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In Reply to: RE: Help..!!!! posted by Ethan Winer on May 21, 2009 at 13:20:52
Ethan's right about the narrowness of the 9" trap as a treatment. That's the reason I said "They're only 9" wide traps so placement will need to be spot on."
A standard 2' wide panel of the sort often used for treatment of first reflections will cover a wider angle of the sound wave emitted from the speaker than a 9" trap at the same distance. Place the absorption device closer to the speaker and it covers a wider angle. You can get the same angle of coverage from the 9" trap by moving it much closer to the speaker. You still need to keep it on the line between the first reflection point on the wall and the tweeter (or inner edge of the front baffle for the front wall first reflection).
I can remember reading a review of the ASC tube traps by Jonathan Scull in Stereophile many years ago. What he did as one of his trap setup experiments was to create a wall of traps placed close to the outer edge of the speaker but aligned roughly along that first reflection path. Instead of placing half of the traps along each wall as Ethan suggests, try placing the 9" trap you're currently using on each side very close to the speaker but along the line of that first reflection path. That will ensure the trap "captures" sound from a wider angle of the speaker's radiation pattern than it will if placed close to the wall, giving you a similar kind of benefit to what you would get using a wider trap closer to the wall.
You may need to fiddle with the trap position a little in order to get maximum effect because of the narrowness of the trap but you should get absorption of a wider angle on either side of the first reflection path with the trap close to the speaker. Unfortunately by placing it that far from the wall you'll probably lose some of its effectiveness at lower frequencies but they're less critical when you're dealing with first reflections so you may still get good results.
Otherwise, if you need wider coverage close to the wall, panels are a more effective and economical approach than cylindrical traps.
You would probably get better results dealing with the side wall first reflections using Ethan's approach but then you lose the benefit of corner placement for your big traps. If you don't want to buy more traps or panels, placing the traps for the first reflections closer to the speakers may be more effective than a single trap on the wall while retaining the benefits of some corner bass trapping. What you're going to have to do if you run with the traps you have is balance the advantages of traps in the corner vs the advantages of wider coverage at the first reflection points and it's your call on which you prefer overall. There are advantages and disadvantages to both approaches.
David Aiken
Follow Ups:
Hello Mr Ethan & Mr. David,
Im very thankful for your suggestions. Does it mean that the wall reflections at the side are causing the midrange to be forward? I Do have curtains( thin ) covering the 1st reflection area, but im sure they do nothing acoustically, as they are almost see-through. I certainly do not mind adding panels, or even substituting my traps at the sides with panels. I have custom-made 2x4' panels exactly copying a DIY recipe from the internet, but have not been using them due to aesthetic reasons. But now im totally frustrated with the imbalance in sound, even after spending so much on the entire set-up, so i really do not mind in adding any amount of absorption anywhere in the room.
And Ethan, i do have the full version of ETF software since a long time, but i have never known how to use it to blend my bookshelf speakers to my subwoofer. I have been reading a lot of posts in AA by you & David & also Jon Risch, & i consider you people as Masters in the Audiophile world. You people have absolutely tremendous knowledge in both acoustics & electronics.
Given the size of my room & my set-up, what else do you suggest to improve the overall balance??? I dont mind making more panels or traps. My only regret is that. even after giving so much time & interest in setting up the whole thing, it still sounds not even close to high-end...
I don't know whether side wall first reflections cause the midrange to image too much to the front. I think there's too many variables involved with those reflections, or any reflections, to predict some effects reliably. The strength of those reflections are affected by the speaker's polar distribution pattern and angle of toe in and also by the reflectivity of the walls.
Definitely try the 2' x 4' panels at the first reflection points. I think your results will improve. That will leave you with the 9" traps spare and if your room is high enough you can try stacking them on top of the traps in your front corners which should help a bit more with control of bass modes.
You said your DIY treatments were unwrapped. If they're fibreglass and you have the fibreglass exposed, I think you should at least wrap them in a thin covering of polyester batting in order to prevent fibre release. Fibreglass may not be considered carcinogenic but fibres in the lungs can cause other problems. I think it would be hard for an adult to get a high enough concentration from exposure to unwrapped traps unless the fibreglass material was deteriorating badly but if you have carpeted floors any loose fibres are likely to collect there and small children playing on the floor are likely to receive higher exposure levels because they're more likely to disturb any fibres trapped in the carpet and also because they're going to have their mouth and nose much closer to the carpet than an adult which means they're more likely to breathe in any disturbed fibres. I'd still prefer to cover them with a cloth cover as well. Jon Risch recommends burlap which you may know as "hessian" which is what it's called here in Australia.
David Aiken
Yes, real fiberglass panels that are at least two inches thick will be infinitely better than thin curtains you can see through.
ETF will show you reflections, but you don't really need that. David and I already told you what you need to do.
--Ethan
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