Position of Subs

[ojc123]

I'm asking for opinions on this so I can make a more informed choice.

 

Our 1950's School Hall s to be refurbished and things are moving on well. I have been involved in every part of the decision making and I'm happy that when it all goes wrong there will be no-one to blame but me.

 

The stage is a typical 1950's letterbox. The Hall is 15m x 15m x 5m. It's to be sorted to improve the acoustics. The stage is 1m high. It has a brick front wall behind wooden panelling. The stage is supported by brick walls running front to back about 3m apart.

 

The PA is under discussion. One supplier suggested line array top/mids above the stage and putting the subs below the centre of the stage and about 30cm back from the edge. I'm happy enough with the line array suggestion as far as it goes. Logic products were mentioned but I'm still waiting for confirmation.

 

Aesthetically, this is appealing because it makes for a clear stage. I've read around a bit and it seems that a single cluster of subs is one of the better ways to arrange them. It's how I arrange them when I hire them in.

 

I wonder what the combined experience of the Blue Room makes of this under the stage suggestion for sub placement from an acoustic point of view?

 

If we were to go ahead with this would it be best to have an open space behind the speakers or a wall built around them? There would have to be a wall built around them somewhere to prevent draughts from understage but it could be tight up to the speakers or it could be some distance away. The front panelling of the stage is to be replaced anyway so a little extra work isn't a big deal.

 

I still have others coming in to make suggestions so this isn't a plan as yet. I'm not entirely sold on this system yet but it is an interesting suggestion. I hadn't considered it before but if I became convinced that the subs under the stage is a reasonable idea then I'd be prepared to make it a part of any plan we make.

 

Thoughts please about subwoofer placement. I'm just looking for ideas to explore and things I haven't thought of, or never knew existed.

 

Thanks in anticipation.

[smalljoshua]

If these companies are going to be selling you this kit, get them to demo the system in the configuration they deem most appropriate. If they do something one way and you're not happy with the sound, change it on the demo day and make up your mind once it's been Demo'd.

 

As far as I'm concerned, Subs under the stage isn't amazing in terms of on stage noise, the stage can resonate unless they're configured as cardioid. A single centre cluster is great for increased levels at the front, but you don't get any benefits anywhere else. Depending on how many units you're going to get, it may be more beneficial to spread them over a distance equally with the gaps so that they couple up to the crossover frequency of the boxes.

 

Josh

[Brian]

I think you're over thinking things.

 

You should draw up a specification of what you want in terms of sound quality, power levels, frequency response etc and let *them* tell you how they will achieve it. They moment you start being too helpful you give suppliers a get-out for when it doesn't work...."but the client said he wanted a cardioid sub array under they stage".

[i_hate_fisicks]

A line array in a 15m room?

[ojc123]

Thanks for that.

 

They're happy to bring in the kit but the under the stage thing isn't possible until we install it. So, if I do decide to go that way then it's going to be with crossed fingers. Resonating is one of the things I'm worried about. That's in my mind with the blocking it off question.

 

E2A

 

This IS their suggestion. I'm exploring it. I'm not speccing any particular solution. I've told them what I want it to do. I'm not fixed on it. It's the under the stage thing I'm thinking about.

[GR1]

In a 15m room it's not really going to be a line array as such more like a couple of speakers hung together. I would suggest hanging the subs too - visually that might not be great in such a small room but if you can't get the subs demo'd under the stage I wouldn't go for them under it. After all you wouldn't put the performers under the stage and expect it to turn out well. [On second thoughts maybe you would if they were ****dy awful !]

[ojc123]

It had occurred to me that flying the subs makes more sense which is among the reasons I'm asking about this proposal. We do have some performers better listened to from the other side of a wall.

[Brian]

Question re: your room sizes...

 

the 15m x 15m figure is just for the hall? So the stage in inset in one side of that space? What width is the proc? And I assume at 5m high it's just flat floor seating? Oh, and what height is the proc?

[chelgrian]

It had occurred to me that flying the subs makes more sense which is among the reasons I'm asking about this proposal. We do have some performers better listened to from the other side of a wall.

 

To be honest in something that small I'd stick with direct radiating rather than any kind of line array. Correct choice of number of boxes and placement is going to be far more important. For example if the hall is 15m wide and you have something hung conventionally on each side of the stage you are probably going to end up with a hole in the middle directly in front of the stage unless you have some kind of centre fill.

[Bobbsy]

At the risk of being contentious, the whole thought of line arrays and subs in a school hall only 15 x 15 metres and with a low ceiling if that 5 metre figure is accurate. What sorts of things will you be doing in the hall that you think you need this sort of system?

 

Unless the school plans a steady diet of bass-heavy hip hop or something, wouldn't one or two conventional speakers--probably 12 inch drivers--provide more than adequate coverage and plenty of bass for 99% of the things I see schools do. You could always rent a sub or two for that extra 1% but, meanwhile, put the extra cash into a better mixer or GEQ or whatever.

[Brian]

Quick drawing of the vertical section attached. Assumptions are that the front row of seats are 2m back from the stage and the speakers are up as high as they will go.

 

D1 is the distance to the front row of ears and is 4m

D2 is the distance to the back row of ears and is 15.5m

 

So the ration of shortest to longest distance is 1:4

 

Theta is the angle between those and is 48 degrees.

 

 

So you need speakers with 48 degrees of control in the vertical which is achievable but not with a 1:4 ratio of distances as the front will be too loud relative to the back. Change to speakers with 24 degrees of control and you get a 1:2 ratio which is *just* about OK but then 24 degrees is difficult to get from speaker which won't, no matter the installer says, be working as a line array.

 

Speakers with less vertical control will simply splash the sound all over the back wall which unless you are treating it, and I don't mean hanging a few curtains about the place, will reflect horribly.

 

A similar exercise in the horizontal plane will, as other have said, show up horrible holes in the front few rows that will need filling with speakers on the front edge of the stage.

[ojc123]

Brian, the 15m x 15m x 5m is the auditorium with flat floor seating. The stage is 6.5m extra in depth. It's 9m across the prosc. The prosc is 3.5m high. A letterbox!

 

Chelgrian, hanging speakers on both sides of the stage does leave a 'hole in the middle.'

 

Bobbsy, it's not at all contentious as far as I'm concerned. We do have quite a few rock bands and a soul/funk band. I completely agree with your analysis. Currently we have a couple of RCF 722A and hire in subs for events. They are perfectly adequate for our events but they aren't positioned in the best place and they look like an add-on. It may well turn out that we recycle the RCFs or replace them with a similar powered or unpowered 12" box. At the moment I'm not ruling anything out. I'm not fixed in my mind about any of this. If we're having a refurb and the money's there for the subs to be installed tidily and in the right place then it seems to make sense to put them in. There's a combination of factors will decide the way we go.

 

Can I reiterate that this is ONE solution suggested by ONE supplier? It's NOT my specification. I'm asking about the sub part of the plan because I'm suspicious of that part of the plan. I'm NOT suggesting the line array WILL happen; it's THEIR suggestion which we'll decide upon when we have other suggestions to deal with.

 

On the basis of the relevant replies about the subs under the stage, I'm fairly convinced that's not going to happen.

 

Thanks for the replies so far.

 

Thanks, Brian. Your assumptions are valid.

I'd done a similar back of the envelope drawing and I'd come to pretty much the same conclusion about the placement. The walls are to be acoustically treated to avoid sound bouncing off the back wall (not just a few curtains.) This isn't going to be straightforward because we will have to compromise among price, acoustics and visuals.

[rich1]

wont get in to the whys and whatfors of subs under stage but if you do

make them build a solid brick wall right behind the subs and fill the gap above subs with rockwool or similar

 

never put subs under a empty stage, 1 it make the stage rattle 2 you will get cancellations because of the reflections from the

back wall of the stage

[ojc123]

wont get in to the whys and whatfors of subs under stage but if you do

make them build a solid brick wall right behind the subs and fill the gap above subs with rockwool or similar

Thanks for that. I'm not convinced it's a good idea to have them under the stage but I thought that it would be a good idea to do something like that if we go down that route.

[themadhippy]

could always use the understage AS the sub cabinet

http://www.royaldevice.com/sala%20audio/covering%20the%20top21.jpg