# Building super-chunk trap, is 703 good enough?



## 91BlckGT

Hi, 

I just bought a few bundles of Roxul AFB 2.5 density fiberglass to build a super chunk trap in the 1' x 2' triangles. In order to put a finished, rigid front on it, I plan on putting a sheet on the 2' triangle face. So the ultimate layering would be a 1'x2' triangle, a layer of polybat and then a layer of rigid fiberglass wrapped in burlap.

Considering I'll already have nearly 16" of roxul behind the rigid glass, would the 705 2" stuff actually be any value of the cheaper 2" 703? Both are unfaced (I believe). I just can't imagine that it would have a measurable difference.


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## bpape

Stay with the 703 and save the money. With a solid slab behind, 705 is a waste of money.

Bryan


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## 91BlckGT

Hmm, if it makes little difference, would using Roxul Rockboard be just as good? I just need something rigid for the front facement.


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## 91BlckGT

Sorry to reply back to my own thread, but I went and re-read through Ethan's site again. 

I guess my plan to put a sheet of rockboard in front of the superchunck traps in the corner, wouldn't perform nearly as well as taking 4 sheets and placing them at the ceiling to wall boundries, will it? I've got REW so I'll take a few before / after waterfall plots to show you guys the different, let me know which one is better.

I've already filled my entire drop ceiling with 8" R32 fluffy stuff, this is just sort of icing on the cake, if you would. 

Here are some pics I just uploaded of the theater:









This first picture shows where the 7.5 ' traps will go, in each corner. The SVS sub on the left (behind the wine-colored curtain) is along the wall, so it will sit pushed up against the trap. That is a full-length brick fireplace behind the curtain, so it does nasty things to the acoustics in the room.










The second picture is a rear view I snapped off, showing the challenges I face in the rear. The room is approx 25' by 13' at the front, and 15' at the rear. ON the right is Office #1, and I have very little room (only 2') to put a mini-superchunk trap. On the left is the bar, and behind it is a fridge, so I'll have only 63" of vertical space for the 4th trap. I might move the fridge though, just no other location for it.


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## bpape

IMO you don't need the sheet across the front if it's framed correctly. You'll never notice any irregularities if the cloth is held off even 1/8". 

Now as far as doing the wall/ceiling corners instead of these others, why would that be better? Are you saying because it's covering more surface area? If so, then that could be good or bad. If its thinner, then you're just getting more mid/high which you may not need. 

I'd want to know the balance of decay times in the room before making a decision like that.

Bryan


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## 91BlckGT

Attached is my low frequency response as measured with REW (all things calibrated at 75dB). This was done with just the SVS running. I had my 80hz xover turned on.

Also attached is the LF_waterfall plot at 300 and 400ms. I'm fairly happy with my response curve right now (it's nearly flat to the 12hz subsonic filter of the SVS, and then it's -3dB down point is 10hz.)

If you need to see other gating (i don't really understand what I'm seeing) I can put other things in there.


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## 91BlckGT

You know, looking at those graphs it doesn't look so bad. However, I think *this* graph will explain what I'm trying to tame. *ick*. It's room mode madness. :doh:

Just moving my listening position a few inches results in wildly different response. I'm sure part of my issues is my screen, which is 3/4" melanin, paired with the bipolar nature of the towers (2 midranges firing both forward and back gives me 10 mids and 5 tweeters in the front sound stage) so things are a bit blurry.

I've considered treating the front wall, but what's the point? After putting the bass traps up, there will be 1 foot of hard surfance on three sides of the screen (top & sides) and 3 feet on the bottom. The screen isn't acoustically transparent, so I can't trap behind it, and even if I did, that defeats the whole purpose of a bi-polar. Granted, stereo music sounds *great* on them, and the sound stage sounds, at least to my ill-trained ears, expansive. But I still struggle to hear dialog at my lower listening levels (i usually watch movies at ~65dB)


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## 91BlckGT

Just a quick update, here are two photos I snapped of the two "almost done" traps installed in the rear of the theater. I had to make due with the limited space back there. Each trap is wrapped in a single sheet of polyfill. Keeps the highs from bouncing off the wood framing. 

I ended up using 1x3" firring strips, with a 3/4" triangle of plywood at the bottom, and a 5/8" triangle at the top. These two are strictly filled with straight 2.5 lb density roxul mineral wool. I ended up beveling the front edges of each of the firring strips to keep a straight face on them.

I have some black burlap to cover them both, just wanted to get all the traps done before I start gluing. 

I'll post some more pics tomorrow of the other two "full-height" traps tomorrow that will show the framing. In those, the top and bottom 16" are Roxul 60 Rockboard (6.0lb density), with the middle being 2.5.

Had I really thought about it, I would have gone 6.0 the whole way. The 2.5 stuff is too floppy and compresses down, so I ended up having to use more than I calculated. Whoops... :dunno:


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## bpape

And how is the sound? I suspect it's helping your bottom end tightness and dialog intelligibility.

Also, have you insulated above the drop ceiling? That'll give you some nice broadband bottom end control too.

Bryan


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## 91BlckGT

bpape said:


> And how is the sound? I suspect it's helping your bottom end tightness and dialog intelligibility.
> 
> Also, have you insulated above the drop ceiling? That'll give you some nice broadband bottom end control too.
> 
> Bryan


I didn't actually hear a difference with the rear traps in place. It might be because the waves are so bad in the room, that 95% of the bass is in the front of the room, and as you walk to the rear it drops off until it's non-existent. Weirdness. I'm hoping the front traps will make a difference. I'm going to do some before / after waterfall charts to measure the change, but I'm waiting until the traps are done so I can ensure that the volume and mic placement are identical.

2/3 of the drop ceiling is stuffed with 8" R32 fluffy fiberglass, starting at the front, and working back even to the hushbox. I ran out of money so I didn't go further.


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## bpape

Interesting that you didn't do the ceiling above in the back and that's where you're having issues.....

You may also find that nulls coming off the rear wall may be part of the issues. 

Bryan


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## 91BlckGT

Here are some more assembly pictures for the curious, and a few snaps of the finished rear traps installed. 

I learned an important lesson: Always use the rigid stuff for the full-height traps. I got by the lightweight mineral wool for the 2' and 5' trap, but the 7' traps had a great deal of compression in the middle from the weight of the rigid wool pushing down on them, so I ended up having to use *more* of it, not less. :gah:


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## 91BlckGT

It's done! Here are some the final graphs...

I can say that the bass certainly *seems* tighter. The graphs don't really show what I hear though. Maybe it's because I "want" to hear a difference. Once thing I did notice is that I still have a null in the middle of the room, just not as bad.

In the two frequency graphs, the blue line is the original untrapped response, and the green is the trapped response. Same goes for the LF waterfall. It would appear that blue has a slightly greater resonance, but that could be also accounted for the slightly less decibal output. I had to turn the volume up a bit (maybe -3dB) to get the graphcs that close.

I should mention that the SVS sub sits pressed up against the left front trap. Not sure if that's ideal or not, but the response is still quite good, so I think that's ok. The bass is certainly tight.


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## 91BlckGT

This next set of graphcs are my two main def techs running. The first picture is rather interesting. From 20-100 there was almost no difference in response, but in the low end (below 20hz) the trapped charts shows significant improvement in low frequency extension. In this case the blue is untrapped, the purple is trapped.

The second graph is again the mains, from 140 up. I don't know what to make of this. The graph is "different", but I can't say it's "better". The final two are the waterfall plots.


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## bpape

Getting there. What I see is still an issue with seating placement and sub placement. Get the sub out of the corner and play with the seating in relation to the length dimension.

Bryan


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## 91BlckGT

Here are some shots of the front traps, i've been neglecting to post those. Here you can see the sub tucked up next to the trap.

I used to have the sub in the middle of the room, between the two couches, and about 3' from the wall. This gave even sound, but there was no low frequency extension below 30hz, it just fell off steeply. Placed where it is, I get excellent low frequency extension, and the response is flat within 4dB, so I thought that was optimal. 

Would the tradeoff here be increased resonance? When placing a sub, should I calculate where in the room the modes occur, and place it there? So, if I have a null at 60hz, would placing the sub help?


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## bpape

Corner loading or wall loading will give more output but in general less smooth response. 

Fixing the null may be sub position related or it may be seating position related. Move the mic a bit with the sub where it is and see what moves. Try a bit left and right and then try a bit forward and back. That'll tell us more.

Bryan


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## trainCatcher

Any further updates on how the superchunk traps are working out? I was thinking about building some myself, but ended up building two 4in thick panels with OC703. I was not blown away by the difference in bass performance when I put them in the corners. The biggest improvement in sound ocurred when the traps were directly behind my speakers - probably due to blocking the first reflection. Bass didn't improve until I moved my sub out of the corner and placed it closer to my seating position


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## bpape

Treating corners is an overall fix. Treating behind the speakers deals with SBIR issues. 2 very different things. 

As usual, rules of thumb are great overall but there are always specifics to deal with. SBIR, nulls off the rear wall, seating position, sub(s) position, xover point, xover slope, phase adjustments etc. will all help with bass control in the room. Some are purely frequency related - some are decay time related - some are both.

When I see a room with that kind of good frequency response down low and good decay times but still struggling with dialog issues, it tells me it's positioning coupled with front wall treatment.

Bryan


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## 91BlckGT

trainCatcher said:


> Any further updates on how the superchunk traps are working out? I was thinking about building some myself, but ended up building two 4in thick panels with OC703. I was not blown away by the difference in bass performance when I put them in the corners. The biggest improvement in sound ocurred when the traps were directly behind my speakers - probably due to blocking the first reflection. Bass didn't improve until I moved my sub out of the corner and placed it closer to my seating position


To be honest, I suspect they are working out quite well! I added two first-order reflection traps on both sides of the listening area, and that's the only change I made. I could still use *more* traps I think, as the bass isn't as smooth as I'd like it to be as I slide across the sofa, but in the prime listening locations I'm happy with it.

Now if my wife would let me crank up the volume...:rant:


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