Acoustical Treatments Master Thread - Page 52

Based on my own tests, I simply do not believe this. I can EASILY point to a sub that's below 80 Hz (and even below 60 Hz). Get near 20Hz, then it becomes hard, but that's because I can no longer hear that frequency anyway.

Martin,

> just did a quick check at the beginning of each track. <<br />
The whole point of that "CD" is to test every frequency. Otherwise you're missing the true extent of the peaks and nulls. I know it takes a while to run all the tones, but if you do that you'll be surprised (and likely disappointed) by how much more deviation you really have.

Look at the graph below, which was measured in a typical 16 by 10 foot room at 1 Hz intervals. As you can see, the true response varies enormously over very small spans.

--Ethan

That's quite impressive. While many studies have shown humans are incapable of localizing anything below 100hz, there are always people on the edge of the bell curves who are capable of amazing things.

Guys,

Often you can localize what seems like low frequencies by buzzes and rattles from nearby furnishings, and by distortion components that are well above the sub's crossover but still are emitted by the sub.

--Ethan

[small voice]Maybe his sub rattles or has port noise.[/small voice]

edit: Sheesh, I should hit F5 more often. Ethan already covered this.

ummmm yeahhhhh Ok

Would you please explain the principles of using a riser as a bass trap? I'd like to understand how it is done. Is it more trouble than it's worth? Or is it just not a good place to make a bass trap.

Also, you said if I had buttkickers it probably couldn't be done. I'd like to understand why.

Sorry about the noob questions.

Well, when it comes to this, I'm a noob as well. But here's my explanation. For bass traps, one can use gobs (a highly precise and technical term) of fiberglass or a resonance absorber, a bass trap designed so that its mass has a resonance at the frequency (or band) that needs to be "trapped" (absorbed). The trouble frequencies strike it and excite it, and the bass energy at those frequencies is removed from the room and converted into motion. If your riser is being driven by a buttlicker, it will have its own agenda and won't be free to absorb energy.

Theoretically, a riser could be used as a trap, but the resonance would change as different people sat on it. To me, that would seem to remove it as a viable trap.

Check my link and go to the riser design & construction. I built it with adding Buttkickers (or Clark Synthesis) in mind, but the darn thing has a fortuitous vibration that seems to be centered at 25Hz. So, when something happening in the soundtrack around that frequency - a quite common occurance in LFE - the riser vibrates. Is it trapping those frequencies? Of course. But that's not what it's for. When the monsters stomp, my audience feels it. With the first movie I played after completing it - Jurassic Park - I realized I needed no Buttkickers.

I'm sure someone (on this thread!) could design a riser to be a trap or just vibrate at a certain frequency (band) for a passive buttkicer-effect. Me? I got lucky.

Pepar:


Think helmholtz.

Doh! Of course. It's called a "resonator" and not a "resonance absorber."

Two styles:
a) ported helmholtz -- with port in tri-corner (all modes active in tri-corners)
b) several 1/8" x ?foot long slots (cut with circular saw).

I have a 13' x 21' x8' dedcated theater. My acoustic treatment will be soffit absorbers plus 4" oc705 bass traps in the front corners. The rear wall opens at 50" high into a larger room.

I think that I want to coat the front wall with 1" oc703 spaced 2" off the wall. If I do this, should I frame each 2x4 piece and cover it with gom cloth? Should I wrap the poly bolster across the frame and into the space between boxes? OR, should I just mount the oc703 to the wall and tuft the gom at the corners of the insulation.

Can someone post a url to an example of a treated front wall.

Loking at this, I will have most of the oc703 behind the screen. I intend to mount the screen 6" off the wall with lightrope for illumination behind it.

I will also place one 2x4 panel at the sidewall reflection points.

Does this make sense? The room sounds reasonable now and has a large leather sofa 9' from the screen (92") I took car to space the speakers 3' from the side walls and the sub is 3' x 5' from the corner.

A little more info on the room.

Not to perfect scale.

The soffit would be 16" x 12 of oc703 along front and side walls.

dipole klipsch speakers are aligned with sides of sofa

92" screen
Any thoughts?

 

down sourh studio.pdf 24.5283203125k . file

Check the theater construction at my link. My room is 13x21x8. And I have a 92" wide screen. I took a different route with a false wall.

I remember devouring members' websites when I was planning my theater whether theirs was what I wanted to have or not. Hopefully, you'll see something you like (or don't like) and that will help.

Thanks
I hadn't considered the false wall approach that you show. It looks like it may cut down on the construction time. However, it means that I will be doing more work inside the house.

Why did you select the 1" material over 2" oc703?

Did you find that the room was too dead when you finished?

The cavity behind the wall is lined with 2" Johns-Manville Linacoustic. The first reflection point absorbers are made from 2" Owens Corning SelectSound Black. I have 2" 703 from which I am going to make Studiotips SuperChunks. Info on them and a WHOLE lotta data can be found here.

The room doesn't seem too dead, but I plan on taking some measurements when I start the trap project.

Thanks pepar and BasementBob. I think I'll forget trying to use the riser as a base trap. The two front corners are available as well as some soffit wall areas and floor wall areas.

I see that some people have their corner base traps starting on top of lets say a 7 inch high stage. When the base travels across the floor to into the corners does it hit the stage front which is usually wood/carpet and deflect? Newbie question...

That's not quite how bass frequencies "travel", but to keep it simple, they wash around something like a 7" stage like a huge wave would.

The size of the waves you're talking about are huge compared to a 7" stage. Also, you can't generally think of bass waves like rays. The higher you get, the more sound acts as you would think - hit something and bounce off. Bass radiates as a sphere and more of a wavefront.

Objects significantly smaller than the wavelength are ignored by the wave.
Objects significantly large than the wavelength cause the wave to bounce off.
http://www.bobgolds.com/Diffusion/Di...nFrequency.GIF

And in between?

pepar:

See the GIF link. Figure 4.2b.

Variations on this happen at the edges of free standing speakers (some diffusion in phase, some diffusion out of phase).

If the wave is even double the size of the object, it will pretty much ignore it. For a 7" height, that's up into the lower midrange.

Bryan

Took the better part of two weekend, but I've finished two bass traps. After taking measurements (hopefully before summers out) I will decide if I need more bass traps. I did buy an extra box of insulation, so I'll just need the wood to build more.

I used my extra OC Acoustic Board {1" thick} on both sides to sandwich in the 4" of Knauff 6lb/ft insulation. Below is a picture of one, but the two are identical.

Nice! How about a side view?

I am getting ready to build two bass traps for the front corners of my theater.
The extremely unprofessional forum thread on bass traps installation (now closed) raised many questions that I hadn't considered.

Experts - please clear this up possibly a moderated thread is in order

I want to build corner traps that have two layers of unfaced oc705. The top bottom and sides will be made from 3/4" mdf. I am treating the trap as if it were a pressurized vessel. That is, the seams will all be caulked and care will be made to ensure that the foam is held tightly against the box. This is all because of the "information" in the referenced thread.

I can not run the corner trap from the floor to the ceiling. It sits on a carpeted riser and has to terminate below the crown moulding.

I don't want to have to build this piece again! It would be much easier to frame out the drywall (single 1/2" layer) so that the oc705 would be held against the frame. I would then forget about the top and bottom also.

Based on current knowledge (the referenced thread insists that the bass traps must be held in proximate contact with the walls and the top and bottom must be in place to maintain the spring constant (at maximum) How does this type of trap really need to be fabricated??? Please don't refer to existing threads. I would like to get some yes/no information from ethan/bpape etc. No catfighting guys.

I don't care about sabines etc., what is your opinion about how the corner bass trap (not superchunk) should be fabricated!

Why was the raytracing method shot down out of hand.

zductive:

You could click on any of the links from "ChristianG's Corner Traps" on down at:
http://www.bobgolds.com/

Similarly on this page
http://www.bobgolds.com/AbsorptionCoefficients.htm
any of the links between "Jon Risch's Absorbers" and "David French's absorption cloud design with aluminum guttering material"
should give you construction ideas.

And there's also ---k--- 's
http://racarris.com/HT/Corner%20Trap/
(with some text about it at
AVS Thread: Upgrades, Treatments & fun w/ ETF )
I think he ended up making three short corner traps and was reasonably content that he had done so. The reason I reference this thread, even though you asked not to, is you wrote
and ---k--- both did that, and posted a bunch of good observations both human and ETF5 graphed.


In an otherwise untreated room, I believe corner traps are an excellent LF bang for the buck/effort. All modes are active in tri-corners, so that's a good spot to put an absorber. (there are other alternatives to corner traps, but I'm unsure how to discuss them without using the temporarily evil 'Sabin' word.)


Don't go gung ho on the sealing. If it has to be portable then simply glue and screw the wood (skip the acoustic caulk).


Sounds a bit like the Dave Portocarrero corner trap at http://www.bobgolds.com/TrapDave/home.htm (bottom two pictures).


Don't bother going gung ho sealing the edges. Just keep the absorber touching the sides if you can. If there's a 1/4" gap some places, don't worry about it. (A 2" hole is just being lazy - fix it).


And of course, last but not least
Studiotips Corner Absorber

Anyway, building a corner trap is not rocket science. The main points are:
a) put 4" of rigid absorption on 2' diagonal as tall as you can and in as many corners as you can and touching the drywall.
b) Cover it with something fire retardant that'll keep the fibers in place when the wind blows on them. If your room has lots of HF absorption already consider something HF reflective, otherwise something you can breath through.
c) any framing you might do is simply to hold (a) and (b) in place.

If you can make observations of what you hear that's different, that's great.
We want pictures!

Thank you Bob for a very reasoned response.

It appears that there is a controversy about the need for the corner panel to come in contact with the corner walls. The discussion is confusing but, the safe way seems to have the glass touching the walls.

I con't think that my original idea of 3/4" mdf on the sides is required even though, it would make the corner reflector stiffer and less transparent to lower frequencies.

The compilation of various traps is great.

I will proceed with caution. Hopefully, I can make a few measurements from my listening position before I continue.

Biggest problem is integrating the corner trap into the remainder of the wall structure. I intend to have 2" oc703 spaced 2" from the walls. Since the corner trap is laid back into the corner, I guess that the best approach would be to have the oc703 run up to the point where it intersects the corner trap.


The problem that I am trying to address is the mismatch between speaker sound when I go through the alignment process. Each speaker sounds different. I believe that it is the result of the differences in high frequency reflections in the room. Time for sidewall reflection baffles!

Thanks for comments!

zductive:


(I'm not a fan of horizontal centers -- I don't care if the drivers are matched -- I prefer the fronts to be identical right down to the model number.)
In my basement I have three identical fronts that are good off axis, and four identical surrounds. There's a scene in Mission To Mars chapter 11, where the mission commander's voice goes through each of the 4 surround speakers one at a time in a seamless arc, as well as the three front speakers, demonstrating that all 7 speakers get individual signals with Pro Logic IIx -- and there's a big timbre mismatch between my fronts and my surrounds.
For my smaller HT I'm thinking of buying 5 identical speakers (two monopole surrounds).

Wall/floor/ceiling reflections (and SBIR) can make speakers sound different (because the reflections don't reflect all the frequencies evenly resulting in colouration) -- but I'm wondering if there's something different about your speakers themselves, or perhaps a blown driver.