Unexpected sound absorption panel result (1 Viewer)

[Josh~H]

I'm almost done with my first DIY sound absorption panel -- I just have to attach the outer fabric. The method of absorption in my case is 7" of R13 insulation and a layer of poly quilt batting.

Anyways, with my 15"x5"x48" w/d/h panel nearly completed, I decided to set it up against the right-hand wall to tame the 1st order reflection from my main L speaker. When I did this, I was surprised to see that the dB level at the listening position actually increased by a couple of dB. I'm using the Radio Shack analog SLM. I can only guess that the 1st (and subsequent) reflections were actually cancelling out some of the direct waves from the speaker, and adding the panel limited this cancellation.

Can any of you acousticians help figure out what could be going on here?

 

[chris_everett]

You got rid of a standing wave! like you suspect, your sound was canceling itself out. Welcome to the wonderful world of accoustics. You probably have much more consistant sound in your HT now as well.

I'm not actually an acoustician, but I'd be willing to bet on that one

Sounds like a very nice panel.....errr, no pun intended

 

[Josh~H]

I'm not sure how much improvement there is just yet -- even subjectively. But if I can quantify the result with my SPL meter, I gain a little confidence. And a couple dB.

I bounced my hypothesis off an engineer at work. He spent a few years doing active control engineering in the field of noise cancellation, so he knows a thing or two about acoustics. Anyhow, after throwing a bunch of theory at me, much hand waving, and a few plots in Matlab, he grudgingly admitted that it was feasible for my crude DIY absorption panel to cut down on the reflective cancellation by a couple dB.

Now that I've had my first taste of apparent success in this type of project, I'm more inclined to continue builing a few more panels to achieve similar effects for the other 1st reflections in my room. Prior to seeing this first panel work out, I was about ready to quit...working with that fiberglass insulation is nasty business.

 

[chris_everett]

2dB on the radio shack meter is pretty significant. It's probably a gain in the lower octaves. Try any good "subwoofer demo" DVD and you will probably hear the results. Also, a scene where you found it difficult to understand dialog may show improvement. One of the good things about the naturally poor acoustics that most of us have in our HT's is that any treatment to the room, even if it's primitive and poorly places, will have a noticeable improvement.

 

[JoshuaR]

Just a quick question... I hear all this talk about poly batting. What is this and where can you get it?

 

[MikeWh]

Polyester batting is used as that thick filler in quilting. Batting is in sheet form. "Polyfill" is loose poly material, that is typically used as a replacement for down feathers in thicker quilts or as the stuffing for stuffed toys. Any fabric store will have either.
Goto: http://www.joannfabrics.com/
And do a search on "batting".

(I also use fabric store polyfill in my aquarium filter... cheaper than from a petstore.)

 

[Terry Montlick]

I'm not actually an acoustician, but I'd be willing to bet on that one

I'd be willing to take that bet!

I think Josh's guess that a reflection had been destructively interfering with direct sound waves is more likely true (especially considering the modest size of the absorber) than that his absorber is now taming a standing wave. It's hard to know for sure without more data, such as room geometry, measured impulse response, etc.

In any case, adding proper absorption can dramatically improve HT sound quality. It's probably the single most important thing you can do for your home theater sound.

Regards,
Terry