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How important is diffuser height if only a few feet from listening position?
Old 14th October 2013
  #1
Gear Head
 

How important is diffuser height if only a few feet from listening position?

My rear wall is only 3-4' from my seated listening position (I'll move the chair depending on the speakers I'm using) and I want to treat the area directly behind me with 1D diffusion and I'm considering these units....

How important is diffuser height if only a few feet from listening position?Acoustic Sound Diffuser Panels Diffusion Treatment Recording Studio Home Theater | eBay

....if I can't find the time to build more QRD's. The seller says the deepest well is 4", so I'm thinking I'm ok as far as minimum sitting distance based on what I've seen when playing around with QRDude (deeper the well, further away from sitting position device needs to be). At 48"x14" I can cut one unit into thirds and have an array of three that will cover a width of 42" but will only have 18" height. I was hoping since I sit within such a close proximity to the device it won't have to cover as much vertical area as something on the other side of the room. Yea or nay?

Thanks,
Michael
Old 14th October 2013
  #3
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Glenn Kuras's Avatar
I don't know all the details about your room but generally speaking when sitting that close to the rear wall you will have larger problems in the low end which thick absorption is the way to go. You can always use a slat design to get some scattering/diffusion action going on.
Old 14th October 2013
  #4
Gear Head
 

Glenn,

Currently on the rear wall I have two 2'x4' bass traps (constructed with 6" thickness rockwool and with a facing of brown craft paper). The traps have about four feet between them and are on stands that have them positioned about 6" out from the wall and angled slightly outboard to deflect mid and high frequency energy to the side walls....which currently only have broadband absorption at first reflection but I will place a few QRD diffusers at rear sidewalls once I get around to building a few more. Reading over the links Jens provided I might be just as well off to absorb reflections off the wall directly behind me or deflect them away from my listening position with something like wedges. I need to read up more on the ISD-gap information he provided a link to and figure how that applies to my situation.
Old 15th October 2013
  #5
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dmsmastering's Avatar
 

Generally diffusers need space and long distances to work proper. I would prefer an absorber with slotted plastic foil in those short situations. You might end up with a high or low (under head) position just to have the diffusor not direct behind your ears.

- Aaron -
Old 15th October 2013
  #6
Gear Head
 

Quote:
Originally Posted by dmsmastering View Post
Generally diffusers need space and long distances to work proper. I would prefer an absorber with slotted plastic foil in those short situations. You might end up with a high or low (under head) position just to have the diffusor not direct behind your ears.

- Aaron -
Would you mind elaborating on the "slotted plastic foil" suggestion?
Old 15th October 2013
  #7
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Glenn Kuras's Avatar
Quote:
Originally Posted by tinnitusintx View Post
Glenn,

Currently on the rear wall I have two 2'x4' bass traps (constructed with 6" thickness rockwool and with a facing of brown craft paper). The traps have about four feet between them and are on stands that have them positioned about 6" out from the wall and angled slightly outboard to deflect mid and high frequency energy to the side walls....which currently only have broadband absorption at first reflection but I will place a few QRD diffusers at rear sidewalls once I get around to building a few more. Reading over the links Jens provided I might be just as well off to absorb reflections off the wall directly behind me or deflect them away from my listening position with something like wedges. I need to read up more on the ISD-gap information he provided a link to and figure how that applies to my situation.
Let me make it a bit simple, keep all reflections -20db out to around 20ms.
Old 15th October 2013
  #8
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jim1961's Avatar
 

Quote:
Originally Posted by Glenn Kuras View Post
Let me make it a bit simple, keep all reflections -20db out to around 20ms.
-30db when possible
Old 15th October 2013
  #9
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Glenn Kuras's Avatar
Quote:
Originally Posted by jim1961 View Post
-30db when possible
When possible make it as even lower. I guess the point to the OP is you want to hear the direct sound from the monitors. Not direct plus effects of things within 20ms (near by reflections).
Old 15th October 2013
  #10
Gear Head
 

Quote:
Originally Posted by Glenn Kuras View Post
When possible make it as even lower. I guess the point to the OP is you want to hear the direct sound from the monitors. Not direct plus effects of things within 20ms (near by reflections).
Yes Glenn, that is correct....and not totally absorb all the high and mid frequency energy, nor have it coming directly at me from the area of the wall directly behind me. So now I'm looking at DIY slat diffusion (I guess this is binary?) to go over the two bass traps I mentioned earlier in the thread and am considering building a third unit exactly the same as the other two (with the slat diffuser face) to situate directly behind me. I'm studying this thread to get some direction in constructing the slat diffuser....

Calculating slat / slot width for a binary diffuser
Old 16th October 2013
  #11
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Glenn Kuras's Avatar
Quote:
and not totally absorb all the high and mid frequency energy
Near/early/first reflections you do want to absorb everything.
Video Early or First Reflection Points - GIK Acoustics
Old 16th October 2013
  #12
Gear Head
 

Quote:
Originally Posted by Glenn Kuras View Post
Near/early/first reflections you do want to absorb everything.
Video Early or First Reflection Points - GIK Acoustics
So, because I sit as close as I do to the rear wall (3-4') the reflection off of it would be considered an early reflection? I sit about ten feet from the speakers, FYI.
Old 16th October 2013
  #13
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jim1961's Avatar
 

Quote:
Originally Posted by tinnitusintx View Post
So, because I sit as close as I do to the rear wall (3-4') the reflection off of it would be considered an early reflection? I sit about ten feet from the speakers, FYI.
3' (6' path) = 5.3 ms
4' (8' path) = 7.1 ms

Yes, these are considered early.

A rear wall distance of 11' (19.5 ms) is required to have rear wall reflections not be early ones.
Old 16th October 2013
  #14
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jim1961's Avatar
 

Quote:
Originally Posted by Glenn Kuras View Post
Near/early/first reflections you do want to absorb everything.
Video Early or First Reflection Points - GIK Acoustics
Redirection can be used instead of absorption. But it can get tricky and a well designed plan must be in order. This is actually preferred by many for it preserves the room energy and when reintroduced at a later timing (20-30ms) keeps the room from becoming dead.
Old 16th October 2013
  #15
Gear Head
 

Ahh, well then that kind of changes everything. So just broadband absorption on my rear wall....say, three 2'x4'x6" panels with one directly behind me and two more flanking that one?
Old 16th October 2013
  #16
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jim1961's Avatar
 

Quote:
Originally Posted by tinnitusintx View Post
Ahh, well then that kind of changes everything. So just broadband absorption on my rear wall....say, three 2'x4'x6" panels with one directly behind me and two more flanking that one?
Or something like this:

My Listening Room
Old 16th October 2013
  #17
Gear Head
 

Quote:
Originally Posted by jim1961 View Post
Or something like this:

My Listening Room
So, you have what appears to be absorption panels behind you (with a few reflective slats?)....and quite some distance away, at that. Why absorption if the boundary layer behind you is so far away? And I see you have the panels angled in toward the listening position. What purpose does that serve, Jim?
Old 16th October 2013
  #18
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jim1961's Avatar
 

Quote:
Originally Posted by tinnitusintx View Post
So, you have what appears to be absorption panels behind you (with a few reflective slats?)....and quite some distance away, at that. Why absorption if the boundary layer behind you is so far away? And I see you have the panels angled in toward the listening position. What purpose does that serve, Jim?
These questions are answered in the thread.
Old 16th October 2013
  #19
Gear Head
 

Quote:
Originally Posted by jim1961 View Post
Or something like this:

My Listening Room
Quote:
Originally Posted by jim1961 View Post
These questions are answered in the thread.
Aww geez, I was afraid you'd say that. Alright, alright...I've been meaning to read the entire thing anyway. Been putting off glazed eyes and melted brain long enough.
Old 16th October 2013
  #20
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jim1961's Avatar
 

Quote:
Originally Posted by tinnitusintx View Post
Aww geez, I was afraid you'd say that. Alright, alright...I've been meaning to read the entire thing anyway. Been putting off glazed eyes and melted brain long enough.
Do what I do and on the first scan, only look at the pictures. They will tell a lot unto themselves
Old 16th October 2013
  #21
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Glenn Kuras's Avatar
Quote:
Originally Posted by jim1961 View Post
Redirection can be used instead of absorption. But it can get tricky and a well designed plan must be in order. This is actually preferred by many for it preserves the room energy and when reintroduced at a later timing (20-30ms) keeps the room from becoming dead.
Not with his set up. He wants to absorb them. But yes you can do what you are saying.
Old 16th October 2013
  #22
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jim1961's Avatar
 

Quote:
Originally Posted by Glenn Kuras View Post
Not with his set up. He wants to absorb them. But yes you can do what you are saying.
If your stuck with the idea of a wall 3-4' behind you, you have to absorb there, agreed.
Old 17th October 2013
  #23
Gear Head
 

Quote:
Originally Posted by jim1961 View Post
If your stuck with the idea of a wall 3-4' behind you, you have to absorb there, agreed.
....and focus diffusion efforts at rear of sidewalls and front wall (if using dipole speaker designs)? And keep in mind, for my situation rear of sidewall means to my immediate right and left, rather than at an angle over each respective shoulder.
Old 17th October 2013
  #24
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jim1961's Avatar
 

Quote:
Originally Posted by tinnitusintx View Post
....and focus diffusion efforts at rear of sidewalls and front wall (if using dipole speaker designs)? And keep in mind, for my situation rear of sidewall means to my immediate right and left, rather than at an angle over each respective shoulder.
Rear sidewall diffusion (in your case directly left and right of the LP) are ideal locations for diffusion IF the energy emanating from them is 20 - 30ms after the direct signal.

Front wall diffusion with a dipole speaker in front of them I have no experience with.
Old 17th November 2013
  #25
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dmsmastering's Avatar
 

Quote:
Originally Posted by tinnitusintx View Post
Would you mind elaborating on the "slotted plastic foil" suggestion?
For example a basotec 100mm absorber behind your head might "sound" very dead... If it feels bad use some kind of thin plastic foil to reflect highs. This will give you a more natural sounding absorber. You can also cut plastic foil strips and hide everything with an acoustic cloth/fabric. The thicker the plastic the deeper the high frequencies which will reflect. Such a thick pp foil for roofs reflected down to around >4k when i did some measures in the past and if i remember right. The thin transparent supermarket foils which crackle loud reflected around >10k. Just to give an inspiration how you can tune/ mod an absorber with different faces to sound more natural.

- Aaron -
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