SDAs and Acoustics

PolksTillIDie

A few days ago I was boxing up some things in my listening-/livingroom and noticed some sound improvement in my setup that seemed to be related to five flattened U-haul boxes I happened to have behind my listening position. Remembering that, today I raised those boxes behind me till they were about a foot above my head and extended to about a foot beyond my head at the listening position to try to enhance the effect.

Holy crap.

The bass has become massively punchier and defined. The soundstage, which was already about 16' wide, gained MUCH more depth and definition.

What's going on here? Relative to the softly dimpled painted sheetrock of the wall, these boxes must be reflective.

Gardenstater

I'd say the opposite and corrugated cardboard should be more absorptive than painted sheetrock.

PolksTillIDie

Now that I think about it, I think you're right. Its absorbing bass that would otherwise be reflected off the back wall. High frequencies are more pronounced too and I was trying to account for that also. But maybe that's just due to undoing the masking effect of the lower frequencies?

Anyway, thanks. Acoustics can be fun.

Gardenstater

Acoustics are fun and pretty heady. I don't think that cardboard was absorbing much bass and the effects you got in that area were probably mostly due to it changing the dimensions of the room effectively. In my room there is a spot I can put my chair which is close to a corner where I love the way the bass sounds and it is no doubt a bass concentration effect.

Got me interested because I'm a nerd and I like to look stuff up. Even triple wall cardboard (NPCC)
is only slightly absorptive with an NRC around 0.063. Walboard is around 0.05 but painted wallboard probably less. If you perforate the cardboard with a matrix of 1/8" holes spaced 1/2" (PCC) apart you can increase the NRC tremendously and the same goes for putting it parallel to the sound (NPCC Parallel Direction).

Your flattened boxes would have an airspace between the cardboard panels and would probably be better than NPCC.

https://www.mdpi.com/2071-1050/13/10/5546/pdf

audioluvr

Did your room look like this?

VR3

I find a panel behind me helps bass tremendously in my room

PolksTillIDie

Thanks for all the replies.

Gardenstater, thanks for the study info. Just scanning the study, it looks like they used single "sheets" of three layer cardboard. My 9 boxes have 9 double layers with some airspace between them, they're 3 in thick when laying flat, but they naturally fan out to about 6 in when leaning against the wall (the way they're positioned now; the position that gave the effect). I can certainly A/B the effect to be sure, and REW and my Umike are just sitting there ...

Audioluvr, is that pic from the Van Cliburn in Ft. Worth I just heard about yesterday?

VR3, thanks for the tip.

Whatever it is, it's no small effect. If I can't find its room absorber equivalent, my livingroom's gonna have that "about-to-move-out" look for awhile.

PolksTillIDie

Did some A/B listening tests and the effect is still there, though when I did some A/B tests with REW, I got some unexpected results. Lemme look at these for a few days.

Stay tuned.

Gardenstater

A good thing to do with A/Bing, at least with my treatments and my room, is to pick some music with more than one vocalist and especially where on is much lower level and see how decipherable and separable they are. Like for example Chris Isaac Wicked Game or Peter Gabriel Mercy Street. That's a great test that isn't going to ever show up on any freq. resp. plot.

VR3

If you want a great track for imaging and such...

Marinas trench - the killing kind

Super complex track with extreme vocal ranges

machone

VR3 wrote: »

I find a panel behind me helps bass tremendously in my room

VR3,
Interesting.
How far behind you?

VR3

I sit about three ft from the back wall and I put the panel right up on the back wall horizontally at head height

PolksTillIDie

Thanks guys for the reference music.

But my issue is the opposite: I can hear this effect in nearly all music. I'm trying to figure out what this cardboard is doing so I can replicate it with something more ... aesthetically pleasing. My default listening position is unfortunately about 1 foot from the back wall. All the favorable sonic changes are at this position and forward until I get to the edge of the equilateral triangle. Its there at the triangle's edge that I get the best imaging, although its improved back at 1 foot from the back wall also.

The cardboard "unit" is about 3 feet square and is about a foot off the floor.

I did a little analyzing with REW and UMIK-1. First I measured spectra with the mic pointed at the wall/cardboard 1 foot away from both the bare wall (green) and then the cardboard leaning against the wall (blue).



I thought/hoped the cardboard had pulled down a nasty room mode I have at about 70 Hz, but the spectra are identical below 100 Hz. But at 10 kHz and higher, it looks like the cardboard is absorbing or scattering this upper range. And since the cardboard is smooth, I assume it is absorbing this frequency range.

We can get to the middle range later, but what do you guys think so far?

VR3

I mean a cardboard box is just a bass trap basically

Gardenstater

Do you listen with only one ear pointed at the cardboard fan shaped absorber or do you listen facing towards your speakers with both ears? I guess what I'm getting at is that I think you should face the mic towards the speakers not the cardboard, especially since it may be acting as more of a diffuser than an absorber.

PolksTillIDie

Yes, I have those graphs too. I was trying to avoid barraging you guys with too many graphs all at once. The point was trying to tell what the cardboard was doing, if anything, ya see ...

Gardenstater

Yep I get that. This might not relate perfectly to your own situation but in my office/listening room I have the speakers up on stands and pulled well into the room over 4 ft from the wall behind them which is half of an octagon, which is said to be a superior shape to a rectangle. Anyway, I never liked the way it sounded when I was in the equilateral sweet spot. The clarity was never there. If I listened off axis near a corner of the room everything was great. If I listened back at the wall as far away from speakers it was good too. In the sweet spot the clarity of vocals and airiness was lacking.

I realized that when I'm off axis and further back I am not getting hit so much with the reflections off of the wall which are out of phase and time delayed to the primary sound wave. Putting open cell foam on that wall has proven this and sweet spot is much better. I have a large piece of mattress foam around3ft x 6 ft taped (wide blue tape stapled to the foam) with a belly in it for air space and the spikey side facing the wall. That has proven the point because subtle vocal separation is so much better in the sweet spot now. It needs to be even thicker.

None of my room treatments are what you would call WAF friendly at all at this point but I can clean them up with more professional looking ones in the future. Work in process.

PolksTillIDie

Yes, I have those graphs too. I was trying to avoid barraging you guys with too many graphs all at once. The point was trying to tell what the cardboard was doing, ya see ...

PolksTillIDie

Yes, I have those graphs too. I was trying to avoid barraging you guys with too many graphs all at once. The point was trying to tell what the cardboard was doing, if anything, ya see ...

PolksTillIDie

Yes, I have those graphs too. I was trying to avoid barraging you guys with too many graphs all at once. The point was trying to tell what the cardboard was doing, if anything, ya see ...

audioluvr

The boxes act as a diffuser as @Gardenstater suggested. That pic I posted was an architect example for a music hall I believe for USC. Basically a square room makes for the worst acoustics.

PolksTillIDie

So, to start, sorry for the multiple copies of my last post. I had hit the "Post Comment" two or three times before I saw the tiny, faint text box saying my post was pending "approval". Apparently whoever approved it thought all of you needed all four copies of the same post.

Anyway, yeah a little mystified by the graph. It appears to me to be doing just the opposite to what I'd expected. I assumed it was absorbing excess bass and maybe reflecting back high frequencies. The graph appears to show the cardboard leaving bass untouched while absorbing highs. I say absorbing instead of diffusing because I'd think dimpled wall board would diffuse more that slick cardboard.

Gardenstater

One thing you can be sure of is that it's doing a combination of reflecting and absorbing but mostly reflecting. If you look at NRC charts of sound absorption materials vs frequency, they always absorb much less at lower frequencies, especially for thin materials. Your cardboard probably is absorbing the most at high frequencies.

What is the shape of your flattened boxes? I thought you said it formed a fan shape, which is why I said it was probably acting mostly as a diffuser. It's like you're adding a small partial wall that is closer to your ears than the original wall and has angular features. When sound reflects it can exhibit constructive and destructive interferences to the original sound wave depending upon the location of the ear and the frequencies. Those sum up in a way for the ear that can interfere with clarity but they can make the sound seem louder or softer. You can't look at a spectral graph and see the distortion, only the resulting amplitude which is why I was recommending listening tests for specific songs.

You could also get a phase change when a small part of the sound goes from the harder acoustic impedance of the cardboard to the air spaces inside and back out into the room.

Just my current understanding. I'm still learning about acoustics and it is obviously pretty hard to nail down. Anyway, have fun experimenting.

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/reflec.html

PolksTillIDie

Yep, I agree that its probably both reflecting/diffusing and absorbing. The fan shape I was referring to was how the flattened boxes appear at the top looking at them from the side. They're just stacked together as a unit and leaned up against the wall, so at the top they fan out a little. That said and just now thinking about it, they *are* tilted back about four inches or so and I suppose this might have an effect as well.

The effect is least at my "practical" listening position 1 foot from the back wall and best (with greatly enhanced imaging/soundstage) forward about 3 feet to the apex of the equilateral triangle, my optimal LP.

So how about another graph?



So in this one the mic is pointed up in omni-directional mode and at the "practical" LP (one foot from back wall). With this one the traces are identical below 120 Hz and above 1.8 kHz. All the significant differences are between these two ranges, small, and probably not responsible for what I'm hearing.

Some phase shift might be what's going on. I just haven't done enough reading on the subject to know how to analyze that yet. But soon, new furniture is going to arrive and I want to understand what I stumbled into so I can reproduce it with more conventional means.