"Buttresses" in a home theatre room: what are they for?

[John Simpson]

Hi guys,

I've noticed in a lot of HT rooms the side speakers are often housed in full-height buttresses -- "steps" in the side walls that protrude into the room.

Is there an accoustical reason for these? Or is it just that the surround speakers can't fit into the wall cavities? I'm thinking they could be there as bass traps, or they may do something to break up standing waves.

I incorporated them into my own room diagrams, assuming they serve a purpose:

http://www.wychwood.net.au/floorplans.html

[bpape]

There are many reasons for columns in the room.

- it breaks up the long surface along the side walls.
- it allows hiding of the side surrounds
- it allows not cutting holes in the walls for speaker wiring that would compromise isolation
- it allows a space for you to run 110v outlets into the room down from in the soffit without cutting holes in the room that will compromise isolation
- it makes a great place to hide some additional thicker bass absorbtion
- etc.

Bryan

[Anthony]

Agree with Bryan above. Breaking up a nice straight wall really helps avoid standing waves from room modes and direct reflection which can cause comb-filtering effects.

Ideally these opnings would be placed at the point of first reflection and to the sides of the primary listening positions and have some absorptive material in them.

If you wanted to get really fancy, you can cover the cavity from the front with slits or holes and make a helmholtz absorber (or no slits and have a full on panel absorber) in the space.

For a lot of people, though, they can't afford to lose the space in the room.

[bpape]

I'd disagree slightly on the location of the columns.

Normally, the rear most columns house surround speakers and should be placed accordingly. That may be directly beside if you're using a dipole/bipole surround and only have one row. If you have more than one, then it ends up slightly behind and between the rows.

The front column would be placed where it's visually appealing and balances the room and the rear column location. If it happens to hit a reflection, that's OK IF you either have the open part low enough to catch it OR have the front wall treated so the reflected sound back to the front is absorbed. If the front is not at a reflection point, then absorbtion on the wall will suffice.

Bryan

[Anthony]

Yeah, I wasn't arguing for asymetricly placed slots/columns. I was just saying first reflection and at listening position would be ideal to have larger cavities for absorption. No side-wall standing waves or first reflections.

Columns are a great way to hide the speaker wires. I have my whole bundle from the front going through a fake column into the soffit and then back to where the equipment is. Nice, clean look.

[MatrixDweller]

In a home theater they are probably there for aesthetic reasons first and practical reasons a far second. From a design perspective they would tend to give a feel of an old public theater or hall.

Without using advanced steel or re-enforced concrete techniques a building that has a high ceiling, and therefore high walls, needs extra support so that the walls don't collapse. You'll see classic buttresses on the outside of churches and castles. They are there to make the wall strong and to make it not fall down.

Modern design may use steel I-beams to re-enforce the wall and roof structures. The higher the wall and larger the roof the bigger the i-beam is needed. The i-beam could extend inside the building rather than outside making the outside square. The inside walls would then need to wrap around the i-beams which could make it look like a "buttress".

A fancy theater or concert hall would want to conceal or decorate the protrusions to make things look better. Throw a sconce or speaker on it, drape it in curtains or make it look like a roman column and it suddenly has a more appealing look and purpose.