Bass Traps under a stage?

E_Redman · Post by **E_Redman** » Sun May 29, 2005 3:45 pm

Sorry, this isn't a studio question, but this site seemed to be the best place to ask...

I have been working at a club that has a massively overspec'd system (what a lovely change!) including 14kw of subs arrayed under the stage. since the rig was installed we have had a few muso's complaining about the amount of bass freq on stage (I've also played on the stage & loved it, but I'm a bass player) so now the boss (system owner) wants to do something about it.

The stage is about 3' high and has concrete on the 3 walls and roof (none of which can be moved...). There is approx 8' headroom on the front of the stage and 6&1/2' above the drum riser. Stage is built from ply and you can feel the floor resonate even with low level music.

The boss was thinking of just downsizing the system but he has no use for the extra speaker cabs and they will just rot away in his garage. I was thinking if we re-built the stage by floating the thickest ply we could find, and put some hangers under the stage and I could keep my huge PA. The Boss is interested, but I still have some convincing to do...

Any thoughts?

knightfly · Post by **knightfly** » Sun May 29, 2005 5:53 pm

Is the stage ported in any way; how does the bass get out from under?
Ever think of just turning down the subs?

What's the framing like under the stage - can it take much more weight?
Any other details that might help us visualize... Steve

E_Redman · Post by **E_Redman** » Sun May 29, 2005 7:17 pm

turn down the subs? are you mad?

the system is really quite well balanced, sounds awesome in the room, the stage sound problem only really rears its head when it's cranked up a bit (probably a bit too much in my opinion...) I figure the stage floor is resonating with the subs, and transfering the low feq's onto the stage at such a level that the monitor system can't cut the vox through. I have not heard any complaints about the sub level on stage when I am mixing, but I aim for a solid, punchy mix at more reasonable levels (around 110db), the problem is when the boss mixes (he is a bit of a revhead) and when Band Engineers show up and try to use every bit of headroom that is available. If it was just a problem with Band Engineers that would be easy to solve...

Hopefully below is a pathetic drawing of the stage, vertical X section... 'A' is a grill to protect the subs from big nasty boots, beer etc (basically the whole front of the stage is a line of subs...), 'B' is an "enclosure" to house the foldback wedges the grey bit on top is another grill to protect the wedges from big nasty boots, beer etc. this may be abandoned if we were to rebuild the stage.

As I mentioned above, I think the stage construction is half our problem because it is not rigid enough. I think we should rebuild the stage from scratch with a more rigid design, and a floating stage floor. So I guess my main question for you blokes is whether or not putting hangers behind the subs, under the stage floor is going to do anything to reduce sub spill onto the stage?

knightfly · Post by **knightfly** » Mon May 30, 2005 8:02 pm

I still need you to accurately describe the floor framing (size of framing timbers, spacing, spans, existing covering), so I can make a wild guess at whether it will support any more weight... Steve

E_Redman · Post by **E_Redman** » Tue May 31, 2005 12:23 am

As it stands, I don't think the stage will hold much more weight. My idea is to build a new stage from scratch. the floor is concrete, so even if the new stage is 10 times the weight of the old one it won't be a problem.

Thanks for your attention Steve.

knightfly · Post by **knightfly** » Tue May 31, 2005 5:44 am

One thing that may be increasing the perceived bass - if the stage is 3 feet above the concrete, then the first axial mode of that space would be at around 190 hZ - this is somewhere between low low mids and high bass, but can be responsible for a STRONG feeling of too much bass.

Building a new stage at the same height won't fix it, if that's what is going on; if that's the case, then maybe building a very deep bass trap BEHIND the subs would help. A front face of 4" rockwool, lined with the same, and hangers in between should do.

In any case, it sounds like a new floor is in the cards; I would over-stuff the joist cavities with fiberglass or rockwool and keep it in place with fiberglass strapping, the kind they use to "band" heavy cardboard boxes with - this would damp the stage membrane to help any ringing that's coming from that source.

Hope this helps... Steve

E_Redman · Post by **E_Redman** » Tue May 31, 2005 8:04 pm

Cool, thanks Steve, thats pretty much what I was looking for. Now for the hard bit, and convicing the powers that be to do it properly...

Much Appreciated.
Ean

Waz · Post by **Waz** » Sat Jun 11, 2005 3:30 pm

I think that what has happened here is that by lining the entire front of the stage with the double loaded subs you have in effect created a line-array of subs that is plastering the center of the room with bass. This buildup of sound is what happens when you have speakers all in the same plane all firing at the same time. The sound wave buildup happens in a 90 degree offset radiating in a 360 degree circle (straight up and out from center stage). Since your floor is made of concrete, the sound just bounces off the floor and further loads the cabinets. I would recommend 1. splitting the subs up and placing them as far left and right as possible, perhaps even placing one in front of the other and using delay to time-align the wave-fronts, or 2. reduce the sub count to just one located center stage and perhaps another one behind it delaying the front sub to time-align them. The drawback to #1 is that you may run into some low frequency cancellation modes as you move across the room. If you wanted to change the orientation of the line array, you would have to stack all of the subs vertically so that the resulting line array would cover the room from left to right. If you do some reading on lin-array theory, you will realize what has happened.

Just my $0.02