A question for the acoustic specialists

[Heapsy]

Hello all,

I'm involved in an installation for a rock and roll type church and after a site visit yesterday I have discovered the stage is not quite what I was expected. Essentially under the stage there is going to be a void, 12m wide, 1.5m deep and 0.75m in height. The floor of the void is concrete, the 4 'walls' are concrete blocks and the lid will be wooden.

I am concerned that there is the potential for a standing wave to form in this space and play havoc with the sound in the room.

What are peoples thoughts, am I over reacting or should this be a concern?

What would you fill this gap with? My instant thought is something like rockwool, is this overkill when all I actually need is to breakup the space?

Let me know your thoughts, thanks in advance

 

Mark

[weatherhead]

You're right the potential is there, however, depending on the power of the p.a. I don't think it'd be too much of a problem. A venue in my town has a roughly similar arrangement, in fact. You may get quite a lot of improvement by just decoupling major sources of low frequency (i.e. drums, large bass amps) from the floor such as is done in some recording studios. Of course, filling the entire thing with rockwool may also help, but it'll be hugely expensive, difficult to do and possibly create acoustic problems of its own.

 

Incidentally, I'm not an acoustics expert, but I'm speaking from my own (fairly limited) experience. For more info on this, I suggest you contact the guys over at the sound on sound forum, they definitely know what they are talking about than I do. Of course, you'll also get some decent answers here ;-)

www.soundonsound.com/forum

[Simon Lewis]

What may well be a problem is sound energy from anything on the stage (kick drum, bass combo, PA subs, expressive dance etc.) causing the stage surface to vibrate. Given there is a large void underneath the "lid" will resonate at various frequencies, and may also "boom" or "echo" when stamped on (rather like a large drum!). This will colour sound heard on and from stage.

 

You need to ensure that a) the stage surface is very well supported at regular intervals, to minimise the unsupported area that can vibrate and b) the void itself is (if possible) filled. Rubble, sand, concrete etc. would be great, but I suspect you'll find that the Ladies Bright Hour keep their biscuit trays and flower arranging kit under there!

 

If filling in isn't possible, you can try decoupling high energy acoustic sources (neoprene absorber under bass cabs, rubber underlay & carpet on top etc.).

 

My 2p worth....

 

Simon

[JakeCTG]

I agree with the other guys that it could be a problem.

 

Your best bet to start with is to bring in either a bass amp or sub and position where it is going to be and see if you get any problems with standing waves, and If you do then consider filling it partly or fully with rock wool depending on how much of a problem it is.

[mervaka]

like simon says (pun intended) I would agree that if allowed, the best way around this problem would be to fill it with something like sand or hardcore? just some cheap, dense building material. go to your local quarry and ask them how cheaply they can fill 13 and a half cubic metres with something sturdy.

[Mr.Si]

You're right the potential is there, however, depending on the power of the p.a. I don't think it'd be too much of a problem.

 

It doesn't take much to excite such a space.

 

As others have said, purely on a structural note you're going to need support for the stage surface, otherwise it'll cave in.

You could make the supports angled too, this'll help reduce the chance of such an excitation, as there won't be many equal opposite surfaces.

Again, you could line these surfaces with 100mm rockwool or some type of foam.

 

Basically, this is a "me too, I agree post" but I thought I'd post anyway.

 

Si

(an acoustic consultant who wants to leave the acoustics industry)

[Brian]

Filling with sand is not a good idea . You'll need 22 tonnes of the stuff and will probably need to reinforce the retaining walls to take the additional lateral load. This might mean you'll need to break into the existing floor to fit vertical reinforcing bars across the joint line.

 

TBH, looking at the dimensions, I don't think you need to worry too much about energy coupling into the void and causing problems. Your bigger problem is that you end up with a 'boomy' stage surface. Solve that and you'll also solve any residual coupling issues. Personally, I'd go with a nice stiff heavy composite floor construction.

 

Get this right BEFORE the stage is built. Remedial action is a) expensive and b) unlikely to happen.

[Simon Lewis]

Filling with sand is not a good idea .

 

Too true - I was talking through the acoustic principles, not the building realities! I had one church with a very similar problem. Filling with concrete was considered, but the time taken to properly harden off was far too long, so a block and beam construction with a screeded top surface was used. There was some structural resonance if the surface was physically stamped on, but nothing that stops them playing rock and roll. This approach was certainly better than the dozens of those stacking portable stage sections that were previously considered...