need advice on acoustical treatments - Page 3

Terry, two earlier threads, same/similar subject:

http://www.avsforum.com/avs-vb/showt...424&highlight=

Followed by: http://www.avsforum.com/avs-vb/showt...7#post19729287

/Ken

That statement is completely uncalled for. I know the individuals involved in that research. I can assure you their interest was, and still is, focused entirely on improving sound reproduction and understanding the factors influencing that quality. The fact that Harman, and Harman's products benefited from that research is not relevant. It is published research ... all of us benefit from that body of work. In a more general sense, the fact a return on investment can possibly be achieved by funding pure research does not invalidate, or reduce the value of, that research.

Further, to suggest a body of work, 20 years earlier, is not relevant simply due to the passage of time is silly. It is not the passage of time which would make earlier work invalid.

you're absolutely right.

[quote=jfeva0049;20630939]hey guys, so i finally got my new speakers up and running and finished my bass traps. what do you guys think of the bass traps? i dont mind criticism.





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lookin' good. Nice to see that they extend all the way to the ceiling. Have you an air space behind them? Why did you decide on a 12" trap width and not wider as it looks like you have the room for wider traps?

More importantly, has it changed the way your music/HT sounds?

What's next on your DIY agenda?

Looks like a very serious good start.

You are not necessarily done with room acoustics.

You might be ready for a more advanced tool like Room Eq Wizard.

what material, thickness, etc did you decide on?

cool! you really need thick traps (or with large air gap) and lots of surface coverage for LF modal sisues - but glad you at least noticed a difference. with decreased modal ringing (eg LF decay times), the bass won't sound so muddy as the energy quickly dissipates (within reason based on the amount of trapping). so when you have multiple thumps of bass, they will be distinct vs all running together in a sloppy mess (like you have noticed).

if possible, do the ceiling corners as well. coverage is key (along with thickness). you have real estate constraints so only so much you can do regarding thickness (which will really effect how low the trapping will be effective, regarding frequency), but better than nothing!

is there a reason why you couldn't do thicker/wider traps in the front? it seems like you have plenty of space and it will be much more effective.
for an experiment, take the rear two traps you built and add them to the front traps (space them side-by-side as if they were a single, wider trap). see if you notice any difference there, and if so, make wider/thicker traps for the front corners.

for the side panels (for specular reflections - NOT LF modal issues), maintain the entire 24"x48" panel area and id recommend 4" OC703/equivilant and space off the wall 2-4" if possible. if you cannot use so much material, try 2" thick but spaced 4" from the wall. spacing from the wall will be almost as effective just as if you had that entire space filled with material - it's almost like getting a free lunch.

the material we are using is a porous (velocity-based) absorber.
a sound wave is forced through the insulation and energy is converted into heat (which reduces the amplitude, which reduces the energy in the wave as it reflects off the boundary and back into the room). less energy reflected back into the room, less interference it has on the original signal and other reflections/room modes/etc.

therefore, to be most effective at absorbing energy, it needs to be placed at areas of high particle velocity in order for maximum absorption. a sound wave has max particle velocity at 1/4wavelegnth. as a wave approachs a boundary, it compresses (builds pressure) and velocity goes to zero. velocity then = 0 as pressure = maximum --- and the wave reflects off the wall and travels back in the opposite direction. think about a pool/billards table. as a ball is approaching the rail, velocity slows (and pressure builds) until the ball comes to a complete stop (pressure=max and velocity =0) for a very brief moment, and then reverses direction (bounces off the rail).

so, right at the wall/boundary, pressure is max and velocity is zero. that is why porous (velocity-based) bass traps are not effective when placed right against the wall. they need to be placed at areas of high particle velocity. now, for LF waves this is difficult because the wavelengths are so long. this is why the bigger the air gap, the more effective the absorption (as you space the insulation away from the boundary, you're moving it closer to a particular 1/4wavelength point where velocity is higher, and thus the absorber is more effective.

however, for specular energy (eg side-wall early reflections), the wavelengths are much shorter than LF bass waves. for higher frequencies, where wavelength is so short, the 1/4wavelength is usually within the thickness of the panel. but for example 700hz has a wavelength of 19.37". the 1/4wavelength of this (19.37/4) is 4.84". so in order for the absorber to be most effective, it needs to be spaced 4.84" from the wall.

treatments cannot just be blindly installed. they need to be placed at areas of high effectiveness. by adding an air-gap, you're spacing the insulation away from the boundary to areas of higher particle velocity for the lower specular frequencies. and the air-gap is free!

now, there are also pressure-based bass traps, and those work by being placed at areas of high pressure (vs high velocity) - like right up against the wall. however, these are quite difficult to build and have limited effective freq. range. the porous velocity-based bass traps are broadband (absorb a wide range of frequencies), and thus are quite effective for relatively little money/work.


for the corner bass traps (for LF modal issues) - the thicker and the more sq. area you attack the better. there is much more energy in the LF waves so it is more difficult to attenuate.

Localhost127 speaks truth. The way you hide the extra depth is to build a frame with sloped sides. Or, you just go all across the wall and nobody sees the depth.

are you referencing corner bass traps or side-wall panels for specular reflections?

for the side-wall panels, if you build a frame on the back of the insulation, that frame provides the air-gap:
here's some old pics:
http://www.avsforum.com/avs-vb/showt...6#post20271296
you can then screw eye-bolts into the frame, add some frame wire, and mount like you would a picture frame. the panel still sits "flush" with the wall (you dont see the air-gap)

4" oc703, 1.5"-2" air-gap (eg whatever the wood is you use for the frame).
enjoy!

also - check this thread as some others on the forum have had pretty good success with the printed (acoustical transparent) fabric.
http://www.avsforum.com/avs-vb/showthread.php?t=1316623
find the goalie's favorite painting/etc. print/order. apply fabric. mount panel. GOAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAL.

(don't recall the whole thread, it may have been covered)

Time for the next step? Measurements?

find the three thumps. The you can at least know what you are targeting, and how that targeting is going.

If you are already measuring, then scrap the thought ok?

I used little foam blocks behind my panels to provide a 2" air gap. You can hardly tell they're there...

I can see nothing has changed in the interim. Dunning-Kruger has it's price.
Ah well.

cheers,

AJ

apparently the sarcasm was lost on you in those questions in that post of mine,
much to the same tone that im asking you --- "so shall i move my speakers to the closet? the bathroom"
carry on, you're commentary (or lack there-of) is proving my point for me.
have anymore bookmarks to share with the class?

Yeah: Loudspeakers and Rooms for Sound ReproductionA Scientific Review

Read it and weep. Then have someone comprehend it for you.
(Hint) It's not about subwoofers, but they will explain that as well.

cheers,

AJ

let me rephrase the question - any more subjective bookmarks you have to share with the class?

Absolutely. Here ya go kids:
Perception
Subjective.


Now there's a big word for you studio guys. Can you even type that on the iphone?
Enjoy your reading class. No promises on the comprehension.
Any more RPG ads to place?

cheers,

AJ

AJ,
how about this. why dont we take a sample-test of the users on this forum to see who actually installed broadband absorption panels to attenuate early reflections, only to then remove them because they didn't like the way the small acoustical space sounded after that.

i really do not have any trouble finding threads or users doing such and commenting on the positive effects. could you please post threads or quotes of people removing treatments because they did not like the perceived difference? in volume? let's put this into practice with real people and real rooms; since measurements alone are not good enough for you.

-deleted

That sounds very scientific. It would definitely trump all the double blinding testing and data accumulated by Toole and the other 50 or so guys over the last 20+ yrs.
Sheesh, why Toole didn't think about sample testing AVS forum members instead, we might never know. Maybe because they already own a bunch of subs?



I'm sure you would have no trouble finding members who had positive effects from swapping cables either. You can find whatever you seek if you really want to.
I have a better idea. Go to one of these shows sometime. Try to find the "treatments", "traps", gauze/bandages etc. in these rooms and then try to figure out why those deaf guys at Stereophile decided to shill a brand with zero advertising in their mag. Seems a bit odd eh? Probably as scientific as your suggestion. But at least it involves getting out of that (padded) closet for some fresh, non-studio air.

cheers,

AJ

btw, any perceptual data to counter Tooles sub ad? No? Not yet?

A nice usb/mic preamp is <$100 and a mic <$40.
Pretty sure you already have a computer.
Plenty free measurement software out there. ARTA, REW, Holm, etc.
You could even cheap out and use the calibration mic that came with your HT receiver if it had one, using your computers soundcard. $0

cheers,

AJ

The Audyssey tower mics, at least, are surprisingly good. Lab grade, perhaps not. But for hobbyist use, it's hard to recommend spending money on mic preamps and calibrated mics, when these are so free (or cheap).