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Bass Traps


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40 replies to this topic

#1 of 41 Gary Hunter

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Posted February 11 2004 - 09:52 AM

I need to add some bass traps to my room and am wondering if anyone has a good discount source for them, or if there are any less expensive alternatives?

I could be willing to try a DIY project if I had clear instructions on how to do it. Thanks for the help.
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#2 of 41 Wayne A. Pflughaupt

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Posted February 11 2004 - 10:54 AM

Have you considered a parametric equalizer like the Behringer Feedback Destroyer? It’ll do a much better job for a fraction of the price.

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#3 of 41 Gary Hunter

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Posted February 11 2004 - 11:04 AM

No, haven't heard of it. Info?
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#4 of 41 Wayne A. Pflughaupt

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Posted February 11 2004 - 12:50 PM

It’s a digital parametric equalizer complete with memories. Made for pro-audio, but it’s too low end to be taken seriously in that arena. However, it’s a remarkable sub EQ and many Forum members have one. So a search on “BFD” or “Behringer” at our DIY and Advanced Projects or Speakers and Subwoofers forums and you’ll get enough reading material to keep you busy for days.

In the meantime, here are a couple of before and after response graphs a member posted a year or so ago. Needless to say, no bass trap can do this.



Before BFD

Posted Image



After BFD

Posted Image



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#5 of 41 Gary Hunter

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Posted February 11 2004 - 03:48 PM

Thanks for the info Wayne. I've enjoyed reading a lot of it already.

A couple of quick questions, though. I am using stereo subs with my mains and an LFE sub. Would I then need 2 BFDs to eq all subs? Do you find that the BFD has eliminated the need for any lf acoustic treatments then?
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#6 of 41 Wayne A. Pflughaupt

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Posted February 12 2004 - 03:27 AM

Gary, the BFD is two-channel.

I gotta tell ya though, with all those subs (I assume placed here and there?), if you’re using all of them at the same time you will have a tough job getting smoothing response, even with a parametric equalizer. Probably the best you can hope for is to locate and eliminate common response peaks. It will be very difficult to get the kind of response you saw in the above graph (which was accomplished with a single sub).

Quote:
Do you find that the BFD has eliminated the need for any lf acoustic treatments then?
The BFD does nothing for acoustics, and neither does a bass trap. The former is merely an electrical means of tuning a room’s response; the latter is a mechanical means.

Acoustics is altogether different. It has to do with treating the “liveness” of a room (I know, that’s not a real word Posted Image ), i.e., controlling echo, reflections etc.

Naturally, since a bass trap is a physical object in a room it will have some affect on acoustics, but that’s not its intended purpose.

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#7 of 41 Pete H

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Posted February 12 2004 - 05:35 AM

Tube traps are fairly expensive. The following website has construction information and lots of useful information.

http://www.geocities.com/jonrisch/

#8 of 41 Peter Overduin

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Posted February 12 2004 - 08:12 AM

Here is a brief quote, and link from an article on the topic of accoustics. It may behoove you to look at it. Good luck with whatever choice you make!

Quote:
And what of "bass traps," those expensive padded cylinders and screens that are sold to naive audiophiles and claimed to absorb problematic standing waves in the low bass? For the most part they don't work, because the wavelengths of low bass frequencies are so large that any "trap" would have to be of ungainly dimensions to become effective. They can be built (there are several in the NRC's listening room) but they are large and ugly. No domestic partner, no matter how tolerant, would put up with them.



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#9 of 41 Rutgar

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Posted February 12 2004 - 08:18 AM

Wayne, What did you use to measure and create the waveforms you posted, to show the before and after shots of using the BFD?

Thanks,

- Rutgar

#10 of 41 BruceD

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Posted February 12 2004 - 08:44 AM

Rutgar,

One sw program you can use with your RS SPL meter is called ETF5

You will also find lots of useful tutortial material on the website as well as demo room examples to help you understand to to best use the program.

Other sw programs include spectraplus, truerta and others I haven't used myself.

Also, there is a website here with an explanation about how to use the MS excel program that was used to create the graphs in Wayne's post. It also explains how to work with the BFD parametric EQ.

#11 of 41 Rutgar

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Posted February 12 2004 - 01:38 PM

Thanks for the information. Since the BFD really isn't very expensive (I found one at a local Guitar Center for $119.00), I went ahead and bought one today. I'll spend the weekend setting it up. I might eventually try the software you suggested, but in the mean time, I'll play with the Excel stuff (since it's free).

- Rutgar

#12 of 41 Sonnie Parker

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Posted February 13 2004 - 11:53 AM

Hmmm... sounds like a very interesting topic.

Hello to you fella's who have taught me so much... Wayne and BruceD.

Gary...

I don't know if I could live without my BFD in my HT room.

Here's my latest graph from the sweet spot:

Posted Image


I've learned a little about taming bass in a small HT room. The BFD was perfect for taming the sweet spot but it doesn't do anything for my heavy bass in the rear of the room. If I tame that area then I create a massive dip in the sweet spot. Even moving from side to side the response changes. It's a serious challenge to get a good balance in all 6 of our seats.


I researched the bass trap idea and for the frequencies I need to tame I believe it will take some serious panels designed to precision to achieve any noticeable results. The rear wall creates a 10db boost in frequencies around 30hz. From what I could gather panels would need to be built from the floor to the ceiling across the entire back wall. I could see how this could get ugly but I could see it dressed out nicely as well. I would hate to do it and it not work. That would take up a lot of precious room if wasted. It may be that I would need some expensive equipment to figure it all out to precision. I need more research no doubt. Getting a bass trap to only absorb the 30hz area sounds like a very serious challenge to me.


There are absorbtion panels that are very inexpensive for those early reflections from your mains. The ones I found I believe work on frequencies 125hz and up. Most are 7lb pcf compressed insulation framed and wrapped in the Gilfourd of Maine material. These are available in several sizes and colors. I have a link to one at my office and will try to remember to post it later. They were very reasonable in price.


Have you completed measurements yet?

There is a link to test tones on the BFD page as well as a lot of other useful links.

#13 of 41 Wayne A. Pflughaupt

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Posted February 14 2004 - 07:54 AM

Great curve, Sonnie! And nice graph. Are you going to keep it up? If so I’ll start using yours instead of the other ones. Yours is smaller and has both curves in one graph.

Also, good to see you here again! You really should do more than 0.31 posts per day! Posted Image

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#14 of 41 Sonnie Parker

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Posted February 14 2004 - 09:13 AM

Thanks Wayne... I try to chime in all along... I know I do a pitiful job overall though. HTF is a big place and sometimes it gets overwhelming to me. Lots of knowledge floating around.

Yeah... I'm gonna keep that graph up as it's on the BFD page. Although boosting that 40hz area doesn't help any with the heavy bass coming off that rear wall if you are sitting on the back row. I'll probably set up another preset to use when we have people on the back row.


Here are my measurements and filters used to achieve the above:


Posted Image


Posted Image

#15 of 41 Paul Spencer

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Posted February 14 2004 - 06:56 PM

I think it is unfortunate that room treatment is not given more attention than it is. It seems that many have gotten satisfactory results using BFD, but I don't think using it without using other methods as well is the best approach.

I have been researching this area recently and I have come to the conclusion so far that the best approach for a HT is to first tame the "modal region" (below say about 200 Hz where the response of the room is dominated by room modes). After the room modes have been tamed a little you can use eq for the listening position.

Bass traps aren't the only way to deal with room modes and yes they do work if done right. I think they are more suitable for recording studios - they use up too much space in a home environment. I understand that you should have at least four of them. I found some info on them here:

Acoustics 101

Posted Image

There are two other approaches to consider:

1. tuned absorbers (ie. helmholz resonators)
2. internal wall damping

You can also use an adjacent space as a large helmholz resonator, and you can put in a series of them to tame different peaks. There are many ways to do this.

Internal wall damping has to do with designing the wall structure so that the walls actually absorb bass more than the midrange and above. This approach is discussed in an excellent book which I recently purchased:

Premium Home Theatre by Earl Geddes

No, this will not replace BFD, but it will tame the problem at the source, which means that you make an overall improvement affecting the whole room. If you have the ability to change the room construction or build from scratch, then this approach makes sense. It's like the difference between airconditioning an insulated vs uninsulated room.

If you do want to try one of those ugly bass traps, do a search for John Risch's DIY bass traps. They are cheaper to build, so its less risky.

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#16 of 41 Anthony*gw

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Posted February 16 2004 - 06:09 AM

what's up guys,i dont know if this has been asked before but can the BFD control fq below 20hz?
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#17 of 41 Rutgar

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Posted February 16 2004 - 08:02 AM

Sonnie, Did you have to use a lot of Gain at 40Hz? Or, did you bring everything down, and then raise the overall level of the sub? I'm asking because many people say you should never boost frequencies with an equalizer.

- Rutgar

#18 of 41 Wayne A. Pflughaupt

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Posted February 16 2004 - 11:49 AM

Quote:
i dont know if this has been asked before but can the BFD control fq below 20hz?
The lowest frequency you can set a filter at is 20Hz. However, a 20Hz filter would have the same effect below 20Hz as it doe above 20Hz. How much so would depend on the bandwidth.

Quote:
I'm asking because many people say you should never boost frequencies with an equalizer.
There are rules to equalizing, but most of them don’t apply to equalizing subs. This is one of them.

Regarding cutting and not boosting, you can find more information on that here.

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#19 of 41 Paul Spencer

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Posted February 16 2004 - 01:53 PM

Anthony:

Why would you want to use eq below 20 Hz? I would use a rumble filter as there is very little output below 20 Hz and it takes so much displacement to get any impact that low that it is not worthwhile - not for the tradeoff of max SPL. Why give up half the output of 99% of the bass for the sake 1% which you will rarely ever notice? This should only be attempted if you are making a cost-no-object sub.

Rutgar:

Sealed subs are more tolerant of eq boost. With them it is quite ok as long as they have sufficient excursion capabilities and the amount of boost should not be too extreme or you get a lot more distortion. With a vented sub you have to be careful of eq boost around tuning.

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#20 of 41 Rutgar

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Posted February 17 2004 - 01:01 AM

Paul, Thanks for the info about sealed subs. I'm using two Velodyne ULD-15's, and they are sealed units. So, I won't be so shy about using a little gain on my BFD.

- Rutgar


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