controlling front wall reflections

[scottjoplin]

Hi Guys,
May I put up another question pls. Since the discussion is about the front wall reflections, how about the rear wall behind our listening seat, should it be totally absorbent instead of
the normally suggested which calls for diffusion. Would appreciate your thoughts on this. I have experimented with diffusion but the benefits don't seem to be as great as diffusion on the front wall
Thanks

Totally absorbent would be the ideal, the sound should flow past your ears and not come back. At low frequencies this is a real pain in the asterix as it's a major source of room modes and absorbing them to any great degree is a real challenge

 

[sumotan]

Thanks Scott

 

[graaf]

Hi Guys,
May I put up another question pls. Since the discussion is about the front wall reflections, how about the rear wall behind our listening seat, should it be totally absorbent instead of
the normally suggested which calls for diffusion. Would appreciate your thoughts on this. I have experimented with diffusion but the benefits don't seem to be as great as diffusion on the front wall

it depends on the distance between the listener and the back wall

when it's greater than 2 m then the back wall reflection is not a problem at all

from bass and room modes perspective a different kind of absorption is needed but it is sufficient to treat either of the two - front or back wall

 

[sumotan]

Hi Graaf
Thanks for the reply. In fact the back wall is indeed about 2m away.
This wall is of cement board & line with 100mm thick insulation material inside.
Was thinking of taking away this whole cement board wall & just line it with cloth like speaker grill.

Thanks again

 

[andy19191]

Totally absorbent would be the ideal, the sound should flow past your ears and not come back. At low frequencies this is a real pain in the asterix as it's a major source of room modes and absorbing them to any great degree is a real challenge

A subwoofer placed against the wall behind the listener with the signal inverted and delayed by the time of flight from the main speakers will absorb most of the axial modes. It usually works well and I believe is a common approach in professional installations like cinemas.

 

[AllenB]

I think the wall behind us should be reflective (but if anything, diffuse), unless you sit close to it. Ie: when there is a timewise gap between the direct sound and the rear reflection, then it can be worth keeping.

 

[scottjoplin]

it depends on the distance between the listener and the back wall
when it's greater than 2 m then the back wall reflection is not a problem at all

My room's too damn small, I'm sitting about 4 feet from the back wall and since I'm using dipole subs, I can't move them nearer to the front wall and if I move any farther away from back wall I start to be in the null, ho hum.

 

[youknowyou]

the front wall is the only wall that you could let untreated with minimal degradation to the sound. but bass traps in every corner is recommended. if the speakers are very close to the front wall, to limit SBIR, some absorption placed directly behind the speakers has been shown to help

the wall behind the Listening Position and the side walls and ceiling/floor 1st reflection points should be treated with absorption. not the entire walls, just the 1st reflection points in relation to your LP and speaker placement

 

[scottjoplin]

A subwoofer placed against the wall behind the listener with the signal inverted and delayed by the time of flight from the main speakers will absorb most of the axial modes. It usually works well and I believe is a common approach in professional installations like cinemas.

What's this technique called? I've not heard of it before and would like to research it

 

[andy19191]

What's this technique called? I've not heard of it before and would like to research it

I know K&H (now Neumann) had their own trademarked buzzword for it which was Plane Wave Bass Array (some measurements and discussion on page 12 here) but this was not recognised by someone on the other side of the pond when I used it. He used a different term which I can't recall and will leave you to discover.

 

[andy19191]

PS He used the the term CABS - Controlled Acoustically Bass System - and here is a PhD thesis of all things. I am pretty sure the method predates the thesis by decades.

 

[scottjoplin]

Thanks for the links andy

 

[bwaslo]

Also called "Double Bass Array" (DBA). Big discussion of it here:
Double Bass Array (DBA) - The modern bass concept! - AVS Forum | Home Theater Discussions And Reviews

 

[Dhruv Meena]

Me tinks that this is not a good idea.
First, the driver is 4" fullrange in a narrow box, with high directivity above 4kHz.
1) sideways near the wall will boost bass below 200Hz (good)
2) but directs high frequencies 90¤ away, (bad)
3) the reflector throws some of high frequency towards the listener (compensates a little the previous) but also makes more interferences from the nearby front wall (bad)
4) increased portion of reflected energy deteriorates stereo imaging very much (bad)

No wonder we don't see more of this kind of systems...

It might work for controlled environment but getting these system right in general market with normal consumer having normal house, i agree with you.


i used to do these panel like thingie on speaker when i was young and it may increase space or spatial cues, it made the whole sound echoey and hollow
and sucks some part of frequency also.
(will do it with my recent TQWT Alpair 11ms i made recently)