Acoustic Panel at 1st reflection point

[CinemaDK]

Just recently started playing with REW measurements, and reflections.

After investigating, I ended up building a 60x100 cm acoustic panel with 50mm insulation (117kg/m3 density) and 25mm air behind. I found several tests and reports stating that just a bit of air behind the insulation would greatly improve dampening features.

I did all measuring with just the left speaker. It is fairly close to the wall, so it has approx 32-35cm distance to it´s first reflection point, which equals just about 1ms. Now there is no doubt that the panel made a difference, that huge spike at 0,8-1 ms disappeared, so good job to myself :-)

Then out of curiosity I thought I would try to do the measurements both with (blue) and without the air gab (yellow) by simply turning it around:

There is a difference, but I lack experience to interpret which is best or if its doesnt really matter?

I'm not going to change the panels anyway, but I am curious

 

[Sonnie Parker]

Yes... it did make a difference, but is it better... the million dollar question no doubt. If they are setup where you can remove and put them back while listening to the same music, that might help you determine.

Check out Floyd Toole's blind testing where the majority actually preferred no side wall absorption. Then again, reading that might cause you to be bias due to the results. We read all this stuff and then our minds start believing it's should be better, so it then becomes hard to be honest with ourselves... crazy mind games.

 

[CinemaDK]

Yes... it did make a difference, but is it better... the million dollar question no doubt. If they are setup where you can remove and put them back while listening to the same music, that might help you determine.

Thanks for replying Sonnie. I am working out of a small room with lots of hard surfaces, and this is a home cinema 9.1 setup (soon to be 7.1.2), so currently lots of reflections are a given. I started this journey because I had a muddy feel to the sound, especially speech. I have very limited listening experience, and close to 0 references to well-sounding setups, which can make it difficult to listen my way forward. And are right I will easily be bias´ed, since I have built up more theoretical knowledge than practical :-)

My interest right now is mainly on the difference between the Blue and Yellow measurements, because I have been researching the "value" of the air gab. But I don't know exactly what is "best" within the ETC curve, from a theoretical view point.

 

[Sonnie Parker]

Thanks for replying Sonnie. I am working out of a small room with lots of hard surfaces, and this is a home cinema 9.1 setup (soon to be 7.1.2), so currently lots of reflections are a given. I started this journey because I had a muddy feel to the sound, especially speech. I have very limited listening experience, and close to 0 references to well-sounding setups, which can make it difficult to listen my way forward. And are right I will easily be bias´ed, since I have built up more theoretical knowledge than practical :-)

My interest right now is mainly on the difference between the Blue and Yellow measurements, because I have been researching the "value" of the air gab. But I don't know exactly what is "best" within the ETC curve, from a theoretical view point.

I think it comes down to what you like, not necessarily how it measures. Some like diffused sound, thus ETC spikes will be higher, and some prefer absorption, which will cause the ETC peaks to be lower. There those that are in favor of more diffusion than absorption, but others like more absorption and/or a combo of both. It's really all about experimenting, and likely it will be hard to find another room that is just like yours that you can hear the various setups, so it comes down to what you like. You do want to make sure you don't have any slap echo in the room, but too much absorption can also act as a tone control on the mid to top-end of the frequency range.

 

[Chris_Adair]

To give you a very straightforward answer, the additional airgap behind extends the absorber’s ability to absorb lower frequencies, so you’d notice a small improvement (decrease) in your RT60 times below 200Hz with the air gap.

With your speaker being so near the side wall I’d expect an improvement in the sound with the panel in place.
By turning the panel around you are also changing the timing of first reflection from the panel, since you are changing the distance.

another thing to keep an eye on is the room response with respect to frequency as linear as you can, to maintain a balanced sound.

REW does the RT60 times for you , and waterfall charts to see decay with respect to frequency.
Enjoy your cinema !

….
There is a difference, but I lack experience to interpret which is best or if its doesnt really matter?
There is a difference, but I lack experience to interpret which is best or if its doesnt really matter?

I'm not going to change the panels anyway, but I am curious
[/QUOTE]

 

[DanDan]

Many of us have been recommending an airgap when space is available. Many have over-egged this based on Kundts Tube and Quarter Wave theories. I have always stuck with 1:1 Fibre:Air But recent discussions are suggesting that for isolated traps at least that more than 50mm Air is not beneficial.

 

[dhai]

"Some like diffused sound, thus ETC spikes will be higher,"
Sonnie, with all respect - diffusion will "diffuse" the ETC spikes,
meaning they get smaller, and nested within more of same.

Cheers,
David

 

[DanDan]

I think he was comparing the Diffused ETC versus an Absorbed one David. I would like to include a note that absorption will dampen soften the high end of the room tone, but the lack of destructive early reflections increases the clarity of the direct path from tweeter to ear. Games of Opposites I call it.....

 

[dhai]

Thank you, DanDan, makes sense. Constructing horn loudspeakers, I've some experience with direct sound ... even within diffractive environments

What shocks me every time in dominantly absorbing studios, is the amount of energy consumed. Amps and speakers struggle hard to produce convincing dynamic peaks. Probably a Game of Extremes in terms of sourcing and sinking energy ;-)

 

[DanDan]

Dunno, the peaks would require the same amount of extra power as the averaged levels. Theory says a 6dB difference from anechoic to fully reverberant, so in real rooms I'd guess about 3dB. The additional clarity can certainly reveal distortion and reflect port chuffing though.

 

[dhai]

Yes, about 4 times the power is what it seems to take - so closer to 6 than 3 dB.
One recent experience, with absorbtion down to the lowest octave:

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Even if it is based on abfusion - not purely absorbtive ! - with some residual reflexions,
the speakers used sound a lot more powerful in regular "hifi" rooms of similar size and
listening distance.

 

[Sonnie Parker]

"Some like diffused sound, thus ETC spikes will be higher,"
Sonnie, with all respect - diffusion will "diffuse" the ETC spikes,
meaning they get smaller, and nested within more of same.

Cheers,
David

Maybe so, perhaps you can't hear them as well... but they certainly "measure" higher with REW.

This is without moving the mic... simply moved in the absorption panels to cover the diffusion panels.

 

[dhai]

Maybe so, perhaps you can't hear them as well... but they certainly "measure" higher with REW.

This is without moving the mic... simply moved in the absorption panels to cover the diffusion panels.

Sonnie, I got your point after DanDan's remark: comparing diffusion with absorbtion, not diffusion with
specular reflection.
And of course there will remain more energy in the ETC. But good combinations of absorbtion plus diffusion
(I call it abfusion) will attenuate and diffuse this energy over time. Your examples are simply not good enough.
Admittedly, this is not easy to achieve, and, to fill in the rest of the timeline, you'd might want more diffusion
in your room.

Have you tried abfusors? If yes, what do you think about the principle?

 

[CinemaDK]

I tried both a 2 inch gap and flush. For my room flush was better, this video breaks it down a bit more:

Hmm, I had searched high and low for some actual experience with the gab then you share this video :-) just the kind of experience I was searching for. Thanks for sharing.

 

[CinemaDK]

Do you have windows in your room?

I have a large 1x2m window behind the speakers/screen yes, and I also have a 2x2m closet behind the listening position (LP) with glas doors. When I did my testing i slide the right panel behind the left, limiting the glas to 2x1m
I am still considering whether to have blanket in front of them or perhaps mount some light weight absorption/diffusion on the other.

Floor is carpeted, and I have sound insulated ceiling.

 

[CinemaDK]

To give you a very straightforward answer, the additional airgap behind extends the absorber’s ability to absorb lower frequencies, so you’d notice a small improvement (decrease) in your RT60 times below 200Hz with the air gap.

With your speaker being so near the side wall I’d expect an improvement in the sound with the panel in place.
By turning the panel around you are also changing the timing of first reflection from the panel, since you are changing the distance.

The video shared by doctor speakers confirmed all the more theoretical stuff I read, and gave me that final piece of the puzzle regarding the airgab. So I am happy with my 2:1 ratio now that the panels are already built.

another thing to keep an eye on is the room response with respect to frequency as linear as you can, to maintain a balanced sound.

REW does the RT60 times for you , and waterfall charts to see decay with respect to frequency.
Enjoy your cinema !

I am just in the early phases of learning to interpret and understand the measurements and different graphs - it turned out to be more time consuming and complex area than initially planned :-) Also what I thought was a more exact science, turned out to be full interpretation and personal experience, bias and taste :-)

The answers here have helped confirm that I now understand the basics of the ETC, and I am happy for answers and help I am getting here.


I have a basic understanding of what the RT60 is used for, but next step is understanding the different values (Topt, EDT, T20/T30, TS).

Same goes for the waterfall, but perhaps now that i have 4 absorption panels, I can move them around and run some tests, and I will try to google for other peoples graphs to better understand it.

Looking at the ones from this measurements I am not seeing much different in either of them, but as I still do not know what I am looking for, that probably explains it

baby steps ...