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Working the Delay Line
8 years ago
General
One of Bruce's other great stories was about setting up sound reinforcement delay lines. Any moderately-sized venue will likely need additional speakers as the speaker arrays at the stage won't be powerful enough to reach further back. At least, not without imploding the eardrums of the audience close to the stage. To keep that from happening, you fly additional speaker arrays further back into the audience to reinforce the sound.
Thing is, electricity travels near the speed of light while sound travels at... well... the speed of sound. There's a rather noticeable difference between the two! So even in an exhibit-hall-sized venue, the electrical impulse to an array halfway back in the room will beat the sound from the stage and create an "echo": actually the mid-venue array is heard first and then the sound from the stage arrives, creating the perceived echo.
The answer then is the "delay line": giving the reinforcement array enough of a delay that those speakers fire when the sound from the stage arrives. And from Bruce's description of the process, it's all done with the ear. A "pop generator" is hooked up to the sound system that will generate a pop or click. And the audio engineer stands back where they can hear both arrays and it sounds like *pop*-*pop*. Dial in some delay, try the pop generator again, *pop*pop*,. More delay... *p-pop*. More delay and it sounds like a single *pop*! But the trick is once you think you hear just one pop, you dial in a touch more delay and the psychoacoustics kick in! Because that last little bit of delay is what fools your brain and makes it believe that all the sound is coming from the stage speakers, rather than two separate sources that are putting out the same sound.
And he is absolutely right. One day I was walking out of the hall headed to dinner. There were rehearsals for the Masquerade going on with someone on a microphone. And about three-quarters of the way back, I decided to turn around and look and listen. Sure enough, as far as my ears were concerned, all the sound was coming from the stage arrays 150 feet away, despite the fact the delay line arrays was only 20 feet in front and above me. I tried to convince my mind that there was sound coming from those speakers and for a half-second, my brain was convinced. Then it reverted back to only hearing sound from the stage.
Bruce says if you cock your head... tilt it to the side and/or turn it to the left or right, it changes the phase of the two sound sources and can make it easier to separate out the sound coming from the delay line array. He also told a story about a concert producer who was convinced the delay line speakers weren't working. So Bruce signaled to someone at the sound board, they turned off the delay line feed and suddenly everything was so much quieter. The producer was convinced after that.
And if you have a big concert venue with multiple delay lines? Once you dial in the set closest to the stage, you walk back to the next set and repeat the pop test. Once the delay is right, the second set will match the first set will match the stage arrays. Lather, rinse and repeat!
Thing is, electricity travels near the speed of light while sound travels at... well... the speed of sound. There's a rather noticeable difference between the two! So even in an exhibit-hall-sized venue, the electrical impulse to an array halfway back in the room will beat the sound from the stage and create an "echo": actually the mid-venue array is heard first and then the sound from the stage arrives, creating the perceived echo.
The answer then is the "delay line": giving the reinforcement array enough of a delay that those speakers fire when the sound from the stage arrives. And from Bruce's description of the process, it's all done with the ear. A "pop generator" is hooked up to the sound system that will generate a pop or click. And the audio engineer stands back where they can hear both arrays and it sounds like *pop*-*pop*. Dial in some delay, try the pop generator again, *pop*pop*,. More delay... *p-pop*. More delay and it sounds like a single *pop*! But the trick is once you think you hear just one pop, you dial in a touch more delay and the psychoacoustics kick in! Because that last little bit of delay is what fools your brain and makes it believe that all the sound is coming from the stage speakers, rather than two separate sources that are putting out the same sound.
And he is absolutely right. One day I was walking out of the hall headed to dinner. There were rehearsals for the Masquerade going on with someone on a microphone. And about three-quarters of the way back, I decided to turn around and look and listen. Sure enough, as far as my ears were concerned, all the sound was coming from the stage arrays 150 feet away, despite the fact the delay line arrays was only 20 feet in front and above me. I tried to convince my mind that there was sound coming from those speakers and for a half-second, my brain was convinced. Then it reverted back to only hearing sound from the stage.
Bruce says if you cock your head... tilt it to the side and/or turn it to the left or right, it changes the phase of the two sound sources and can make it easier to separate out the sound coming from the delay line array. He also told a story about a concert producer who was convinced the delay line speakers weren't working. So Bruce signaled to someone at the sound board, they turned off the delay line feed and suddenly everything was so much quieter. The producer was convinced after that.
And if you have a big concert venue with multiple delay lines? Once you dial in the set closest to the stage, you walk back to the next set and repeat the pop test. Once the delay is right, the second set will match the first set will match the stage arrays. Lather, rinse and repeat!
It's kind of funny to see this type of thing put into effect in the local movie theaters with the giant 50' tall screen that has about 60 rows between the projector and the screen. They are supposed to be tuned to the room so that you are able to hear stuff happening in the movie *behind* where you physically sit.
The acoustic trickery is cool though, because I suppose if you actually had a venue large enough there would be enough accumulated delay that what you see *visually* is out of sync with what you hear *audibly*!
Very cool stuff!
I find your last point more interesting though... you _could_ have a venue large enough where the mismatch between the speed of light (visuals) and the speed of sound (audio) is noticeable. It should be possible to keep the audio in sync with the visuals (little or no delay), but then you're inviting echos from the other speakers that are closer to the stage. And yet, if you delay the further back arrays to match the sound coming from the stage, it will definitely be out of sync with what you're seeing (but as out of sync as it "normally" would be, if you were able to hear what was at the stage directly). Kinda a conundrum.