Broadband absorption stands for reflection points

[psykx]

Hi there,
I have just moved into a new room in a shared house and in the next couple of months I'm going to be setting up a project studio. My room is pretty large W5m X L5m X H3m and the back wall is a large bay window so I shouldn't have too many bass problems once treatment is sorted.

I have a problem with first reflection points, in that on one side there is a door and on the other there is a book case built into the wall next to a fireplace. My idea for a starting point of my room treatment was to build two 4" x 2" broadband absorption panels on stands that I could place on either side of me at an equal distance so tame reflections.

I'm not really sure how thick they should be, 6" seems to be often touted around here but in this position is it useless? or should I use 8" because the corners will be difficult to treat in this room?

or is putting reflection point stands a bad idea all together?

[Sebg]

Do you really mean

place on either side of me at an equal distance so tame reflections

. These are not the first reflection points.

Otherwise, absorbers on stands are fine. Make sure they are stable and (relatively) easy to move.

[DanDan]

If the reflections are tamed, do those panels need to be an equal distance?
Two such panels is a very small amount of the total room area, don't expect anything other than an improvement in stereo imaging.
Two of your dimensions are the same so do expect serious bass problems.
DD

[psykx]

I ment an equal distance from me on each side, and roughly an equal distance between me and my monitors, which should be the mirror point. The distances I gave are pretty rough as I didn't have a tape measure and I used a paper ruler to measure a belt and did it that way. I think the distances could be out by a foot in either direction, maybe more.

I think I was hoping that bass traps in this position would be effective at absorbing bass before it had a chance to bounce around the room.

[DanDan]

Even at 8 inches deep, just two of these will have very little surface area compared to the room. Bass waves will hardly notice them.
DD

[psykx]

Originally Posted by DanDan

Even at 8 inches deep, just two of these will have very little surface area compared to the room. Bass waves will hardly notice them.
DD

That can't be a fair analogy as if the room was 'outside size' then the ratio would be smaller and the room would sound better. The bass waves aren't going to 'see' the whole back wall of my room so it could sound ok, and if it doesn't I will treat it as much as I can.

Really what I want to know is is it worth making the first reflection point stands deeper than the standard couple of inches for first reflection points?

[avare]

Originally Posted by psykx

Really what I want to know is is it worth making the first reflection point stands deeper than the standard couple of inches for first reflection points?

The standard, if there is one by that word is 4" MINIMUM for 1st reflection points. As far as making them deeper goes, it will help the low end. As Dan wrote, the room will need much more, but it will less much more.

Andre

[DanDan]

psykx, I am afraid I don't understand what you are saying in that last post. Your overall question does bring up some interesting points though.
e.g. An airgap behind a panel does increase it's performance dramatically, but this was established with a very large area of material. A small patch is likely to receive less and less benefit as it gets farther from the boundary.
Also to quote a ridiculously vague rule of thumb, one would need to change at least 20% of the surface area to notice an acoustic change.
So Two small panels, what to do?
Worth using at all? yes, for HF side reflection control.
Where to put them? Hmmm. I would plump for installing them on the boundaries, even if one is a door. I reckon the HF reflection will get hit twice there. 4+4 is as ever, our minimum recommendation, but practical matters often decide otherwise.
If your interest is Bass, it is focussed very strongly at the corners, all 12 of them. With little material, one would obviously place it at the focus points.
Two 2x4x8' traps straddling the floor or ceiling tricorners behind your speakers might be useful.
Two large SuperChunks would definitely be worthwhile.
If you go this thick try to keep the density down.



DD

[localhost127]

Originally Posted by psykx

Really what I want to know is is it worth making the first reflection point stands deeper than the standard couple of inches for first reflection points?

you start with a design requirement: how many dB do you wish to attenuate the first-order-reflections in comparison with the original signal.

you can then verify whether your broadband absorption panels for specular reflections have indeed attenuated the reflections down to your required level with the ETC response. if not, add more thickness to the panel, or angle panel to change angle of incidence and verify with ETC again.

[psykx]

dandan I mean that using the amount of wall covered with bass traps as a guide to how much treatment you need is a bad idea, because very large spaces need much less treatment than very small spaces to sound "good".

I had not considered the air gap on either side of the traps, maybe that would have an effect.

My thinking was that as bass radiates from a speaker in a roughly uniform direction. placing a trap near the speaker should absorb more of the bass than one further way. Imagine how much trapping would be needed to cover a speaker 8" in trapping compared to the rest of the room.

Also surely trapping bass reflections before they reach you could increase the quality of the sweet spot? rather than the quality of the room?

you can then verify whether your broadband absorption panels for specular reflections have indeed attenuated the reflections down to your required level with the ETC response. if not, add more thickness to the panel, or angle panel to change angle of incidence and verify with ETC again.

I'm not sure how many dB I wish to attenuate the first-order-reflections by, surely I want to remove as much as possible, given that the rest of the room won't be treated particularly well in the beginning? Will changing the angle increase the amount of reflection?

[DanDan]

Very creative ideas. I kinda wish some of them were true.
There is an element of truth in the large spaces idea, just an element. However I don't think we are dealing with large spaces here.
The gap and proximity to boundaries and locating at quarter wavelengths are all significant. However Andre has previously shown that fibrous panels are only about a third as effective as one might hope, when away from boundaries. The idea of surrounding speakers with fibre has again an element in it. However the fibre would need to be enormously thick to be useful. Note that John Sayers sometimes buries speakers in an absorbent corner. A bit of everything going on there.

Bass Waves will simply ignore the little dot of fibre due to their relative sizes.

DD

[psykx]

well you know what they say about a little bit of knowledge. I did a search for what andre proved but couldn't find anything, can you give me a title or a link please?

[avare]

Originally Posted by psykx

I did a search for what andre proved but couldn't find anything, can you give me a title or a link please?

Which are you referring to?

[psykx]

Originally Posted by avare

Which are you referring to?

this bit.

However Andre has previously shown that fibrous panels are only about a third as effective as one might hope, when away from boundaries

[DanDan]

I am trawling memory here. I believe Andre provided a link to probably a BBC document. I reckon they were examining the behaviour of fibre placed away from boundaries, at the high particle velocity regions of 1/4 wavelenght etc. etc. One might expect spectacular absorption there but the test indicated otherwise. It can probably be found in the Q4Avare thread.
Not sure about the word proof, but I believed it when I read it.

There have been a lot of innovative, probably incorrect views of what might be happening posted in this thread. While I kinda like the energy, one could go through the whole field of acoustics in a point by point manner. Time better spent RTFM IMHO.

Regarding the interesting or possible scenarios, REW is free. psykx, I recommend you get that and try some tests. The first big one will be to detect any effect from two small panels in a room.
The HF is easy, see my Desktop Reflection Revealed thread.
Searching for an LF effect will get your creative juices flooding, I guarantee it.
DD

[avare]

Originally Posted by psykx

this bit.

Dan got it already. Thanks Dan. It is from a BBC RD paper. Links can be found in the Q4 Avare thread.

Well proven,
Andre