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In Reply to: RE: Combined Bass Traps on Ceiling? posted by David Aiken on January 16, 2009 at 14:16:08
Thank you, David, for your very detailed post!
I agree, the cart may be ahead of the horse on this one. I plan on mocking my system up this week, and I will be doing it with much shorter cables or maybe I'll just run them along the floor.
As a follow-up to your post, what kind of tests should I do to determine what kind of traps/acoustical treatment I need? Should I find a way to measure the response of the room? Or should I just listen and see what I'm hearing and/or missing?
Thanks again!
Ethan can help you more on the testing front than I can. I've got no measurement software and the RS meter doesn't cut it.
Basic recommendations:
Bass traps will help in almost any room. Placement at corners is best and going floor to ceiling if you can, or from one end of the room to the other if you're doing them along the ceiling. It's hard to do too much bass trapping, especially in corners.
If you want an expansive sound then some diffusion on the front wall between the speakers and absorption on the back wall seems to be the recommendation from Rives.
If you're interested more in soundstage and imaging, then absorption on the front wall at the first reflection points and either absorption or diffusion on the rear walls at the first reflection points.
Absorption at the side wall first reflection points tends to control the width of the soundstage and add a little to the precision of the imaging. Floyd Toole recommends leaving these points untreated but says that some people, especially musicians, are sensitive to these reflections. In my room I treat them. The soundstage isn't quite as wide but it is more precise and the imaging is definitely more precise in my room.
First reflection points on the ceiling: I can't comment personally on this one because I've never tried attaching anything to my ceiling apart from some thin foam tiles which kept falling down and didn't seem to do much anyway, but then thin foam won't do much anyway.
Some people argue for diffusion over absorption at first reflection points. The closer you are to a diffuser, the less effective it will be. Diffusion takes distance and space in which to develop and first reflection points tend to be relatively close to you unless you've got a sizeable room. With the exception on diffusion on the back wall I'd tend to side with absorption at first reflection points unless you're going with the Rives style approach of a live end of the room in front of you, in which case I think you should use absorption rather than diffusion on the back wall. In general if you want to add diffusion I'd tend to place it towards the rear of the room and further away from you than the first reflection points on whichever surface you place them on.
You want carpeting on the floor between you and the speakers, and drapes covering windows if you have a lot of window area and you find the sound getting bright. If you've got windows at first reflection points, I'd definitely recommend curtains, especially if it's on a side wall and you're thinking of leaving the first reflection points untreated.
Some people talk about deadening the room too much and turning it into an anechoic chamber. You couldn't turn a room into an anechoic chamber without covering all of each room surface with very thick, very effective absorption. The approaches I outlined above leave most of the room's reflective surfaces untreated and you're not going to get anywhere near an anechoic chamber with that sort of treatment approach. Still, some people like much liver rooms than others and I have no doubt some people would object to even the basic treatment approach I've outlined. Individual tastes do vary. Some people would like the room even more absorbent than the approaches outlined above.
One thing is important, however. If you're going to use absorption apart from bass traps in corners, make that absorption as broad band as possible. It seems many people have a tendency to use relatively thin absorption which is only effective at high frequencies so what that does is to dramatically alter the tonal character of the reflected sound, absorbing the highs and making it sound dull. The reflected sound contributes a lot to what you hear and that will end up making things sound dull or dead but the problem isn't too much absorption, it's the use of narrow band absorption. If you use broad band absorption the tonal character of the reflected sound isn't changed very much, only the level of the reflected sound from that area is reduced. That's what you want to achieve—a reduction in level with minimal change in tonal character.
David Aiken
Thanks again David! That certainly will get me started.
Okay, little update... I actually listened to the system yesterday. I didn't have a belt for my LP12 (left it at the other house - like having a Ferrari and no key!), but I did have an Oppo SACD player and a couple SACD discs I'm very familiar with.
I set the system up with the equipment behind the speakers. I'll take a picture this evening, but they are open racks that certainly provide some diffusion. There are five low racks that run pretty much the entire distance of the back wall. The old Sound Organisation two-shelf units. No damping or sound absorbtion of any kind. Not even a carpet. Big-assed Acoustat Model 3s about 3' from the front wall, 14" from the side walls.
I have to say I'm impressed! Be bass was way better than I thought it'd be. It booms a bit, but it's very low, around 30 Hz. Sounds a bit unnatural but I think it's better than having to listen to a 100+ hz bloom. Not sure how I'll correct it, but I think putting some damping between the speakers and floor will be a start.
I'll move my 9x12 area rug in there next. I'm sure it'll help things a bit.
At 30 Hz bass will be low from most speakers because they're rolling off by then, but there's also very little musical content below 30 Hz apart from organ, the bottom 29 Hz note on a piano, and I think a couple of low brass instruments. Bottom note on a double bass/electric bass is actually 41 Hz. Unnatural sounding bass is often a different issue involving issues other than level.
I'd start with the area rug. Damping between the speakers and floor may or may not be a good idea and the only way to find out is to try. There are various ways to provide damping and not all are equal so it's hard to draw conclusions from one experiment. You may get lucky first time or you may not. As far as the unnatural sounding bass goes, damping may help that or it may exacerbate it. Unfortunately there's only one way to find out and that is by trying it. There's more than a few things in audio where that's true.
I definitely think low racks are better than high but I don't think you'll get much in the way of diffusion from low racks. They'll certainly have some effect on reflections from the area in which they're located but it will probably be more in the way of changing the angle of reflection rather than diffusing the reflections that occur.
David Aiken
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