Diffusion v absorption

[spiritofmusic]

Currently running my system in an 18x48x8 dedicated room, w generally good acoustics. I had been using a suite of GIKS...bass traps and 4x2 absorber 242 panels, three on front wall, two each at side wall reflection points. I did take them out but was advised to reinstall them when a visitor remarked my central image was phasey and unfocused. I didn't really get his comment at the time, but kinda got the idea when things became more palpable on reinstall.
A year on, after a major breakthru on bass linearity in my room (filling a large floor void w acoustic rockwool), I tried out removing the GIKs again, and the haziness was immediately apparent, lack of focus and sucking the life out of the mids.
My visitor a year back was dead right.

My Q is not whether this experience proves my room benefits from help...it does.
It's whether what I'm describing indicates that I should stick w absorption panels, or would get more mileage from diffusion, or look for panels that do both (ASC and Acustica Applicata come to mind)?

 

[Duke LeJeune]

Imo it is well worthwhile to get a professional diagnosis and professional guidance. I suggest award-winning Jeff Hedback of Hedback Designed Acoustics, who specializes in remote diagnostics and design (in other words neither the pond nor the pandemic are barriers), and who is still affordable.

I used to be an avid amateur room acoustics guy, until I listened in a room that Jeff had done (remotely) and realized the gap between what I could do and what he could do was enormous. At the risk of oversimplifying, room acoustic treatment is kinda like crossover design: Just as good crossover design calls for the right values of inductance, capacitance, and resistance in the right places, so too good room treatment calls for the right kinds of diffusion, absorption, and reflection in the right places.

Here is an article Jeff wrote for GIK Acoustics, the word "diffusion" is in the title but the article is about much more than that:

https://www.gikacoustics.com/diffusion-by-jeff-hedback/

Jeff was interviewed by Larry Borden of Dagogo back in 2012:

https://www.dagogo.com/an-interview-with-jeff-hedback-of-hdacoustics/

 

[spiritofmusic]

Ok, he sounds qualified Lol. Quite how you choose the blend remotely seems fascinating.

 

[pjwd]

Currently running my system in an 18x48x8 dedicated room, w generally good acoustics. I had been using a suite of GIKS...bass traps and 4x2 absorber 242 panels, three on front wall, two each at side wall reflection points. I did take them out but was advised to reinstall them when a visitor remarked my central image was phasey and unfocused. I didn't really get his comment at the time, but kinda got the idea when things became more palpable on reinstall.
A year on, after a major breakthru on bass linearity in my room (filling a large floor void w acoustic rockwool), I tried out removing the GIKs again, and the haziness was immediately apparent, lack of focus and sucking the life out of the mids.
My visitor a year back was dead right.

My Q is not whether this experience proves my room benefits from help...it does.
It's whether what I'm describing indicates that I should stick w absorption panels, or would get more mileage from diffusion, or look for panels that do both (ASC and Acustica Applicata come to mind)?

HI Marc

I am slightly hesitant to wade into this but here goes:
As an alternative view you could just look at the acoustics as first reflections and reverberation control. In my past life I worked on a few projects with an excellent acoustic engineer (James Heddle) and this is what we did.
As I understand it all forms of diffusion alter the phase of first reflections - also repeated diffusers add comb filtering to this mix - this was avoided to keep reflections in pase with direct sound as much as possible. ( those wing diffusers may work as I suspect they scatter most of the sound sideways but I am not certain )

In the attached picture of a control room for a recording studio - flat reflector/ absorber panels were arranged on side walls to keep reflections specular but avoid them in the listening plane - baffles on the ceiling absorbed and prevented ceiling reflections to the listener - in all four corners of both wall and ceiling there were substantial bass traps with absorptive faces.
The bass traps and general absorption controlled reverberation time and the reflection/ baffles reduced early reflections smearing the sound. But there was till enough reflections going on to provide a nice reverberant tail ( not overdamped )
The spaces worked really well - in studios we took the same approach but looked at the microphone as the listener and instruments as the source so it a bit more complex


As I recall in your room you have a sloped ceiling that would reflect a lot back to listening position - probably saved by some beams that act like baffles - I would suggest your first move would be to add more ceiling baffles and nullify any remaining reflections - then try bass traps in rear corners ( nehind speakers ) and go from there with these principals in mind

This link is a paper that the acoustic consultant wrote on the project that explains the ideas

https://www.acoustics.asn.au/conference_proceedings/AASNZ2016/papers/p143.pdf

at the very least it is more grist for the acoustic mill

cheers
Phil

 

[spiritofmusic]

So, more absorption on the ceiling eaves...as opposed to diffusion? Or look for a product that does a bit of both?
It was fascinating hearing a hollowness or phasiness removing absorbers on the front of side walls...I just couldn't surmise from that whether absorption was all I needed or it indicated need for pure diffusion.
Certainly most of the big installs I see rely on pure diffusion on front wall.
+1 re paying more attention to eaves.

 

[pjwd]

So, more absorption on the ceiling eaves...as opposed to diffusion? Or look for a product that does a bit of both?
It was fascinating hearing a hollowness or phasiness removing absorbers on the front of side walls...I just couldn't surmise from that whether absorption was all I needed or it indicated need for pure diffusion.
Certainly most of the big installs I see rely on pure diffusion on front wall.
+1 re paying more attention to eaves.

not so much absorption as baffles to stop the reflections - how absorptive they need to be depends on how bright the room is - that is kind of a second function - I have used a 12mm recycled PET board that is pretty cheap and has some absorptive qualities - I expect there is an equivalent in UK - and you actually don't need many but the geometry is based on a "mirror on the ceiling" just like folks talk about for side walls - will be more interesting than a flat ceiling - can you point me at a picture of your room and some dimensions and I will show what I mean

 

[spiritofmusic]

Willdo. It is fascinating that a shape I know is frowned upon by many ie eaves ceilings...esp mine where the outer eaves come down to 4' above ground level, has checked out so well...and thus my 18" deep steels at 7'6" centres are maybe breaking things up.

And now you're suggesting...break them up a lot more.

 

[spiritofmusic]

The main Q that an opinion would be great. If removing pure absorbers on walls leads to more hollowness and vagueness, does that suggest 100% diffusion would do better? Or a mix w absorption? Or stick to 100% absorption?

 

[pjwd]

Willdo. It is fascinating that a shape I know is frowned upon by many ie eaves ceilings...esp mine where the outer eaves come down to 4' above ground level, has checked out so well...and thus my 18" deep steels at 7'6" centres are maybe breaking things up.

And now you're suggesting...break them up a lot more.

not really - just remove any remaining early reflections that occur between beams - the beams are the reason the room isn't the disaster folks would assume - suggesting the theory works

 

[spiritofmusic]

And pure diffusion on walls? Esp front wall?

 

[pjwd]

And pure diffusion on walls? Esp front wall?

ha.... wine time here so loosing focus - I am not a diffusion enthusiast as it stuffs up phase - I have removed qrd and curved diffusers and heard the difference - unless they also reflects sideways and not back into the room - which I am not sure many do

side walls are the same reflection story as ceiling and baffles may be the go if the room is not wide enough - or a false wall that slopes down to floor at reflection point - spaces like the shoebox Boston town hall probably owe their reputation in part to the attached pilasters along the wall - essentially baffles reflecting back sound to stage rather than into audience

over and out for me this evening

 

[spiritofmusic]

Enjoy...thanx for yr input, appreciated.

 

[spiritofmusic]

PJ, my room is over 18' wide, a reason why descending eaves down to side walls only 4' high is not an issue. I'm 6' tall myself and can stand 2' to the outside of my spkrs before I headbut those eaves.

I was seriously considering on walls replacing my absorber panels w diffusers, but you've got me thinking about replicating my structural steels break ups w more slats on the eaves.

There are also some neat inexpensive diffuser options from GIK and Thomann.De that I could try out on my eaves.

 

[pjwd]

PJ, my room is over 18' wide, a reason why descending eaves down to side walls only 4' high is not an issue. I'm 6' tall myself and can stand 2' to the outside of my spkrs before I headbut those eaves.

I was seriously considering on walls replacing my absorber panels w diffusers, but you've got me thinking about replicating my structural steels break ups w more slats on the eaves.

There are also some neat inexpensive diffuser options from GIK and Thomann.De that I could try out on my eaves.

Marc
a rough idea of what I mean - you only need to have them where the ceiling is reflecting to your seating position - the left shows the red reflections and the right roughly the baffles ( blue)

as you can see the hot spot is in the middle of the ceiling
you could do the same on side walls where you have absorbers - you will not need many as the beams and columns are already doing the work

if you are interested let me know the ceiling height in the middle of the room and the locations of chair and speakers relative to beams ( looks like they are close to lining up) and I will look at a section to see roughly how many you need ( when I have a moment ) - pretty simple

if not no worries

Phil

cheers

 

[spiritofmusic]

Phil, I will once I know your hourly consultation fee, thanks Lol. I'll measure up once I get home from work. Much appreciated.

 

[spiritofmusic]

Phil, PM re dims sent

 

[Zero000]

I reckon after putting up that 100 inch ALR screen between my Duettas has resulted in a significant uplift in sound quality in the new room. Whatever it's doing is a good thing.

I was very happy about it last night.

 

[spiritofmusic]

Well, gotta be preferable to a big slab of glass flat screen tv.

 

[Zero000]

Yeah it's very inert/dead to the touch and doesn't vibrate though I suspect it might move a tad. I will feel it later when the SPL is high to see if I can detect anything. Also I have my gear very low down near the floor. As a consequence imaging is improved considerably since nothing intersects the speakers at ear height.

Good stuff.

 

[pjwd]

I reckon after putting up that 100 inch ALR screen between my Duettas has resulted in a significant uplift in sound quality in the new room. Whatever it's doing is a good thing.

I was very happy about it last night.

yes that would absorb some sound at the reflection points - depending on how tight it was and how far from the wall( more the better) - In my situation I built a 150mm box and filled it with dacron insulation and stretched screen over the front - a broad band absorber to damp down the room - not ideal for reflection angles but not much you can do about a screen - I agree it was a big improvement
I did look at perforated screens but picture quality was lower

Are these ALR screens newish on the market

Phil