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RE: Treatment for rear wall behing speakers

Posted by David Aiken (A) on November 7, 2007 at 14:05:28

In Reply to: RE: Treatment for rear wall behing speakers posted by Vinyljunkie100 on November 7, 2007 at 08:38:02:

Maybe I can help here. I've got a L-shaped room and the toe of the L is behind me and to my left, like the space through your open archway. My right speaker faces a wall to my right just behind the plane of the listening space and the left speaker faces a wall 3 metres further back than that.

Pretty much everyone here has been very strong on symmetry for very good reason. The soundstage tends to become unbalanced when things are asymmetrical. In my room, if I remove the acoustic treatments the soundstage pulls strongly to the left where there is a stronger reflection from the wall since I also have an open arch to the hallway on the right of my right speaker. Those problems can be dealt with but they can sometimes be difficult and it's often hard to predict what will work best in advance. Dealing with asymmetric rooms can be a real puzzle.

If you rotate the room 90 degrees to the left, which I think would have some advantages, these are my suggestions:

1-rotate the bass traps on the wall behind the speakers 90 degrees as well;

2-cover the windows with heavier drapes than the sheers already there. It is also possible to get noise absorbing curtain liners intended to block outside noise by absorption. They absorb internal sound as well. Use them if possible. You won't see them, they're a second curtain hidden behind the first one, so people won't notice and that's a good thing with acoustic treatments in many ways. You can open the curtains when you aren't listening so that the room is more light but the room will be dim if you do a fair amount of daytime listening.

3- the right side (the wall currently behind the speakers) may need some treatment and you've got some there already. It may be enough so start off leaving things how they currently are and spend some time—I mean a couple of days—just listening to things with the speakers and bass traps moved 90 degrees to the left and some absorption on the windows behind the speakers. In the short term before you get heavier curtains there, you can simply try some gel thingies on those windows and just hanging a blanket or two from the existing curtain rail to get an idea of what things are going to be like. That also allows you to shift things back to where they are now without having made any big expenditures if you don't like it.

4-if you find the soundstage pulling to the right, then there's too much reflection from the right side. Locate the early reflection point (location will depend on where the sofa or chairs end up) and treat the area there along the lines Jon Risch suggested. If the reflection point falls on one of the doors (I think that's unlikely), use a free standing acoustic panel in front of the door.

5-leave the discs where they are on the left. You may need to put some absorption there if the soundstage pulls to the left but if you do simply use a free-standing panel placed in front of the discs at the site of the early reflection point. You're going to have to juggle what you do on the left and right at the same time because one will affect the other but the trick is to use absorption to balance things. You won't succeed very easily if you try increasing the reflectivity of the side that's lower in reflectivity but simply using absorption on both sides will balance things and it also helps produce a stronger soundstage into the bargain.

6-that leaves the wall behind you, currently the right wall. There's going to be a bit of experimentation here and it may take some time to get things right. I'm going to assume that you aren't going to want acoustic treatments in the adjoining space. The options that I would be trying, based on guesses of how things will sound, are:

a) play with speaker toe-in;

b) bass traps in those 2 room corners also;

c) absorption on the wall area next to the archway;

d) a free standing panel behind you to block some of the reflected sound from the other room. It would need to be a fair sized panel, the full width of your sofa if you keep the sofa, but otherwise around 3' or so wide at least, the size Jon Risch recommends as a minimum for his DIY panels.

You may or may not end up having to use one or more of those suggestions but I would bet on having to use one or two. Speaker toe-in will make a difference and I have my speakers pointed directly at the listening position because that puts me on axis and ensures any reflections from behind me or from the wall to my right that the right speaker faces are going to weaker because they're coming from further off axis than the direct sound. That fact helps a lot and actually lets me get away without having to treat the wall my right speaker faces. Playing with toe in is simple, free, and easily reversible. The only down side is that it can be time consuming.

It's hard to predict what that room that will end up behind you will do to the sound. It will create a problem of some kind, but it also gives you a longer dimension for one speaker to fire into and that will provide a lower standing wave frequency which, while it sounds problematic, need not be and it can actually help since the lower the lowest standing wave frequencies, the earlier the room's sound starts to smooth out. The problem with standing waves really isn't that they exist, it's that there's only two or three per octave at low frequencies. When the number builds to 5 or 6 per octave there's enough overlap to start smoothing out the dips and peaks in room response and this happens earlier (ie at a lower frequency) when the lowest standing wave frequencies are lower in frequency.

So there's some suggestions. The 90 degree rotation may actually end up being relatively simple with the exception of what you do with the open entrance that you end up with behind you. Even that may not be too much of a problem but that's impossible to predict and it is going to be a matter of trying the rotation to find out.

Hope the above ideas help somewhat.



David Aiken

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