just got this photo. I'll try again tomorrow. Hopefully you will be able to hear the minima as I walk around the room.
And here is the video, I just realised I could have made the minima much more pronounced by covering the microphone with my finger.
Tags: sound, interference