measuring bass trap effectiveness

[Nate]

First a little background... I recently won seven 4'x2' bass traps in a raffle. I need to decide how many, if any, I am going to keep. Those I don't keep I'll give to a guy who needs them (and will get me a good price on a 1080p projector in the future). My room is small, 10.5'Wx18'Lx8'H. More on my room here. Anyway, here are some possible locations for the traps, though I'm not sure how I feel about how they look against the striped grey color scheme...



I just stumbled upon REW and I set it up tonight with my RS SPL meter (and cal file). I don't have a subwoofer yet, but my AV123 MFW-15 will be shipping this month. For now it is just the Rockets: RS750 LR, RSC200 center, and RSS300 surrounds (all AV123). Here are some graphs (sorry they are so tall)...

BLUE is with zero traps.
PURPLE is with six traps.





So the frequency response was hardly affected. I was confused about this until I did some digging on these forums. Turns out traps affect the FR little under 60-80hz (explained here). Instead, they reduce the decay which makes the bass tighter. So here are the appropriate graphs...

BLUE is with zero traps.
RED is with three traps.
PURPLE is with six traps.







Like a true scientist, I spent so much time fretting over measurements that I forgot to do some subjective listening. This will happen first thing tomorrow (it is past 3am already!).

Now for some questions! Thanks for making it thus far BTW.
1) Do you think these traps make a significant difference in SQ?
2) Remember all the graphs above are without a sub. Do you think I would see a larger difference when my sub is installed?
3) How are my trap locations? Does putting bass traps at the first reflection points help with high frequencies?
Somewhat related questions...
4) Do I want to buy a DSP1124P or FBQ2496?
5) I have read the RS SPL meter is ok for EQing a sub. Would I gain much with an ECM8000 mic?

Thanks for your help/input/etc!

[brucek]

Sorry Nate, your pics don't load - can't see them ........

[Otto]

Hmmm. No problem loading pics here.

[Nate]

Weird. Maybe my domain was down for a second. Seems ok now!

[brucek]

No joy here - guess someone else will have to comment.....

This is what I see , for example....

link.jpg


brucek

[Otto]

OK, I'll offer a couple comments.

First, let me welcome you to the Shack, Nate.

1) Do you think these traps make a significant difference in SQ?

Not sure. Foolishly, I've never played around with any traps or room treatments. Although you've done your measurements, you'll have to do those listening tests! You may have to go back and forth a few times. Do let us know your thoughts.

all the graphs above are without a sub. Do you think I would see a larger difference when my sub is installed?

Not sure about that.

FWIW, I will say that you don't need to measure to such high frequencies. The SPL meter isn't going to be very accurate, and you start running into reflection issues that don't really represent the true frequency response of the mains -- that's why it's so jagged and all over the place. For bass measurements, going to about 200 Hz should be fine.

3) How are my trap locations? Does putting bass traps at the first reflection points help with high frequencies?

Normally, I think of bass traps as going in corners, but I know that others can be used throughout the room as you have done. Clearly, yours are not designed for corners, and I'd experiment with whatever recommendations the manufacturer gives you.

I do believe that mounting bass traps at first reflection points will help tame high frequency issues. That is, if the bass traps are "soft" such that they will absorb the HF components, which I think they are. Although they may be designed for bass absorption, I think they will also do something of a job on HF reflections.

You may get a better response to these types of questions in the "Room Treatment" forum here at the Shack; there are a number of experts there, as well as experienced users.

Somewhat related questions...
4) Do I want to buy a DSP1124P or FBQ2496?

I'd generally recommend the 1124 to anyone with a sub, a PC and a desire for good bass response. It will help tame gross issues with your room. For the price (<$100), they can't be beat. Of course, you may have a magically flat room already, but we don't know because you don't have your sub in. Still, it's very unlikely that that's the case.

5) I have read the RS SPL meter is ok for EQing a sub. Would I gain much with an ECM8000 mic?

The SPL meter is generally fine, but you will get a better cal file with the ECM8000, so your measurements will be more accurate. You'll also need a preamp of some type. If you're just using it for sub EQ, you'll be OK with the SPL meter unless you want to experiment.

Good luck!

[terry j]

Hi Nate

most people prefer it if the graph limits are the same on all posted graphs, and the standard we use is 45-105 db, 200 hz upper frequency limit.

Of course in your case it might be an exception if you wished to show the effect of the treatment in the higher frequencies in which case use smoothing to avoid the comb filtering. My REW is not open, but from memory you can choose the amount of filtering in the graph adjustment tab or something.

I infer from your post that you use the RS meter, if so then it is of no use at all above a couple of thousand hz, so another reason to bring your graphs 'back to the fold' I suppose. That meter IS fine for bass eqing with at least some sort of cal file, found here of course!!

Not sure why the waterfall graphs are so dense, mine certainly don't look like that!! Have a play, often you will find for comparison purposes it's best to have two (or more) graphs overlayed, you can use the all measured tab (I think it is). For the waterfall, again have a play and you will see it has cool features like one or the other can be 'dimmed' again making for ease of reading.

Please take all I've said with a little grain of salt, there are many experts here (and Brucek is one of the best) and I AINT one of them. I don't think I've led you too far astray though, just a stop gap measure till the real boys come into the game.

EDIT
Gee, I hope that didn't come across as belittling Otto's help!!! Nate, you will find there are many helpful people here and Otto is certainly amongst them.

[Otto]

terry j wrote:

Gee, I hope that didn't come across as belittling Otto's help!!! Nate, you will find there are many helpful people here and Otto is certainly amongst them.

Not at all, terry! I don't know enough about room treatments, so I'm happy to hear anything about this stuff! Thanks for the kind words.

[Nate]

I'll try posting in the Room Treatment forum. Sorry, I had not seen that earlier! Maybe an admin could move this thread there, since we've already discussed some things here?

Brucek, I've never had someone be unable to reach my domain! I wish you could see the pics. Maybe because the pics have commas in their names? What browser/OS do you have? Does http://n4te.com work for you? How about:
http://n4te.com/misc/newht/rew/
If those work, then these direct links to the pics in the order they were posted above might work...
http://n4te.com/misc/newht/sketch/basstraps1.jpg
http://n4te.com/misc/newht/rew/fr,all,15-7k.jpg
http://n4te.com/misc/newht/rew/fr,all,15-600.jpg
http://n4te.com/misc/newht/rew/lfwat...aps,15-750.jpg
http://n4te.com/misc/newht/rew/lfwat...aps,15-750.jpg
http://n4te.com/misc/newht/rew/lfwat...aps,15-750.jpg
If those really still don't work for you, please let me know and I'll upload them to an image hosting site, or I'll attach them to a post.

Thanks for the welcome Otto! I have gone back and forth, carrying traps and putting them in different places, even involving the wife in listening tests. Since it isn't a simple A/B test -- there is carrying 6 or so traps between listens -- we have found it difficult to say the traps noticeably improve SQ. We used a couple songs but most testing was done with an LFE demo DVD (Saving Private Ryan, LOTR, U-571, etc) since we mostly watch movies. Both the old lady and I feel like the sound is slightly more detailed with the traps, but we aren't sure it isn't placebo effect either.

I posted the FR graphs out to higher frequencies to try to show the effect the traps have on FR not just on the lower range. I posted waterfalls up to 750hz because I found that the traps reduced decay the most above 300hz+. The traps seem to do little to the FR and decay below 300hz. I'll be sure to post using the standard sizes next time, or I'll post a disclaimer why I didn't.

Hi terry j! Thanks for your input. I saw the smoothing option, but couldn't figure out why anyone would want to reduce the resolution of their graph. Using it for high frequencies makes sense! I've read the RS meter is only decent up to ~7k, so I only posted up to that. I realize the high stuff in the FR graph is mostly useless, but there are some slight differences in using traps around the 400hz range.

I think the waterfall graphs are so dense because I've graphed out to 750hz, since I saw differences using the traps out to there. I wasn't sure about the "window" setting when generating the waterfall graphs. I think I set it to 300ms.

I read the help and found the waterfall overlay settings, but the FR of my readings is nearly identical, the only difference being the decay is longer for one. This doesn't make for a very pretty waterfall graph if both are shown at once, even with transparency. I made a quick animation that shows the difference in decay with the traps...



I meant to ask before, what exactly does this impulse graph mean (blue is zero, pruple is six traps)...

[Nate]

Hi, I mis-posted about my room treatments... I posted in the REW/BFD forum and I think this forum (Home Audio Acoustics) was more appropriate. My thread detailing my bass trap installation and with graphs of the results can be viewed here...

http://www.hometheatershack.com/foru...ctiveness.html

Maybe an admin can move that thread over here to this forum? Sorry!

In a nutshell, frequency response is nearly identical with 6 traps installed, but I see this difference in decay...



How does the decay compare with typical rooms with 0 and with 6 traps? I'm trying to determine if I should keep the traps are sell them.

I have gone back and forth, carrying traps and putting them in different places, even involving the wife in listening tests. Since it isn't a simple A/B test -- there is carrying 6 traps between listens -- we have found it difficult to say the traps noticeably improve SQ. We used a couple songs but most testing was done with an LFE demo DVD (Saving Private Ryan, LOTR, U-571, etc) since we mostly watch movies. Both the old lady and I feel like the sound is slightly more detailed with the traps, but we aren't sure it isn't placebo effect either.

Maybe we just need more listening, maybe our untrained ears just don't know how to pick out the differences. Or maybe the difference isn't really there? That is why I started measuring with REW. The piece I'm missing is how much decay is bad decay, and how much difference in decay is audible? What exactly should I look for when listening?

Thanks for your help!

[bpape]

Hi Nate.

A few suggestions to let you and everyone else see better what's really happening.

1. Please set the scale across the bottom to 0-300Hz
2. Please set the scale across the bottom to logrithmic
3. Please set the scale across the side to say just 56-96.

Bryan

[terry j]

AAhh, thanks Brian. I missed the linear scale rather than logarithmic...couldn't work out why it looked 'odd'.

nate, your in good hands now, the experts are arriving so I'll sign off ha ha.

[Nate]

Sure thing, here we go...





And just in case it is important, here is 300-750.

http://n4te.com/misc/newht/rew/lfwat...ps-300-750.jpg
http://n4te.com/misc/newht/rew/lfwat...ps-300-750.jpg

[bpape]

OK. Thanks,

Looking back at the pic of your room and the treatment locations, I can see why we're not getting much improvement. Realistically, none of the treatments are really placed anywhere that will impact bass response much. They're placed for reflection control (which is good and also important.)

You'll need to hit the front corners pretty much floor to ceiling, kill the front wall - especially behind the front speakers, and also hit the wall directly beside the main speakers. I'd also consider doing some of the wall/ceiling intersections in a couple places in the room. Keep in mind that to be effective for bass, they need to be a minimum of 4" thick.

Also, when you get a sub, there's a lot we can do with placement, xover, and phase adjustments. Unfortunately, the best place to lock things to the screen and give good imaging is rarely the best (or even an acceptable) place for smooth bass reproduction.

Bryan

[Nate]

Thanks for your input Bryan! Right now I have the 2'x4'x4" traps. To be crystal clear (sorry if I'm a bit slow ), are you are saying that there isn't a way for these to be effective bass traps for my room? To do the corners in front, I assume I'd have to cut insulation into triangles and stack it, then cover it with some black material. The traps are worth more as is, so I'd have to sell them and buy more insulation.

If that is the case, maybe I should use at least some of the traps for reflection control. They are 4" thick when I've read 2" would do, but I do have them on hand. How many should I use and where? I think I can answer my own question... at the first reflection points. I think 2" thick traps would look better there since the room is so narrow. The 4" traps in front of the seating area make the room feel quite a bit smaller.

I guess the question I really need to ask is, should I use the 3 traps in the back of the room? These locations have the highest aesthetics. Are these locations effective for either bass or reflection control? The two on the side walls are pushed into the corners (this was true when the "6 traps" readings were taken for the graphs above).

See, I want to use the traps to improve my room's acoustics, but if they are going to make a negligible or very small difference then I guess I'll sell them and put them to better use that way. I feel so lucky to have won these traps and I think they are of high quality, they certainly look very professional. I really wish they could be made useful in my room!

Is reflection control measureable with my RS meter? Would I see the difference in the FR or the decay?

In the following two graphs of 300hz to 750hz, I see more differences with 6 traps (purple) than I do sub 300hz above...





How significant are these differences? How should I expect to hear the difference?

I just won a DSP1124P on ebay for $55 shipped and I picked up the phono-to-mono and MIDI-to-USB adapters today. All I need is the freaking subwoofer!

[JimP]

Nate,

I think what Bryan is saying is that you can move your existing panels and get more use out of them when it comes to bass trapping.

I'm just learning about this stuff myself, but I think its pretty basic about doing your 4 room corners first (straddle corners), then behind your front speakers and then on the sides at your first reflection point. For bass 4" minimum, for first reflection 2". Please correct me if I'm wrong.

[bpape]

Exactly. Move them for better benefit and also plan on adding a few more. Smaller rooms require proportionately more bass control and absorbtion in general than larger rooms.

In all honesty, pretty much ignore everything above 300Hz until we get the bottom end sorted out.

Bryan