A famous engineer on a Federation Starship once said “Captin, I canna’ change the laws of physics…”
Scotty may have been talking about the amount of time he needed to restart the warp engines, but if he was an audio engineer, he might have been referring to time alignment. Sound travels really s-l-o-w compared to the speed of light, the speed of electricity and certainly warp speed.
We’re all familiar with the number: 1,087 feet per second. That’s 343 meters per second, plus or minus a bit depending upon temperature and humidity. Unless I’m trying to gauge how far I am from a thunderstorm, I find these numbers useless and feel that it’s a lot more “real-world” to express the speed of sound as roughly 1 foot per millisecond. That’s a number I can relate to. If I am sitting ten feet from a loudspeaker, it will take roughly 10 milliseconds for the sound to reach me. If I’m 100 feet away from the P.A., I hear sound 100 ms (milliseconds) after it has actually happened.
When I was a kid, I had a friend named Tommy who was a little bit nuts. I’m sure he and the other neighborhood kids said the same thing about me. I can recall clearly one sunny summer Saturday afternoon when I was walking home. I was roughly two city blocks away, and I could see Tommy holding a baseball bat. He was standing next to a fire hydrant (or “johnnie pump” as we called it) and was smacking the top of the hydrant with the bat. I was several hundred feet away but could easily see what he was doing. I’ll never forget that what I saw did not match what I heard. The sound of the bat hitting the hydrant did not reach me until an eternity after I saw it happen.
This later plagued me at baseball games, where I typically sat several hundred feet away from home plate (the crappy seats plagued me even more but that’s another story…). I’d see the ball get hit but wouldn’t hear it until “a few seconds” later. Ditto for my experiences sitting in the cheap seats at concerts, where the sight of a drummer hitting his snare drum always preceded the sound of the snare drum coming through the P.A. Real-world examples that light travels much faster than sound.
It’s All in Your Head(phones)
Here’s a practical situation everyone can relate to. You’re at a sound check and the drum tech is helping you go through the drum mics. Your headphones are sitting around your neck. You’re checking the snare mic, and when the snare is hit, you hear the sound fairly soon after you see the stick hit the head. When you press that solo button, you’ll definitely be disconcerted to experience hearing the snare feed sound in your cans before it reaches the P.A. system. Why? Although it’s the same electrical signal feeding the P.A. and the headphones, it takes longer for sound from the P.A. to reach you due to its slow speed. That’s annoying, but it’s not obnoxious.
Fast-forward to showtime, and that same phenomenon becomes obnoxious. You solo a channel in the headphones and can’t hear it clearly because now the P.A. is cranked up and everything in the cans is early compared to what’s playing out of the P.A. You’re trying to hear if there’s a buzz on the bass DI, but it’s very hard to tell what the heck is happening due to the delayed sound from the P.A. That’s why many digital consoles provide a delay on the headphone output: so you can time-align the phones with the P.A. system. You add enough delay to “hold back” sound in the headphones until the P.A. catches up.
Once the cans are aligned with the P.A., what you hear in the headphones plays in time with what plays over the P.A. system (it still doesn’t match what you see). It may not change your life, but it can certainly lower your brain damage over the course of the show. If I know that the FOH mix position is 50 feet from the P.A. system, I can dial in a delay time of 50 ms for the headphone output. That’s the reason you often see the option to set delay time in feet or meters as opposed to ms. You can measure the distance from P.A. to FOH, and that’s your delay time, plus or minus a bit for variations in temperature and humidity. If you don’t have a tape measure handy, you can walk it off and get pretty close.
[It’s a bit beyond the scope of this discussion, but sound travels faster as temperature increases. The speed of sound expressed as 1,087 feet per second is in dry air at around 0 degrees C. As temperature increases, so does the speed of sound but it’s not profound. At 25 degrees C, the speed of sound increases to around 1,138 feet per second. Humidity also affects the speed a sound but to a much lesser degree.]
Sub Alignment Issues
We’ve examined the idea of using fill speakers under a balcony and the need to delay those speakers so that they are aligned with the main P.A. hang (for a refresher, see T&P in the July 2012 and December 2015 issues of FRONT of HOUSE). What’s equally important is to make sure that the full-range cabinets are aligned with the subwoofer cabs.
I recently played a show where the entire P.A. was flown from the ceiling. The rig consisted of left and right hangs, each with five mid-size line array boxes. Augmenting the full-range boxes were four sub cabinets, also flown. These were configured (rather oddly) as hangs of two, centered between the left and the right, with one pair hung directly in front of the other. Both of these boxes were “recessed” from the full-range hangs, meaning that the subs were late relative to the full-range cabinets.
If the boxes are carefully aligned, this type of arrangement can deliver cohesive sound. It would require that the full range hangs and the front subwoofer are all delayed relative to the subwoofer farthest from the FOH position. I estimated that the full-range boxes would require a delay time of approximately 16 ms, and the front sub approximately 5 ms.
Such small delay times may seem insignificant but they can easily cause phase issues in the low end. I was told that the full-range cabinets were delayed to the subs but what I heard did not sound coherent: I suspect that the full-range cabs were aligned with one of the subs, but that the sub closer to mix position was not delayed to the rear sub.
The result was an extremely incoherent low-frequency response that varied wildly throughout the room (granted the room wasn’t exactly designed for good sound…). Solid low-end at the mix position translated to absolutely no kick drum in the middle of the room — far worse than I typically hear in the rooms I work. And if I got the kick to be present in the middle of the room, the people at the rear of the room (myself included) would be sick to their stomachs from excessive low end.
I speculate that a few milliseconds of delay could improve — if not fix — the issue. But you don’t visit someone’s home and tell them what color they should paint the living room, if you get my drift. Nonetheless, a fraction of a second translated into sub-standard (ha!) audio — and could easily have been prevented.
Steve “Woody” La Cerra is the tour manager and Front of House engineer for Blue Öyster Cult.
