[ringring7]

I am in charge of the sound at church, we use an old venue. The problem I am having is that there is a lot of bass in some parts of the room and not a lot in others. I assume there is some bass bouncing off the back wall and causing nodes or something.
Bottom line is I need some info on bass traps or ways to remove this massive variance in size.

We have a live band playing and we use subs- though the whole system is never terribly loud, id guess about 85db?

The room is about 50 feet wide, 40 feet deep, and roof slopes between 10-25 feet. Ill try to draw it below...
                      -------------
                    /
          ------/
------/



-----------------------------
back                            front(speakers and band)

Id appreciate any tips you got!

[Mr Content]

Hi 7, I know what you mean. I have noticed the same problems in a lot of churches we have visited. It make a different with how many people are their as well. we have a big church, about 800 on Sunday morning , but only about 350 to 400 in the evening service. so the mixing is quite different. we decided that we needed to make some big floor to ceiling bass traps and place them in the first reflection point of the bass. this worked to even out the bass, and also to tighten it up heaps, so much that we can run the subs a notch or 2w higher now. I can recommend them, cheap to make as well.

Mr C aa

[bpape]

A few questions...

Where are the subs located?

Seems as though the ceiling is backward to normal setup.  Usually, the ceiling would get higher as you move away from the stage to better allow the sound to propogate and act like a horn.  In the current setup, it's getting smaller so you're fighting the horn.

You're bordering on large space acoustics.  I'd want to know a bit more about the construction also.

Bryan

[Ethan Winer]

I am in charge of the sound at church, we use an old venue.

Where are you located, and what's your budget? Any meaningful advice depends directly on both. I can tell you that the general solution is absorption that's effective to low frequencies.

--Ethan

[Ted_D]

ringring7,
I would like to help your church and also get feedback on the effect of my Bass Stations in a larger(ish) room. Send me a PM and I'll forward a couple of Bass Stations to your church for you to use on extended loan. We can work out the placement when we talk.

Yours in music,
Ted Denney III
Lead Designer, Synergistic Research Inc.

[nathanm]

I bet the Bass Station would be a great match for a church since they both operate on the same principle.

[ringring7]

Thanks for all the replies!!! 

Ok. Well first of all I live in New Zealand, so sending me stuff would be....expensive. But thank you for the kind offer.

Yes- the room does seem to me to be backwards. But there is nothing we can do, we are here for a few years until we build. I think it was designed that way for purely acoustic performers..such as actors or acoustic instruments...that way its a bit like a sound shell- its a pain any how.

The room also has a few "internal" rooms and offices added which compounds the issue. I would guess that the best place to put some big bass traps would be on the back wall. Problem is that there are office windows and doors there. I had though we could make some portable bass traps- that would look like large partitions that could go a metre or 2 in front of said windows and doors....are bass traps effective if they are not secured properly? Should I try and get bass traps on the corners first? Or aim for the rear wall first? I thought about just some fibreglass insulation still in its packet put in the corners? Or other...

The subs are located on either side of the stage which is about a quarter of the room width from each side. Ill try a top down drawing...

--------------------------------------------------------
old bar-    |--backstage storage....slightly lower area-
              |------------------------------------------
-------  --|-|                  STAGE                              |
        --/      |___________________________________|           |
     --/        SUBS     |        smaller stage        |     SUBS          |
   --/                      |_____________________|                      |
                                                                                        |
                                                                                        door to different part of building
                                                                                        |______
                                MAIN AUDITORIUM AREA                       |          |
                               ROOF IS STILL HIGH HERE                      |          |
                                                                                        |          |
                                                                                        |          |
                                      ROOF Begins tiered sloping               |          |
                                                                                        |          |
                                     ROOF lowest here                           door      |
                                                                                        |          |
----------door--------door---door----------|                          |______|
                     |                 |                    |
                     |                 |                    |                                entrance             
-----------------------------------------------------------------------

The main speakers are on top of the subs. They are a 3 way active, 4th way passive speaker. The subs make it a 4 way active system. Kinda cool.
There is some sort of "Acoustic treatment" in place alreay on the ceiling. It absorbs the mid and highs, the rooms is not very live at all.

Budget is about $1000 NZ, about $600 US.

Some more info on DIY bass traps would be good...specifically what the covering of it should be.

[Ethan Winer]

Acoustic treatment for large spaces is approached differently than for a room in a home. But in either case, to solve bass canceling you'll put bass traps where the bass builds up the most. Your guess that it's caused by reflections off the rear wall makes sense. Is the bass boomy there? If so, you'll put large amounts of very thick fiberglass along that wall. You can cover it with almost anything. Plastic bag type wrapping works.

In truth, treating a large venue for a specific problem like this is tough to do remotely. With that small budget you'll need to do a lot of experimenting. My best suggestion is to buy a large number of fiberglass bales (leave them in the plastic wrapping) and stack them along the rear wall. Let us know what happens.

--Ethan

[ringring7]

would some rigid fiberglass suspended on the backwall help better than bales of fibreglass?

[bpape]

It would have to be relatively thick.  Say 6" or more.  To do it on the cheap, you might consider just using some R30 framed and covered in cloth.  Putting the paper face out will help minimize the upper mid and high frequency absorption.

Bryan

[Russell Dawkins]

Would this not be a case where multiple subs located in widely differentiated parts of the space would yield more benefit per dollar than absorption? I have read of numerous instances where the "swarm" approach advocated by Duke at Audiokinesis and Earl Geddes was so effective than bass trapping could almost be eliminated. Something to do with bass standing waves being distributed and excited less by the fact that each bass source contributes less to any particular set of nodes. It does make intuitive sense to me.
Another similar approach which might prove easier to implement would be the one advocated by Klein und Hummel for their studio systems. They call it the Plane Wave Bass Array. Here is their description:

Multiple smaller subwoofers (2 - 4 depending on the width of the room) distributed along the front wall create a plane wave that propagates from the front to the back of the room. This has the effect of reducing the interaction of the plane wave with the side wall. The consequence is less excitation of the side-to-side room modes and a better time-domain response. From a sound quality point of view, the low frequency reproduction will be tighter and cleaner. This is called a Plane Wave Bass Array™ (PWBA™).

I'd be looking at getting a slew of Golwood 18s if I were you!  http://tinyurl.com/5uyo5c

I have to emphasize that this is (informed) conjecture on my part.

[Duke]

Nice job with the illustrations, ringring!

I've never set up in a big room like that before, so this will be just guesswork.

Since the diagram says "subs", does that mean more than one in each location?  If so, and if possible, move some of them to significantly different locations (like closer to the wall behind them, and off to one side or the other).  I prefer as much asymmetry as possible in at least two, and preferably three, dimensions. 

Also, "too much bass" can also be "lack of highs".  If the room has too much material that is absorptive at mid and high frequencies, but not in the bass region, the tonal balance will be biased towards the low end of the spectrum the farther away you get.  You might add some ceiling-mounted, angled, time-delayed, bass-shy speakers back where the bass is too strong, to restore proper tonal balance.

Is it feasible to leave some doors open at the far end of the room, or elsewhere?  Doors that open into adjoining rooms can often act as bass traps to a certain extent.

I agree that low frequency damping in the areas of bass build-up is desirable, but some of these other steps may help as well.

Duke

[bpape]

Agreed. 

It's hard to say without hearing.  The multiple sub locations can certainly help with response issues but if the problems is with cancellations, then addressing the ringing and decay times would be appropriate.

Bryan

[youngho]

Would this not be a case where multiple subs located in widely differentiated parts of the space would yield more benefit per dollar than absorption? I have read of numerous instances where the "swarm" approach advocated by Duke at Audiokinesis and Earl Geddes was so effective than bass trapping could almost be eliminated. Something to do with bass standing waves being distributed and excited less by the fact that each bass source contributes less to any particular set of nodes. It does make intuitive sense to me.

It has to do with driving the modes in different phases. Because the standing wave pattern of modes results from constructive and destructive interference as the incident and reflected sound waves interact with each other, additional waves at these frequencies will disrupt these patterns. The simplest example of this is the mode cancellation discussed by Floyd Toole in his white papers at the Harman website, where one subwoofer is placed at one end of the room and another is placed at the other end, then both are driven by the same signal in phase, resulting in cancellation of the modes corresponding to this dimension. For the Geddes method, at least one of the subwoofers should be off the ground.

In small rooms, bass trapping will still be beneficial because room modes exist at frequencies above the range reproduced by subwoofers but still below the transition region or Schrodinger frequency. I don't know about a medium sized room like the church. Geddes does recommend bass trapping.

Another similar approach which might prove easier to implement would be the one advocated by Klein und Hummel for their studio systems. They call it the Plane Wave Bass Array. Here is their description:
Multiple smaller subwoofers (2 - 4 depending on the width of the room) distributed along the front wall create a plane wave that propagates from the front to the back of the room. This has the effect of reducing the interaction of the plane wave with the side wall. The consequence is less excitation of the side-to-side room modes and a better time-domain response. From a sound quality point of view, the low frequency reproduction will be tighter and cleaner. This is called a Plane Wave Bass Array™ (PWBA™).

From what I've read, the bass array can be a little tricky and cosmetically undesirable, since they generally require the subwoofer drivers to be placed midway between floor and ceiling, at the midpoint (for two drivers) or quarter-wave points (for four drivers) on the front and back walls. The rear driver(s) are driven by a signal that is electronically delayed to basically "absorb" the signal from the front driver(s). The mid- or quarter- placement is necessary to address odd-order modes in lateral and vertical dimensions. There was an interesting discussion about this and Geddes' method on DIYaudio recently.

Young-Ho

[ringring7]

By "subs" i mean 1 18 inch speaker in a box.

The bass is only bad at some places in the room, so I dont think its a lack of highs.

I think I will try moving the speakers and adding bales of fibreglass. Will keep you posted.