sub in/under stage.

[Tim Padrick]

Having them on the stage may be nice for the players, but it presents a problem out in the congregation:

http://www.peavey.com/support/techno...ncellation.cfm

http://www.padrick.net/LiveSound/CancellationMode.htm

Now I see that this was an old topic. Oops.

[ScottyGunn]

Might be an old topic, but its new to me!

We have flown subs -about 40 feet up off the main floor. Is that an effective place for them? We could not really put them under the stage in the middle because of the steps going up to the stage. That would mean putting them on the sides and possibly getting some cancellation or power alley effects. The room is about 90' wide and about 80' from the front of the stage to the back wall. Our mains are also flown and connected to either side of the subs.

[liko81]

Scotty,

Subs are a tricky thing to get right. When you start talking about installed sound, you want to put the speakers in *just* the right place to make sure the space itself is helping and not hindering your efforts. This is doubly true for subwoofers, which produce low frequencies which interact much more strongly with various "boundaries" in your space. As such, almost any positioning of subwoofers can be "right" or "wrong"; it depends totally on the space. Flown subs are a common solution, because they accomplish several key goals for an initial effort expenditure.

Beyond the most obvious advantage of not blowing out the first few rows with enough bass to give the back rows a good kick in the pants, flying a subwoofer gets it away from both the floor and the walls. As I implied above, bass frequencies (<80Hz) "couple" with flat surfaces in buildings (boundaries); the surface will reflect, and also absorb and transmit, these low-frequency sound waves, acting as a natural amplifier.

However, these low frequencies are pretty omnidirectional; regardless of the direction the cone travels, the waves it generates are pretty much equal in all directions. This means that subs are also subject to something called "boundary cancellation"; the sound going backwards or sideways couples with the wall and is re-emitted back outwards, where it combines with the sound waves initially travelling in that direction. At a certain frequency, dependent on distance from the boundary surface, the waves will cancel out.

To prevent this, you must place the sub at a distance from all surfaces such that the sound waves in the critical 40Hz to 80Hz range don't cancel. It has to do with phase of the two waves, and the critical number is 120. If the two waves are closer in phase than 120 degrees, they'll combine constructively. Further than that, they'll combine destructively (cancel out). At the other end, a wave further than 240 degrees from another will begin combining constructively again.

Now, the speed of sound at sea level is 1126 ft/s, so the length of one sound wave at 80Hz, which is the threshold of directionality and so the point where you want to cross over from subs to mains, is 14 feet. Within one-third of that, or about 4.5 feet, two waves of that frequency will combine (as will all other lower frequencies).

The wavelength of a 40Hz tone (the threshold of human ability to distinguish pitch; below this, tones are "subharmonic" and are felt more than heard) is 28 feet. At distances further than about 19 feet, two waves of that frequency start combining again. Some higher frequencies will start cancelling at this distance, being beyond 480 degrees, but at this point you're more than a full wavelength out of phase, and far enough away from any wall that the power of the reflected wave is dissipated.

As Brad Weber stated below, we have to halve these distances for our purposes, because the sound is going out to the wall and then bouncing back, the distance the wave travels is double the distance from the speaker to the wall. So, that's the "no zone". Between, let's call it 2.5 and 10 feet, frequencies the subwoofer produces will cancel with the coupled reflections. We want the sub outside this range as measured from the nearest point of all walls. Looking at where that would allow the sub to be placed on the floor of the average church sanctuary (e.g. behind the chancel, or sitting in someone's lap or in the middle of the center aisle), flown subs become an obvious choice.

This same distance calculation also comes into play for hanging pairs of subs, if that's your goal. Again, you generally want all drivers of all subs to be within 5 feet or further than 20 feet apart. Spacing subs that far apart tends to create delays and echos (remember you can't really "aim" subwoofers like you can main speakers). That generally leads to a single center cluster of one or two subs (timed to coincide with the mains, as Brad again mentioned below), or to a "line array" of subs whose waves will combine in a special way instead of the waves on either end cancelling.

The disadvantages are obvious. A flown sub has to be, well, flown, meaning you're screwing into your ceiling with eye bolts to create tie points. It's also right up there in the middle of your sanctuary space; first, that's not the greatest solution for an old historic cathedral, or even a church with multiple levels like a loft where the sub could block a needed view of the chancel. Also, while quarter-inch cable (about the diameter of a pencil) has a working load of 1,400 pounds (a single sub cabinet is between 50 and 150 lbs and you'd use three or four cables to fly one), people do get uncomfortable sitting under a big black wood box hanging 20 feet over their heads.

[ScottyGunn]

Wow, liko! Thanks! That's a pile of info. The speaker cluster is suspended at an angle with an estimated, average distance from the peak ceiling of 3-4 feet. Could that be causing some cancellation in the LF? We are looking at the probability of lowering the whole cluster a few feet and increasing the angle to keep more sound on the floor and away from the giant back wall, which is treated with acoustic paint, but no other treatment.

[Brad Weber]

I agree with liko81's general comments regarding boundary cancellation and multiple device interaction, however I disagree with some of the specific numbers. In particular, the total path length of the indirect signal is to the boundary surface and back, so the desired distance from the boundary is apparently one-half the distances noted. So further than 10' or so being preferred to minimize the effects at 40Hz. On the other end, a subwoofer crossed over at 80Hz actually has useful output above that and if a speaker is close to a boundary then I tend to try to keep them within a foot or less. Thus for subwoofers I typically try for either very close to any large surfaces or a minimum 10' from them.

However, that also assumes the 'boundary surface' is sufficiently large and reflective at 40Hz and in many cases the effects of boundary cancellation are much less pronounced at very low frequencies.

The other side of this is that with the speakers 40' overhead then the speakers are likely far enough away that anyone hearing themselves through the speakers may perceive that as a separate event or echo. Any latency added to the audio path such as by digital mixers and/or digital signal processors can add to the natural delay to make the speakers appear even further away on terms of delay relative to any direct sound.

I personally like flying subs especially if flown along with the mains. That typically allows for better coverage and better alignment between mains and subs. And both theory and measurements have shown that the 'boundary loading' and increased bass often associated with subs on the floor is not what people think it is. At very low frequencies the floor is already acting somewhat as a boundary for the listeners and thus any added gain from putting subs on the floor is greatly due to an increase in the indirect sound from the subs, the sound reflected off the stage walls, ceiling, etc. rather than due to any increase in the direct sound. With modern speakers I would rather use a higher output subwoofer.

[liko81]

Brad, I made the edits to correct the math and mention the additional considerations of distance delays.

[Tim Padrick]

Originally Posted by ScottyGunn

Might be an old topic, but its new to me!

We have flown subs -about 40 feet up off the main floor. Is that an effective place for them? We could not really put them under the stage in the middle because of the steps going up to the stage. That would mean putting them on the sides and possibly getting some cancellation or power alley effects. The room is about 90' wide and about 80' from the front of the stage to the back wall. Our mains are also flown and connected to either side of the subs.

Owing to the high and low pass filters and cabinet design, most subs have at least 10ms of delay. The tops have much less. So if the tops and subs are next to each other and say next to a performer, they are already behind the performer time-wise. But since we want the performer behind the speakers for mic bleed/anti-feedback reasons, this gets fixed - at least for the tops. The subs may still be behind.

Now lets fly the system 40' up. There won't be any seat in the house where the tops are slightly closer than the performer, but things may still be OK. (with the subs still behind the tops). For a seat in the front row, the tops may be 20' farther away than a performer who's fully downstage (and a little more for the subs).

So it's impossible to know for sure without modeling or 'giving it a try').