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Hi Guys,
I’m looking for some advice on QRD set up.
I’m setting up a 2 channel listening room and would like to know which of the two alternative rear diffuser arrays would work best on the attached JPEG.
The plan with the diffusers directly behind the listening position seems the most conventional and would allow me to use 24 inch QRD diffuors grouped in fours which I understand to be the best way to use QRDs.
The plan with the angled diffusers to the side only allows either three 24 inch QRDs or reducing them down to 18 inch losing some lower frequency diffusion. The thing I like about this set up is the absorption behind the listening position should give clearer imaging (something I particularly like). It also should act as a bass absorber with bass passing into the large room behind.
(The BAD panels on the sides are a necessary alterative to QRDs due to lack of space.)
Any help would be appreciated.
Mark
Follow Ups:
I've said it before and I'll say it again. There is no way to treat a room that will be perfect for everyone. Different people like different types of result and my feeling from many discussions of absorption vs diffusion here over the years plus my own limited experiments with diffusion in my room leads me to believe that there are strong individual differences associated with the use of diffusion in listening rooms. Some people like it, some don't, and I suspect there will be differences of opinion on the amount and placement of diffusion even amongst those who like it.
My advice: try both and pick the one you like the most.
My experience is also that knowing what a particular treatment will do with the sound in the room is different to knowing whether or not I will like the sound. Over the years I've done things with my room or my system that I thought would be improvements, and even had the sound change in the way in which I expected it to sound, only to discover that I didn't like listening to the result. I think that the experience of listening to a particular treatment in a room is a very different thing to trying to decide in advance whether the sort of change in the sound which that treatment will produce will be good or bad.
The only way to assess the result of a particular change is actually to listen to it, and the only way to know which of 2 options will be the better is to compare them. Those aspects of our perception which contribute to our feelings of like and dislike, and even love and detest, don't always agree with our intellectual assessments of what will be a better or worse sound, and I think the disagreement often hinges on a question of degree. A bit too much, or too little, of something we think is a good thing, whether it be absorption or diffusion, can be the difference between like or dislike, love or detest. The problem is often getting the amount right, just as it is getting the amount of seasoning right in the dish you are cooking. You have to taste while you're cooking if you want to know whether you're getting things right. In my view you have to listen when you're treating a room in order to know whether you're getting things right. And just as not everyone agrees on how much salt, or chili, or whatever you need in a particular dish in order for it to taste great, my experience is that not everyone agrees on how much absorption or diffusion you need in a room in order to get great sound, or on where it should be placed. It's your room and it's your opinion which counts, and I think you need to try to listen to both in order to get the answer to the question you're asking.
David Aiken
Thanks David,
Great analogy, in fact that is exactly how I cook !
That makes perfect sense, As I intent to keep the panels portable, I will experiment to see which gives me the sound I like (I tend to prefer good imaging).
What are your thoughts on grouping QRDs ?
From what I have read and tried to understand, QRDs work better when grouped together in four or more (using a Barker code for more than 4) which would suggest that because of my limited space, I would get better results from grouping four 18 inch QRDs together rather than three 24 inch. My only concern being I would lose some lower end diffusion.
Really appreciate your input, I live for sound and music.
Mark
pun intended—actually thoughts on diffusion but still.
I don't think losing diffusion at the lower end is likely to be an issue. Decay time is longer at lower frequencies because they get absorbed more slowly, and diffusion in a room increases as decay time increases though modal activity isn't diffuse in one sense. Even losing some bass diffusion with a given layout, you're still going to have more diffuse mids and highs, a longer decay time at low frequencies and some modal activity at low frequencies. In fact you'll have that sort of picture whatever you do.
I don't think that whatever audible differences there are, it would be the difference in lower end diffusion that would lead you to choose one approach over the other. I think it's more likely to be what's happening in the mids on up and how that affects things, including how that affects imaging and soundstage, regardless of whether you prefer those precise or not.
Just be aware that you need space between the listening location and the diffusors. Sit too close and you'll hear some problems because the diffusion won't have integrated over the whole diffusion bandwidth and audible anomalies can occur. If I remember correctly, RPG suggest a minimum of 10' between diffusors and listening position somewhere on their web site. That distance varies a bit based on diffusor type and bandwidth but I suspect it's a good guide for QRD type diffusors like you're considering building.
If you can't provide that sort of distance you may well be better off considering absorption rather than diffusion. If you go the absorption route, consider using a number of 2' wide panels, place 2 at the rear wall first reflection points, and experiment with different placements for the others. I would expect you to get best results with some space left between panels and the alternation of absorption with reflection from bare wall will tend to promote a bit of diffusion anyway but nowhere near as much as you would get from using diffusors.
Once again, however, how much absorption you would use with this sort of approach and how you space the absorbers out is going to be a bit of a personal matter in my view.
David Aiken
Thanks David
That's a really good point about the lower end frequencies, although I would be going from 600hz with a 24 inch QRD up to 900hz with a 18 inch, so I guess that's the lower mids ?
I have about 9 ft between the listening position and the diffuser so I should be ok based on 600-900Hz.
The layout (image attached) would give me 4 ft wide absorption immediately behind the listening position which opens out into a biggish room behind so I was hoping this would give me some additional bass trapping .
It doesn't work that way.
Sound will certainly exit the room through the opening and some will come back through it, after reflection around the space on the other side of the opening. What you have, though, is a coupled space and the results of that are unpredictable. The sound that re-enters the room through the opening is going to have some of the acoustic "signature" of the other space.
There are some plusses to be had. For a start you not only weaken the basic front/back axial mode for your room but you gain another mode based on the length of the distance between your front wall and the wall opposite it in the other space. The modal response of a room smoothes out as the number of modes per octave increases and gaining another mode, especially a lower one than your room would otherwise have, results in the "modal density" increasing faster than it would if you didn't have the opening which is beneficial, as is weakening one of the room's natural strong modes.
On the other hand I've heard of coupling spaces causing suckouts and simply causing bass response to start rolling off at around 100 Hz in one case. If the "sound" of the space behind is particularly different in character from the "sound" of your room—say if one space has a relatively normal or even "warm" character and the other sounds "hard" or "bright", the difference in tonality of the sound re-entering your room from behind might be disturbing.
I don't know any way of predicting whether the coupled space will work for good or bad. I've got 2 openings in my room, but my room is L-shaped, the right wall is the wall with the bend in it, and one of those openings is in each of the two "right" walls. One is into a hallway and the other into a large open plan area that the hallway enters so I'd hate to try and figure out what's going on in my room but it doesn't seem to create any major problems for me. On the other hand, a former dealer (now retired) who I know had a showroom with coupled spaces and he was never satisfied with the bass response of speakers in that room. Measurements showed it simply started rolling off from 100 Hz or so on down.
Basically all I think you can do is try but if there's a big tonal shift going on behind you with a difference in character of the sound coming from the rear sides of the room and that from the opening behind you, you may want to try either using absorption rather than diffusion behind you so that the sound coming from the coupled space dominates and weakens the difference in character, or place some sort of absorbing panels as a screen across the area behind you in order to weaken the sound re-entering the room, weakening the difference in the other way.
If you want to actually get genuine absorption including at bass frequencies then what you need is an opening to outside the house, to open space, so the sound exits and doesn't return.
David Aiken
My plan was to close off the area entirely using solid diffusers rear left and right, and 4 ft absorption panels in the centre which is directly behind the listening position.
I was hoping this would give me a good amount of bass trapping, effectively using the whole room as a bass trap.
I expected some sound to re-enter the room, but having travelled through 4 inches of fibre glass (the absorber) twice that it would have been weakened and delayed. I was also hoping that the difference of the sonic signature of the rear room would be less apparent due to this process.
I guess it's a case of 'suck it and see' :/
Unfortunately, yes.
I think that's definitely the case with odd rooms—non-rectangular shapes, left/right wall assymmetries, openings, etc—and also with asymmetrical system placements when those are required for various reasons. There may be software somewhere which can help predict what will happen in some cases but even then, knowing how the sound will behave doesn't exactly correlate with how we perceive the sound and whether or not we will like the result.
There are guidelines from many sources on how to treat a room but in my view they should all be treated as starting points rather than absolute recipes. Even if you follow one of those sets of guidelines you may well find that you prefer a bit more or a bit less absorption in some areas, or more or less of the wall exposed, or absorption instead of diffusion or vice versa at some point. Part of that occurs because not every room is the same, there are large differences in rooms, but part of it is simply a matter of taste.
We don't all agree about what is the best concert hall sound. Different people prefer different halls and even different seats in the same hall. If we can't agree about what acoustic delivers the best result for live music, what are the chances that we'll all agree on how a listening room should sound?
My view is that there is no "one size fits all" solution when it comes to room treatment. There are several recommendations for room treatment setups out there from reputable sources and they aren't all the same. Most people can probably find a recommendation that will go close to suiting their room and tastes but I'm prepared to bet that with a bit of fine tuning and slight modification they can get results that they like even more.
I think sucking it and seeing, or at least hearing, is an essential part of the treatment process. I don't think you can get the best results for your needs without doing a bit of that.
David Aiken
Thanks for the all information David.
I'll post my findings as I go.
Mark
Can't help on grouping QRDs—no experience with QRD diffusors.
David Aiken
No problem, thanks for your previous post it really helped.
If there's anyone else out there knows anything about this subject I'd appreciate their input.
Mark
Since you have (or plan to buy) diffusors from RPG, you should ask for their advice. Nobody knows their products as well as they do.
--Ethan
Hi Ethan,
Unfortunately I don't have the budget for commercial QRD diffusers so will be making my own using the QRDude calculator.
Mark
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