Need some bass trap advice

[toddsa]

I'm a little like a fish out of water here... I'm a home theater guy, not a studio guy... but I'm looking for some advice.

I'm grappling with a low frequency problem in my dedicated home theater... primarily a massive peak in dBs around 30-40 Hz (very boomy!)... my room is 12 feet wide, 17 feet long, 8 feet high.

I have some constraints... such as, I can't move the seating... and I've tried moving the sub around to no avail.

I've incorporated a parametric equalizer to some success... but the extreme boominess that I describe above still exists outside of the primary listening position that I equalized!

So, I'm turing to trapping.

I currently have 8 panels of 3" thick 703 2'X4' boards that I had been planning to cut into triangles and place around the corners of the room in stacks that are 3 to 4 feet tall. These stacks would be made of triangles that are 17"X17"X24".

The more I've read, the more I have come to decide that these triangles will probably help even out the response in my room for sounds 80Hz and up... but not for the low - low - 30-40Hz frequencies that I really need to attack.

I feel like I'm running out of "reasonable" options....

I'm wondering if I will be pretty much stuck with the 30-40Hz peak unless I take some really radical steps in terms of alterations to the room.

Aside from the triangular corner traps... I've also thought about building 6 inch thick panels that are 2 feet X 4 feet and angling them in the corners of the room. Would this give me more low frequency coverage?

Also, I have been reading about perforated panel absorbers...or a perforated resonating absorber. The site I've looked at is here: http://www.stereo.net.au/forums/showthr ... bass-trap)

I can run the calculations, no problem... but I have some questions about the construction of these (and the size of them) if anyone thinks they are worth a shot.

Thanks!

toddsa

[Soundman2020]

Hi Toddsa, and welcome!

my room is 12 feet wide, 17 feet long, 8 feet high.

Assuming that your room is rectangular, theory predicts that your 1,0,0 mode is at 33 Hz, and your 0,1,0 is at 47 Hz, so those might well be the issue, especially if you have no bass trapping at all in your room as yet.

I've incorporated a parametric equalizer to some success... but the extreme boominess that I describe above still exists outside of the primary listening position that I equalized!

Yup, as you discovered, EQ is not the solution, because it is not the problem! Room modes are a time-domain issue (energy built up and released over time by the room itself): Eq helps reduce the build up, but can do nothing at all about the release, since that depends on the room itself, not on your sound system. Your sound system pumps energy into the room modes, but once the sound stops, the room carries on for a while, releasing the energy. That's the "boominess" you are hearing, and no amount of EQ can fix it.

You cannot use EQ to fight a problem that is in the time domain. As you noticed, you might be able to use it to take the edge of the problem for one specific location in the room, but it is not actually treating the problem at all, since it does nothing for the rest of the room, and in fact might make things worse at other points in the room. Aside from the fact that your system is now misrepresenting the original sound tracks!

I currently have 8 panels of 3" thick 703 2'X4' boards that I had been planning to cut into triangles and place around the corners of the room in stacks that are 3 to 4 feet tall. These stacks would be made of triangles that are 17"X17"X24".

That's basically a "superchunk" bass trap, which is a good idea for your room anyway, since it is a small room and will need bass trapping, regardless of whatever else you might need.

The more I've read, the more I have come to decide that these triangles will probably help even out the response in my room for sounds 80Hz and up... but not for the low - low - 30-40Hz frequencies that I really need to attack.

Well, yesssss.... and noooo... Theoretically, that will cover quarter waves down to about 250 Hz for waves arriving head-on to the front face, and down to 140 Hz for waves arriving along one of the walls, but that just tells you about peak absorption. They will still treat lower frequencies (albeit not so effectively) down to maybe one octave lower, but even so, this is an acoustically small room, so you undoubtedly have many other problems on that very area, and they will need treating anyway. So I'd go with the superchunks, for sure. It might not cure your major issue, but it certainly will help somewhat, and will cure other issues that you haven't even noticed yet, because they are masked by the 30 - 50 Hz issue.

To treat lower frequencies effectively, you already figured out the answer: panel traps (membrane traps). Here too, you are looking at very large devices. A sheet of 3/4" plywood over a 12" air gap would get you down to that ball park. But it isn't just the tuning that you need to worry about: you also need to position the device in the correct location in the room. Panel traps are pressure devices, meaning that the operate on the pressure component of the sound, not the velocity component. So if you place your panel trap at a point where that particular mode exhibits a velocity peak, and a pressure null, it wont work! Also, if this is, for example, a 1,0,0 axial mode, then placing the device on the ceiling would also do nothing, since the 1,0,0 mode never even sees the ceiling. The device would have to be positioned right up against the front or rear wall in order to address that mode. Perhaps you could position it behind your screen, at the front of the room. Then there is also the size issue: Make it BIG...

That would be my approach: Do the superchunk bass traps anyway, then if you still have that "boominess" issue at 30 - 40 Hz, do a couple of large panel traps tuned to the specific problems that you have, and put them in the correct location for those modes.

Maybe others here can offer better advice, but that's the way I would go about it.

- Stuart -


For example,

[Eric Best]

Are you having trouble with clear understandable sound or do you have way too much rumble?

You also might want to check around 141hz. Two of your axial modes coincide there. It is also the frequency that I usually think about when I think of a room being boomy. Also if it is a standard drywall constructed room, shouldn't frequencies that low pass through that construction and not excite those frequencies?

Try a total cut there with your EQ to see if it makes a change (it won't sound right or fix the problem, but it will give you info). Also try around 282hz because the modes line up there too.

[toddsa]

Yes, the problems are definitely in that range of 30-40... I have the REW software up and running and the peak starts at 30 Hz and extends to 40Hz (my speakers are calibrated at 75dBs and the peak in that 30-40 range is up to about 95dBs). This creates a really boomy bass that makes it sound disconnected.


I've been doing some further reading... especially on the vast amount of material provided by Ethan Winer. I'm now considering scraping the idea of a "super chunk" and just framing and straddling the corners with 3" thick 703 that has a reflective material in front of it. Why? I am definitely working on a budget... and according to the results of Winer's tests (http://www.ethanwiner.com/density.html) the more panels I get in the room will trump fewer thicker panels. Also, according to the results of his experiment (linked above), 703 with some sort of reflective facing is actually more effective at lower frequencies (down around 40 Hz).

So, I'm going to give it a run.

Because of the layout of my room, some sort of resonator might just be too large for my application. I appreciate your input on it. As you know, with limited space, there is a very fine balance between aesthetics and function. If this were a recording studio, it would tilt totally toward function. Not to mention some of the space limitations I have also.

So, my plan, right now is to build these panels... one for each corner... two for where the ceiling meets the wall at the front and back of the room, respectively, and - just for the fact that I will have some spare material - some small rectangles for the upper wall/ceiling corners of the room. Eric, if my memory serves me correct, the area around 140Hz fluctuates some, but according to my REW software that frequency range is pretty flat. My equipment is all at home, so I can check later.

I'm getting the feeling that adding bass traps will have a positive impact - maybe it won't totally solve all of my problems... and maybe it won't have the biggest impact at the 30-40 Hz range, but it is really my best option. Kind of a win-win.

My biggest hope is that I won't over-deaden the room. But I **think** that adding some kind of reflective material to help keep these bass traps from soaking up to much of the mids and highs will help. I have already treated the room with 2" thick acoustic panels at the first reflection points on the walls and ceiling. So I definitely don't want to dampen it to much.

I'm struggling with what kind of relfective material to buy --- I've read everything from brown Kraft paper... to plastic sheeting... to tin foil??? I guess just about any thin/rigid product will do?

Does it sound like I'm heading in the right direction?

[Soundman2020]

I'm now considering scraping the idea of a "super chunk" and just framing and straddling the corners with 3" thick 703 that has a reflective material in front of it. Why? I am definitely working on a budget... and according to the results of Winer's tests

I'm looking at those tests and not understanding how you used them to arrive at your decision: There is nothing at all in those test that deals with superchunks! In those tests, Ethan is only comparing different types of OC products, and different distributions around the room. He NEVER tested superchunks in that series, so how did you arrive at the conclusion that superchunks would be LESS effective, just by looking at that test? In effect, you are saying "I have decided that apples are tastier than oranges" but you only ever ate apples, never even looking at an orange, in arriving at the conclusion.

Sorry, but your conclusion is invalid, based on the research you are citing.

For a test that actually DOES compare apples to oranges, you might want to look at this:

http://forum.studiotips.com/viewtopic.p ... hunks+test

The conclusion is pretty obvious: superchunks outperform corner traps by a wide margin, across the entire spectrum tested.

the more panels I get in the room will trump fewer thicker panels.

That's what we are tying to tell you, but you don't seem to be listening! Your room is SMALL! it WILL need bass trapping, and lots of it: There simply is no doubt about that. The BEST place for bass trapping is the corner of the room, due to the "free boost" you get there: 12 dB for corners between two surfaces, and a whopping 18 dB for "tri-corners" (between three surfaces). Put more everywhere you can, by all means (you WILL need it), but the corners is where bass trapping it is most effective.

Also, according to the results of his experiment (linked above), 703 with some sort of reflective facing is actually more effective at lower frequencies (down around 40 Hz).

The reflective facing has nothing at all to do with the low frequency absorption: That is there for an entirely different reason. The absorption would be just as good without the reflective facing. The purpose of that is to NOT absorb too much of the high frequency range. Since those are large devices, and they are very effective at absorbing across most of the spectrum, if you DON'T use that reflective facing, then the room would end up too dead, lifeless, dull, and ugly. The reflective facing is to reflect highs back into the room, keeping it lively, while allowing lows to pass through and get treated. There might be some type of membrane trap effect, yes, which might be responsible for the dip at 42 Hz, but that tells you nothing at all about how it might react at 30 Hz, or 35 Hz, or 38 Hz, or any other frequency. Once again, as Ethan himself warned: you cannot necessarily extrapolate those specific results to other frequencies.

So, I'm going to give it a run.

Good luck with that! Treating a room based on poorly conducted research arriving at invalid results from a partial understanding, does not sound like a smart thing to do...

... is actually more effective at lower frequencies (down around 40 Hz).

Ummmm.... Nope! Check the graphs at studiotips: Both superchunks and ordinary panels across corners have peak absorption at around 100 Hz. As Ethan himself carefully pointed out in his tests, his room has NO modes at 100 HZ! The absorption then falls off rapidly down to 63 Hz, which is the lowest frequency that they were able to measure accurately. And that brings up the next point: How were you able to accurately measure that your frequency problems are peaking at 30 and 40 Hz? Very few speakers, sound systems, measurement mics, and DAW interfaces can give accurate flat response at such low frequencies. Then there are the known issues with software trying to calculate FFTs on such minimal information. Please list the test equipment that you used for to get these readings, and also the procedure. I'm not convinced that you actually did measure what you think you measured...

Because of the layout of my room, some sort of resonator might just be too large for my application.

In that case, you are out of luck! Treating low frequencies requires large devices, no matter what principle you base them on! You can't wish your way around the laws of physics, just because you don't like what they are telling you. The simple fact is that sound waves are not greatly influenced by objects that are significantly smaller than the wavelength itself: Sound waves just don't "see" objects that ate much smaller than they are: they ignore them, go around them, and carry on as though they weren't even there. The simple fact is that a sound wave of 30 Hz. has a wavelength of roughly 37 feet. A quarter wave of that is 9 feet. Anything smaller than about 9 feet wide/high is simply invisible to a 30 Hz sound wave. Simple fact of physics. (Assuming that you really did measure correctly, and that 30 Hz really is your problem). 40 Hz is 28 feet, for a 7 foot quarter wave. simple fact. Conclusion: You cannot treat your 30 Hz and 40 Hz low frequency problems with small devices! They HAVE to be big if you want them to have any effect at all.

So, my plan, right now is to build these panels... one for each corner... two for where the ceiling meets the wall at the front and back of the room,

That will help with bass trapping in general, yes, and you DO need it, yes, but it will not treat 30 Hz issues very effective at all.

I'm getting the feeling that adding bass traps will have a positive impact - ...
My biggest hope is that I won't over-deaden the room....
But I **think** that adding...

Unfortunately, sound waves don't care about your feelings, hopes, thinks, dreams, or aesthetics: the only thing they care about is the laws of physics.

maybe it won't totally solve all of my problems... and maybe it won't have the biggest impact at the 30-40 Hz range, but it is really my best option. Kind of a win-win.

Well, that kind of is exactly what I told you yesterday! One more time: You have a small room, therefore it WILL need bass trapping, and LOTS of it. No question. One more time: this bass trapping will, undoubtedly, improve the overall response of the room, and to a certain extent, even low frequencies, since it will even out some modal issues. It might even deal with the real higher frequency issues that you think you are seeing as lower frequency issues, when they really aren't.

I have already treated the room with 2" thick acoustic panels at the first reflection points on the walls and ceiling.

That is too thin to be effective across the full range of spectral frequencies for that room. 4" would be more like it, provided that it was also spaced 4" away from the walls. Even that will only cover down to about 800 Hz or so. But 2" up against the wall is only giving you coverage down to about 2 Khz.

Does it sound like I'm heading in the right direction?

For general bass trapping? Yep. Exactly as I explained yesterday. For your specific issues at 30 Hz and 40 Hz (assuming that you really did measure those correctly)? Nope. Those can only be treated with large tuned devices.

The most frustrating thing about this whole thread that you most likely will go ahead and install those panels anyway, no matter what folks here tell you, and then be so thoroughly happy with the drastically improved way it sounds afterwards, due to the overall greatly improved bass response, that you will tell everyone about how your then treatment solved an issue at 30 Hz or 40 Hz, when in fact it did no such thing! Your remarks will then be repeated and perpetuated around the internet, as these things do, reinforcing yet again false conclusions based on invalid analysis of questionable incomplete research!


- Stuart -

[toddsa]

Wow, "Stuart", thanks for that entirely insulting and back handed response. I especially enjoyed reading that last paragraph that - in as many words - painted me to be a village idiot. Really appreciate that.

Let me break this down for you into its most simplistic terms. I am on a budget, which you could have inferred from the statement that simply said "I am on a budget." Rather than wasting your time pounding out a diatribe that, to your credit, must have taken quite a while to compose... you could have simply said "Your best option is going to be the superchunks... you are aware of that, yes?" or "Let's back up for a second. Let's talk about your budget and see what kind of options - if any - might work within your constraints."

The fact is, I have put quite a bit of money into my home theater over the years. I also have a life that lives outside of that room that is very demanding on the wallet and I live in a very expensive area of the united states. So when I say "I am on a budget" that means I have a budget. Which means that I am not able to spend $450 on cases of 703 for something like bass trapping at this time... I am not willing to commit $80K to additions to my home to build the perfect theater room... and, no, I don't have the ultimate in equipment. I am willing however, to put $90 toward a case of 8 panels of 3" thick 703 that might be able affect some change. If that's all I have at the moment to put towards a project for a room that is meant to watch a movie in every once and a while, then so be it.

You spent so much time telling me how wrong my inferences are that you never stopped to try and understand my conclusions. Of course I understand that superchunks are a better option than 3" 703 panels straddling corners. I'm not an idiot. But what you need to understand is that the link I provided shows that more coverage is better than less, thicker, coverage... meaning that I will probably have better results with what I have - spreading it out to the corners of my room - than taking what I have and superchunking one - maybe two - corners.

Also, if you would look at the results, the evidence shows that the FRK facing does improve the 3" panels impact from 40-200Hz... which means that its presence, for whatever reason, does have an effect on dampening bass - in that range - outside of providing a benefit reflecting some highs.

The silly thing about your response is that you were so eager to point out how wrong and misguided I am , that you failed to stop to try and understand my basis and where I am coming from. For future reference, you would do everyone a favor to tone down your condescending tone, your use of bold letters, and your hyper response to a non-emergent issue.

To conclude this exchange, in the fine fashion that you most offered up at the end of your post: What I think the most frustrating thing about this thread is, you will probably go off telling people how some idiot didn't listen to your advice and that he is off ruining the acoustical world that you so righteously defend on internet. Yet you, hiding behind a keyboard and a screen, will feel justified in having been rude and condescending to a person thinking that you are educating them, when you in fact did nothing of the sort.

[BriHar]

Hello Toddsa,

You might also want to try posting on Gearslutz.com, in the Acoustics forum
http://www.gearslutz.com/board/studio-b ... acoustics/
You may even get an answer directly from Ethan as he often posts there.

[toddsa]

Thanks for the heads-up. I'll check it out.

Best,
toddsa

[Soundman2020]

... thinking that you are educating them, when you in fact did nothing of the sort.

Yup, I noticed that! Like they say, you can lead a horse to water, but you cannot make it drink...

Whatever....