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post #31 of 79 Old 01-24-07, 10:09 PM
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Re: Bass traps or EQ

Welcome to the world of the believers. Setting things up right and treating your room appropriately are the most cost effective upgrades you'll ever make. You're seeing and hearing that for yourself right now.

You'll find yourself listening to things you haven't heard in a while and hearing them in a whole new light.

Enjoy!

Bryan

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post #32 of 79 Old 01-29-07, 08:11 PM
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Re: Bass traps or EQ

[EDIT: Oooo000PS! I just realised I hadn't seen the entire thread before I posted this . . . looks like I'm a little late in my replies! DOH!!! I was going to just delete the post, but on second thought I'll leave it here in case anybody else might happen across it and find it helpful. Anyway . . . congratulations, Fred! Nice job. ]



Quote:
F1 fan wrote: View Post
Bryan ,Iam aware of the broadband coverage but I was wondering what kind of thickness was needed to get any benefit in the 50hz range.My space is limited but I did some checking today to see what the maximum size of trap I could squeeze into the corners.It looks like the max would be a 4' tall triangle shape with a 16" face pointing into the room.The maximum thickness would be somthing like 10 or 11 inches in the center.This is a lot smaller than the Tri Traps but will it be reasonably effective if not 50hz at least for the several modes falling around 150hz?
If you have the right density of material, you might be able to make something in that size that will help out at least down to the 150 Hz range, and maybe marginally down in the 50 Hz range. The 150 Hz range is more important for music anyway. For example, the low E on a bass guitar or bass fiddle is right around 40 Hz, but most of the energy of the sound of that note actually comes from the 1st harmonic, at 80 Hz (which is why you can hear the bass notes even on tiny speakers that roll off much higher than that.

There is no getting around the fact that you need mass to deal with low frequencies, so more would be better. You'll want to make sure you *at the very least* treat all four vertical (wall/wall) corners if it is at all possible. If you can't do that, then you'll want to look at treating horizontal (wall/ceiling or wall/floor) corners if you can.

Typically, for bass traps, you should use at least 703 or 705 rigid fiberglass panels (705 offering more bass absorption than the 703, due to a higher density), and it needs to be at least 3 to 4 inches thick. The thing is that these come in 2' x 4' sheets . . . which means that you'll have to do some cutting to get them to a 16" width for the face of the traps, and cutting fiberglass or rockwool is not exactly a whole lot of fun.

With 703 or 705, you don't necessarily need to actually fill the whole corner with fiberglass, as having an air gap behind the panel actually increases the amount of absorption over having the same panel with no air gap behind it. You may be able to get a small improvement by stuffing some lower density fluffy fiberglass behind the 703 or 705 panels.

You could also just fill the entire corner space with 703 (I'd say the 705 isn't necessary if you are going this route, and the 703 would be more cost effective). For this, you'd get a bunch of 703 panels and cut them into triangles (here again, having a 24" face is going to make for less cutting and less wasted material), and just stack the triangles until you reach the desired height.

You can also get a bit of improvement in the bass absorption of these panels if you use the FRK version of the 703/705 across the face of the traps. The FRK panels have a kraft paper facing, which will act as a limp mass membrane. Mind you . . . you will need to sandwich several panels of 703 or 705 to acheive the 3-4 inch thickness you need for low frequency absorption, but you will only want the FRK facing on the outermost panel!

You don't want to have layers of kraft paper in between the 703/705 panels, as this would actually DECREASE the effectiveness of the panels, because you will impede the gas flow through the thickness of the fiberglass. In large part, the absorptive behaviour of the fiberglass material is acheived via the tortuous path the sound pressure is forced to take through the fiberglass, both on its way in, and then on its way back out, as it is reflected back through from the walls behind the trap. If you have multiple layers of membrane, you quite drastically change the way the air pressure moves through the absorber, to a detrimental effect. The preceding is, of course, a grossly over-simplified explanation of how it actually works, and I started to write a more detailed one, but then I decided not to bore you with the details.

Last edited by [email protected]; 01-29-07 at 11:55 PM.
post #33 of 79 Old 01-29-07, 09:05 PM
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Re: Bass traps or EQ

Scott makes good points.

Don't be discouraged. A solid chunk of 703/705 with a 16" face and 10" thickness in the corner will absolutely have some effect at 50Hz - certainly way below 150Hz. 10" of 703 will have close to a 1.0 coefficient down to around 100Hz. Is it going to be 1.0 at 50? No. Can it be .4 or a bit more? Yup.

In short, you do what you can.

Bryan

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post #34 of 79 Old 01-29-07, 09:08 PM Thread Starter
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Re: Bass traps or EQ

Hi Scott, thanks for chiming in with a very informative post.Im glad you didn,t delete it.And I assure you a more indepth explanation would not have been boring.
Does this sound reasonable?with the next traps I build I was thinking of using higher density mineral wool (6lbs/cu ft instead of 3) to maybe get better absorbtion at lower frequencies.I would like to use OC 703 or 705 but unfortunately it is not readily available in Canada.I will be cutting it into triangles and stacking them into the wood frames.Yes I agree it is not the nicest stuff to be working with.

By the time I'm done I should have about 75 percent of the vertical wall corners treated.

Thanks again for your input.
Fred
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post #35 of 79 Old 01-29-07, 09:22 PM Thread Starter
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Re: Bass traps or EQ

Quote:
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10" thickness in the corner will absolutely have some effect at 50Hz - certainly way below 150Hz. 10" of 703 will have close to a 1.0 coefficient down to around 100Hz. Is it going to be 1.0 at 50? No. Can it be .4 or a bit more? Yup.

Bryan
1.0 down to a 100hz and .4 at 50hz .I like those numbers Bryan
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post #36 of 79 Old 01-29-07, 09:23 PM
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Re: Bass traps or EQ

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Now I have to build two more and see if that brings even greater rewards.
There is absolutely NO QUESTION that it WILL bring greater rewards. Not only will you have more actual absorption in the room, but you will be treating the room modes from FOUR problem spots instead of only two.

There are some very good animations that show how room modes work, and how sound pressure displaces the air particles in a room, here:

http://www.isvr.soton.ac.uk/SPCG/Tut...ding-rooms.htm

You can see how the modes gather in the corners and then shoot back out into the room -- this is where the sound pressure gathers velocity before being reflected back into the room. I think the above-linked animation is helpful in understanding why it's important to treat the corners of your room as adequately as possible, and that dealing with this reflected sound is really the only way to effectively deal with room modes.

If you are able to impede enough of the velocity of the wave front at those critical spots to reduce the amplitude of the reflected sound, you will reduce the amount by which the reflected sound will distort the direct sound (from your speakers), which it does by either canceling out the direct sound (where the reflected sound is out of phase with the direct sound) or by enhancing it (where the reflected sound is IN phase with the direct sound).

When you EQ your room, you aren't changing the way the sound travels through the room or the way it interacts with the room boundaries. So the problems are still there. I don't deny that you can get a *subjective* change in the overall frequency response of the room (though the actual effect of EQing a room is very positionally specific down to within less than a cubic inch, and it really doesn't fix the modal ringing -- it might reduce the amplitude of the modal ringing at a given frequency along with the amplitude of the direct sound, so there is a subjective change, but the ringing is still there in roughly the same ratio to the direct sound, and it's still acoustic distortion), but you aren't really fixing the problem -- you're simply masking it . . . a bit like putting a band-aid on a broken arm.

Conversely, if you do as much as you reasonably can to fix the REAL cause of the problems, and ONLY THEN put a band-aid on the smaller problems, you'll be able to get a lot closer to what the artist and the recording engineer intended you to hear. But keep in mind that EQ in itself is an introduction of a distortion of the sound source (in a number of ways, including the introduction of phase errors to the direct sound!), and, again, it cannot change the physics of how sound travels and interacts with room boundaries.

This concept is ESPECIALLY important when it comes to small room acoustics, because, unlike larger rooms (like concert halls, etc.), there is no delay (that your ear can discern) between the reflected sound and the direct sound -- particularly at lower frequencies, and the sound doesn't get a chance to decay on its own before it hits the next boundary problem spot (i.e., it's once again gathering in a corner to be reflected again) as it would in a large room (e.g., the size of a high school football stadium or a good sized concert hall), so you have to focus on reducing the *amplitude* of the reflected sound. If there is enough delay between the reflected sound and the direct sound, then your ear/brain will be able to distinguish the two sounds (to varying degrees, of course, also depending on the relative amplitude of the reflected sound to the direct sound). This stuff is hard wired into our brains (known as psychoacoustics), and it is part of the way our ear and brain are together able to determine the relative location of sounds, and hence the reason we can get a three dimensional image from only two speakers.

Sorry to ramble on about this, but the EQ v. acoustic treatment comes up so regularly, I thought it might be helpful to some people to discuss it in the above terms. Obviously each individual/room/family, etc. will have various different criteria as to the meaning of "reasonable" when it comes to treatment, given factors like WAF, multi-purpose use of a room, aesthetics, budget, etc., but I think a lot of people try to use EQ as a replacement for proper acoustics treatment because (a) it's the path of least resistance, and (b) they don't understand the above information. Or they will continue to spend thousands in upgrading their electronic gear, when a small investment in a bit of acoustic treatment will give them a FAR greater return.

As a recording engineer, I always try to get the sound right at the source first. That will usually mean dealing with the acoustics as much as possible up front, and then finding the right mic, preamp, and, prehaps more importantly, mic position for the particular instrument I'm recording and the acoustical conditions I'm dealing with. In pretty much all cases in which I have the option, I'll change mics, mic positions and, where possible, the acoustic conditions before I reach for EQ to "fix" the sound.

As a professional musician (drums) who has played in literally hundreds and hundreds of different rooms of all types and sizes over the years, and who has performed in said rooms on everything from the very finest instruments to student/entry level drums, I can tell you that for the most part I'd rather play an entry level drum kit in a nice sounding room than a very expensive top level instrument in a bad sounding room.

Last edited by [email protected]; 01-29-07 at 11:47 PM.
post #37 of 79 Old 01-30-07, 03:09 AM
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Re: Bass traps or EQ

Yes, I agree with Bryan that you should be able to get some absorption down to 50 Hz with your current design. I didn't mean to say that it wouldn't help at all (perhaps I shouldn't have used the term "marginal") . . . I meant it more in relative terms, that larger traps would help more if you could do it.

But perhaps even more so, I wanted to point out that you can be less concerned about a 50 Hz room mode than you would want to be about modes at ~80 Hz and up. One reason is that the majority of the speaking range of even the lowest instruments is in the 80 Hz and up range, because we hear the first harmonic louder than we hear the fundamental of these instruments. If you play any given music CD through a spectrum analyser, even stuff that is fairly bass heavy, you'll see that there is a lot less energy down around 50 Hz and lower. And that is because, again, the "speaking range" of these instruments is at the first harmonic.

Actually, the first stage of mastering for music recordings often involves applying a high pass filter at around 30 Hz or so, which gives a noticeable roll-off curve from 60 Hz down to 30 Hz.

But your room (as with most small rooms) is more likely to have much bigger problems in the 80-500 Hz range.

I ran ModeCalc on your room and took a couple of screen grabs of the results, and you will see that you have MUCH bigger, and much more noticeable issues in that range. And if you look at the one that gives you a graphics plot, you'll see where you have numerous modes from each of the dimensions bunched up right next to each other. It's that kind of stuff that can really make the overall frequency response of your room look like the Swiss Alps (as most small rooms do!).

[EDIT: Yikes! How to I upload a picture here? I thought I had done it right, but I don't see the pic! ]

If you get some of those modes sorted out, things start to become much clearer, and because there is more power there it can punch through any mud/rumble that can come from a mode in the range of 50 Hz or below.

If it's a home theater system, and you like to watch movies with lots of explosions and earthquakes in them, you'll have stuff with more energy down in that range . . . but who's gonna care if an explosion or an earthquake rings a little longer than it's supposed to. Noam sayin'?

Quote:
F1 fan wrote: View Post
Does this sound reasonable?with the next traps I build I was thinking of using higher density mineral wool (6lbs/cu ft instead of 3) to maybe get better absorbtion at lower frequencies.I would like to use OC 703 or 705 but unfortunately it is not readily available in Canada.I will be cutting it into triangles and stacking them into the wood frames.Yes I agree it is not the nicest stuff to be working with.
Just to clarify, when I say 703 or 705, I'm basically using these terms as generic references to rigid fiberglass or mineral wool panels of essentially 3 lb or 6 lb density, respectively. Performance should be roughly equivalent, if I'm not mistaken.

I'm not absolutely certain off the top of my head (I'd have to look up the data, etc.), but I would guess that the 6 lb density fiberglass/rockwool, etc might be overkill for using it in this way, where you are filling the entire corner. (Maybe Bryan or others can give you some more precise info on this.)

I think I'd almost rather see you spend the difference in cost between the 703 and the 705 on getting more of the 703 and making larger (taller) traps if you can swing it (i.e., lovely wifey would permit, etc.).

If you were making panel traps to straddle across the corners, rather than filling the entire corner space with rockwool as you are here, then the denser 705 panels would make more of a productive difference, I think. The design you are using (filling a corner) and a panel trap have similar, but subtlely different, behaviours in play that cause them to perform the way they do.


Yikes . . . it's late and I'm gettin' sleepy. But I do have a couple of suggestions I could offer on how you might improve on what you've got there without adding too much complication. More on that tomorrow.
Attached Thumbnails
Bass traps or EQ-modecalc-freds-room.jpg  


Last edited by [email protected]; 01-30-07 at 03:25 AM.
post #38 of 79 Old 01-30-07, 03:21 AM
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Re: Bass traps or EQ

OK. Got it sussed now. Here's the second ModeCalc screen grab. Attaching both in one post made for a very W-I-D-E thread!

Looking at these numbers, you can also see why using REW to analyse the frequency response of your room is going to give you a much better read than using test tones of even as fine as 1 Hz increments!
Attached Thumbnails
Bass traps or EQ-modecalc2-freds-room.jpg  


Last edited by [email protected]; 01-30-07 at 03:35 AM.
post #39 of 79 Old 01-30-07, 07:22 PM Thread Starter
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Re: Bass traps or EQ

Scott, thanks so much for taking the time to post the Mode calc results.
I see what you are saying about problems well above the 50hz range.Things do seem to get a bit ugly from 150-500hz.
As you suggest I will stick with the 3lbs/cu ft mineral wool for my remaining traps.
Fred
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post #40 of 79 Old 01-31-07, 02:49 AM
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Re: Bass traps or EQ

Fred, see my comments in this thread (both of my recent posts there) regarding a similar design to yours.

Also . . . I might suggest that, instead of building a frame for your trap that consists of wood panels, you would probably do better with a more sparse frame, more closely resembling a triangular box kite.

Triangular frames (open) at the top and bottom, and support struts at each corner (I'll attach another quick and dirty sketch here as well).

The wood panels up against the wall are redundant, and waste a certain amount of space that could be used for either additional absorption material or an air gap, either of which would increase the amount of absorption you are getting from that space.

Further, you want as much absorption material exposed to the room as possible. Having the top surface exposed will also increase the amount of absorption you can get with these traps.

Better yet for you, if you can swing it, would be to simply go from floor to ceiling with the "superchunk" style traps you are building (again, see my comments in the other thread.

First picture is open top frame, second is side view of the trap frame. Again, very rough sketches here. Also (just to be clear), with regard to the side view, the support struts can be flush with the corners of the top and bottom frame, rather than being inset as the diagram shows.

Last edited by [email protected]; 01-31-07 at 02:59 AM.
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