Seeing Sound Waves

[jhuntington]

I noticed something interesting in the crowd at the pre-inaugural concert during Garth Brook's version of "Shout": you could actually see evidence of the sound waves moving through the long, narrow crowd. I made a little video and posted it here: http://www.controlgeek.net/blog/2009/1/21/seeing-sound-waves.html

[calebprime]

Cool.

I especially liked the fast-motion at the end, you can really see the delayed wave.

[iiwo]

That's a really cool video. If it is the delay of the sound causing this, that would be an awesome tool to use in class!

Now comes the but. With a group that large, it is not unusual to place speakers at regular intervals along the area where the crowd will gather. This keeps the sound tech(s) from having to blast the guys in the front in order for a whisper to reach the back. The signal in the cables to the speaker would have a slight delay, but I don't think it would be that much.

Was anyone there and/or have video from which the location of other speakers were present? Assuming there were additional speakers that is?

[malaka]

One more vote for "cool". I love science...

[jhuntington]

Now comes the but. With a group that large, it is not unusual to place speakers at regular intervals along the area where the crowd will gather. This keeps the sound tech(s) from having to blast the guys in the front in order for a whisper to reach the back. The signal in the cables to the speaker would have a slight delay, but I don't think it would be that much.


I actually cover this in the blog entry below the video--digital delay is intentionally added by the sound engineers to match the acoustic arrival time from the previous speaker systems. I've done this a lot on big systems--see the photo page from my website...

You can also see in the video in one of the shots the delay towers for this concert, and many, many more (check out the satellite photo) for the inauguration itself.

John

[Horatius]

That's a really cool video. If it is the delay of the sound causing this, that would be an awesome tool to use in class!

Now comes the but. With a group that large, it is not unusual to place speakers at regular intervals along the area where the crowd will gather. This keeps the sound tech(s) from having to blast the guys in the front in order for a whisper to reach the back. The signal in the cables to the speaker would have a slight delay, but I don't think it would be that much.

Was anyone there and/or have video from which the location of other speakers were present? Assuming there were additional speakers that is?



Did you read the blurb with the video? He discusses these issues.

[Miss_Kitt]

Wow! Very cool, thank you!!

[In My Spare Time]

I actually cover this in the blog entry below the video--digital delay is intentionally added by the sound engineers to match the acoustic arrival time from the previous speaker systems. I've done this a lot on big systems--see the photo page from my website...

You can also see in the video in one of the shots the delay towers for this concert, and many, many more (check out the satellite photo) for the inauguration itself.

John

This is correct. Audio engineers call them delay towers. In addition to allowing for a more reasonable sound pressure level (SPL) up front, sound absorption rates vary with frequency, with higher pitches tending to die of sooner. The delay towers, therefore are adjusted to compensate for this and would have a very strange sounding frequency spectrum on their own, with a significant overemphasis on the highs.

Looking forward to watching your video when I get home tonight. Sounds fascinating.

[Kilgore Trout]

I saw a documentary somewhat recently, I think on PBS, about The Stones holding a concert in Brazil to a huge audience and they noted the computers used to run the speakers that were set up at intervals in the crowd.

[iiwo]

I actually cover this in the blog entry below the video--digital delay is intentionally added by the sound engineers to match the acoustic arrival time from the previous speaker systems. I've done this a lot on big systems--see the photo page from my website...

You can also see in the video in one of the shots the delay towers for this concert, and many, many more (check out the satellite photo) for the inauguration itself.

John

I have now. I was in something of a hurry earlier and thought I'd read the whole page with the first paragraph and watched the video, managed to miss the second part down below. My bad! That explanation totally makes sense and I feel silly. :blush:

[In My Spare Time]

Just watched it. WOW! Very impressive John.

[BenBurch]

VERY COOL. Elementary school science teachers should use that clip!!!

[zooloo]

thank you
:)

[MattusMaximus]

Excellent - I'll be using this in my physics classes when we discuss sound waves. Thank you! :)

[ChrisC]

Nice! Thank you.

[jj]

Well, strictly speaking you're showing the speed of sound, not sound itself. It is, however, a great demonstration of something I've had to pound into more than one would-be acoustic architect's head.

It is possible (with the cost of modifying the sound waves) to actually visualize higher-level sound waves using a near-condensing atmosphere... But it's tricky.

[MRC_Hans]

The delays are, of course, necessary, because otherwise the sound arriving directly from (the speaker towers near) the stage would be out of sync with the sound from the repeaters, and you would get an echo effect. This effect is often very prominent on old film (and sound) clips of events where speakers are simply dispersed over a large area.

I remember another example of this effect: In the old Simon and Garfunkel Central Park Concert album, there is a point where the audience attempts to clap their hands in time to the music. It fails because the crowd is simply too vast.

Hans

[MRC_Hans]

BTW, when I saw the title of this thread, I thought it was about something else: There is a certain disorder which makes people see colors, and sometimes shapes, in response to sounds. It is, of course, not the actual sound they see, but signals somehow spill over to their visual cortex.

Hans

[fagin]

You learn something new every day. Thanks.

[jj]

The delays are, of course, necessary, because otherwise the sound arriving directly from (the speaker towers near) the stage would be out of sync with the sound from the repeaters, and you would get an echo effect. This effect is often very prominent on old film (and sound) clips of events where speakers are simply dispersed over a large area.

I remember another example of this effect: In the old Simon and Garfunkel Central Park Concert album, there is a point where the audience attempts to clap their hands in time to the music. It fails because the crowd is simply too vast.

Hans

Digital delays that do this cleanly are one of the biggest wins in modern concert reproduction. You can use delays, sync the towers, and use lot less power up front, (cutting down the issues with feedback well beyond the cancellation that is also a wonderful new digital technique), and keep a decent intensity all the way to the back, without echo echo echo echo echo echo...

Then again, drivers (speakers) have improved, horns have improved, amplifiers have improved, ... )

[MattusMaximus]

I showed this video to my students this week, and they went nuts for it. Epic win.