Sound as a switch?

I was hypothesising if was possible to use sound as a switch. It falls under the idea of when a cell phone is placed next to a speaker it creates a buzzing noise and if I could detect that sound through the arduino and start a motor. Would it be possible to use the speaker on the arduino and have it detect the sound the speaker creates as sort of an input, would I need to buy a special component or something? (Also any extra information on the topic would be extremely help full)

I used sound as a switch in the post below to trigger a flash to take a photograph of a balloon bursting:

Oh wow thank you so much. Just out of curiosity is it possible to get a signal back from a speaker that is connected to an arduino? I`m just a bit hesitant to test it since I had completely fried my board last time doing a similar test.

There is a nice guide to connecting stuff here:

http://arduino.cc/forum/index.php/topic,154549.0.html

Okay awesome thanks again :smiley: