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[Phillip_Graham]

Jens Brewer wrote on Sat, 25 April 2009 00:29

Phillip Graham wrote on Fri, 24 April 2009 17:33 That is why in cardioid sub setups you reverse the box closest to the boundary, and not farthest away.  This insures the canceling boxes are closest to the center of the "virtual array", and that the vertical lobe behavior of the array is also the cardioid pattern you seek. Phil, in this example, you're talking about a 2 box vertically stacked cardioid arrangement, right?  One sub facing 'forward' and the other faced opposite.  I just want to be clear on that since the first thing I think of when I think of cardioid subs is two boxes on the same plane spaced apart by x' with delay added and polarity reversed on the cabinet closest to the audience.  Or have I pictured it wrong?

Yes I am speaking of an array of 3-4 subs stacked, with the bottom one (against the floor boundary) turned around backwards.

Depending on the sub design/size, typically one rear-facing box will balance 2-3 front firing boxes.

From a practical matter of floor space, I think one almost always has to do cardiod arrays this way, where all the boxes are in the same plane.

[Kevin.Windrem]

Christian Tepfer wrote on Fri, 24 April 2009 11:24

... Spacing in between the subs eases the beam as well...

MAPP seems to indicate increasing spacing between subs (with no delay tapering) NARROWS coverage (until the spacing gets too large, then you start seeing cancellation). 12 tight packed subs is already pretty narrow.

[Mac Kerr]

Kevin Windrem wrote on Sat, 25 April 2009 13:35

Christian Tepfer wrote on Fri, 24 April 2009 11:24 ... Spacing in between the subs eases the beam as well... MAPP seems to indicate increasing spacing between subs (with no delay tapering) NARROWS coverage (until the spacing gets too large, then you start seeing cancellation). 12 tight packed subs is already pretty narrow.

Once you add the walls it gets more complicated. These are predictions for an array of 10 700HP subs tight packed standing on end. It is in a room 100' wide and 150' deep, with the subs 30' off the back wall. All the plots use a 1/3oct band centered on the specified frequency.

The first screen is done with no walls and no delay at 63Hz:



This screen is with no walls, but with a 1.5ms delay arc, no delay on the center, 1.5ms on the next boxes out, 3ms on the next, etc.



This screen is with the walls turned on, and with the same delay. that nice smooth room response doesn't look so good now.



This next screen is 31Hz with walls on, and no delay:



Here is 31Hz with the delay added, not so different from no delay.



Here is 125Hz with no delay:



And lastly, 125Hz with the delay on, hopefully this is rolled off.



How the array behaves is very dependent on frequency. It is also very dependent on the environment. The difference delay makes with no walls is easy to understand, and seems very controlled. When you add in the reflecting surfaces of the room the whole picture changes. It is easy to see why it is not so easy to make this all work in the real world.

Mac

[Ivan Beaver]

So where would I put FOH to get a good idea of what the rest of the people are hearing?

There are so many variables involved.

Getting good low freq coverage is one of the harder things to predict and to implement.

I would argue that in Evans case-put FOH where it gets pounded really good-so the engineers don't kill the subs trying to get bass when they are located in a null.

It would do ALL mix engineers to walk around the rooms they are in-not just the general are of FOH, and get an idea of what the rest of the people are hearing.  It can be VERY eye/ear opening-if they care about what anybody but themselves hears:roll: .

You may or may not be able to do anything about it, but at least acknowledging it is a BIG first step.

[Patrick Tracy]

Mac, that was awesome. It coincides pretty well with how I have been imagining the problem but haven't had the tools to investigate. However, I've walked a lot of rooms.

There's a lot of talk about power alley on this forum, and outdoors I've definitely heard the problem. Indoors I suspect it's a different deal, depending very much on the distances and frequencies involved. My audio instinct tells me that power alley may come in handy to keep LF off the side walls near which there will be standing waves anyway. Of course I realize there will be situations where it hurts more than helps. If you (or anyone with the tools) had a spare moment to do the predictions I'd love to see an example or two of split subs in a club sized room.

[Mac Kerr]

Patrick Tracy wrote on Sat, 25 April 2009 21:52

If you (or anyone with the tools) had a spare moment to do the predictions I'd love to see an example or two of split subs in a club sized room.

What do you mean by "club sized room"? Here is a look at a pair of 700HPs per side 30' apart. This is the same 100'x150' room.

No walls:


With walls:

[Patrick Tracy]

Mac Kerr wrote on Sat, 25 April 2009 20:24

Patrick Tracy wrote on Sat, 25 April 2009 21:52  If you (or anyone with the tools) had a spare moment to do the predictions I'd love to see an example or two of split subs in a club sized room. What do you mean by "club sized room"? Here is a look at a pair of 700HPs per side 30' apart. This is the same 100'x150' room.

That did nicely. In general terms it's what I expected as one possible outcome, that power alley could be mitigated by the room. I know better than to generalize those results to other rooms/placements/frequencies.

[Edit] Thanks!

[Mac Kerr]

One of the things I have noticed looking at all these Mapp plots is that it is not so much a "Power" alley, as it is a series of cancellations. The power alley in the last set I posted is not that much above the level of the sides. It is the almost complete loss of low frequency where there are cancellations taking place. Perfect summation only nets +6dB, but perfect cancellation is infinite.

Mac

[Klaus {nojunk} Zimmermann]

Mac Kerr wrote on Sun, 26 April 2009 04:59

Perfect summation only nets +6dB, but perfect cancellation is infinite.

in theory, IRL cancellation will go down appr. -30dB (max)...

[Art Welter]

Mac,

I would think the ceiling height would be as important as the wall relationship as far as LF room response.

It appears you can specify room floor dimensions, but what is Mapp deciding to make the ceiling height?

Art Welter

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