Seeing Sound Waves

I noticed something interesting during Garth Brooks' performance at the pre-inauguration concert on Sunday: I could actually see the sound waves. No, I wasn't hallucinating. To see the sound for yourself, check out this little video I made:

Note: Video quality updated October 11, 2010

What does this have to do with sound systems or entertainment technology? Many people don't realize that sound, which travels only about 1,130 feet per second (depending on the temperature), actually travels way, way slower than light, which travels at roughly 186,000 miles per second. This accounts for why when you see a lightning flash, you generally hear the thunderclap seconds later.

And so, when we have a sound system where we want people to hear some distance away from the stage, sound system engineers have to take this into account. Speakers at a distance from the stage (like at the concert pictured in the video) must be intentionally delayed to get them to line up in time with the sound emanating from the stage.  This happens because the electrons in the cables transfer the audio signal out to the delay speaker positions effectively, for our purposes, "instantly", while the sound waves, vibrating air molecules, can take many seconds to travel.  And if the "delay" speakers, remote from the stage, are not aligned in time (even milliseconds matter), the audience will hear either destructive interference called "comb filtering", or perceive an echo. (Interestingly, from what I saw on screen, it appears that the large video displays on the mall were actually delayed to match the slowness of the sound.)

Keep all this in mind the next time you are at a large concert!

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