Building proper bass traps

[Vaughan Odendaa]

Hi there,

I have been pondering over this for some time now and I think that I need to do this.

What I want to do is build a trap that will straddle the corners from ceiling to floor. Not stacked, just a single trap that is big enough to cover that amount of surface area.

I have consulted with Owen Corning South Africa (believe me, I hit him with 703 and 705 and he doesn't know what on earth I'm referring to ). I tells me that there are ready made sizes of up to 2.4 m and 50 mill thick. Also, he tells me that this is the rigid fibreglass.

Now what I can't believe is the price. For one panel board, it's about R250 ($20 or so). This obviously will exclude the fabric. I need four of these. Now I have a guy who will laminate a wooden board at the back of the fibreglass to prevent high frequencies from being absorbed.

I assume that this is correct ? For each panel, an equivilant sized board of reflective material needs to be bonded behind the fibreglass to absord HF.

Now since the fibreglass only comes in 50mm thick pieces, I was thinking of purchasing two of them and then laminating them together. That's 100 mill. Around 4" thick. Is that enough or should I perhaps shoot for even thicker ?

Lastly, I want to straddle each board across the corners and I might shoot for an inch or two off the wall, if that's possible. Cover all the boards with suitable material and I'm done.

Is that a sensible plan or what ?

--Sincerely,

 

[Vaughan Odendaa]

Anyone ? ?

--Sincerely,

 

[Buckle-meister]

Some food for thought

What I want to do is build a trap that will straddle the corners from ceiling to floor. Not stacked, just a single trap that is big enough to cover that amount of surface area.

Why? Why one trap? Whilst stacking traps on top of one another may not inspire confidence in terms of stability, neither does one panel standing twice as tall and at least with two panels you'd have the option of placing them separate from one another if at any time you wanted to.

...I have a guy who will laminate a wooden board at the back of the fibreglass to prevent high frequencies from being absorbed. I assume that this is correct ?

Not having room treatment with reflective backing I cannot say for sure, but my understanding is that the reflective backing is simply thin shiny paper. The quote above gives the impression that you're going to stick this paper onto a 'wooden board' of unspecified thickness. I don't think that's right.

For each panel, an equivilant sized board of reflective material needs to be bonded behind the fibreglass to absord HF.

The reflective material doesn't 'absorb HF'; quite the opposite. It's reflective so that high frequencies aren't absorbed so as to prevent a room from becoming too 'dead' sounding.

...since the fibreglass only comes in 50mm thick pieces, I was thinking of purchasing two of them and then laminating them together.

I don't think the panels should be 'laminated' together; simply placing them back to back with both held overall in a frame would achieve the same result. 'Laminating' them together could potentially present an impervious barrier to certain (high) frequencies passing through the panels. It'd also prevent the 'laminated' fibres from vibrating in sympathy with the sound passing through the panels and therefore reduce the amount of energy lost, which is exactly the opposite of what you're trying to achieve with absorption.

...I want to straddle each board across the corners and I might shoot for an inch or two off the wall, if that's possible. Cover all the boards with suitable material and I'm done. Is that a sensible plan or what ?

I'd say so. Given the very large thickness of absorption you'd need to truly tame bass, you should also consider using multiple subs and/or EQ. I suspect most folk would prefer to see/have multiple subs in a room than however many panels would achieve an equivalent effect.

Read this to see how Sploo made his own traps.

Hope this helps.

 

[Vaughan Odendaa]

Why? Why one trap? Whilst stacking traps on top of one another may not inspire confidence in terms of stability, neither does one panel standing twice as tall and at least with two panels you'd have the option of placing them separate from one another if at any time you wanted to.

Sorry, I was not clear before. I would be using two traps, one for each corner from floor to ceiling, straddling the corners. If I use a wooden frame to hold the rigid fibreglass in, is that enough to reflect HF back into the room ?

I assumed that one would need to bond a wooden panel behind the fiberglass to really have a good effect in terms of reflecting HF.

The quote above gives the impression that you're going to stick this paper onto a 'wooden board' of unspecified thickness. I don't think that's right.

The board would be thin. You mention paper. I''m not sure what you are talking about. Perhaps you could explain.

I don't think the panels should be 'laminated' together; simply placing them back to back with both held overall in a frame would achieve the same result.

Has this not been done before ? I was thinking about laminating two 50 mil thick boards together for a 100 mill thickness. So you are saying that this is not the correct way to go ?

Is there a way of laminating and still achieving the goal of reflecting some high frequencies back into the room ?

'Laminating' them together could potentially present an impervious barrier to certain (high) frequencies passing through the panels.

Oh, okay. Thank you very much for your advise. I hope you can elaborate on some of the points a bit more. Again, thanks.

--Sincerely,

 

[Vaughan Odendaa]

Anyone there ?

--Sincerely,

 

[Buckle-meister]

Anyone there?

Calm down. Some of us have lives outside of Audioholics.

I would be using two traps, one for each corner from floor to ceiling, straddling the corners. If I use a wooden frame to hold the rigid fibreglass in, is that enough to reflect HF back into the room ?

If you have a frame, it wont do anything apart from frame the traps. Are you meaning 'frame' or 'panel'. The former borders something whereas the latter is infilled.

I assumed that one would need to bond a wooden panel behind the fiberglass to really have a good effect in terms of reflecting HF.

I don't know. As I said, I don't have HF traps myself. However, I can say that when I ordered my own, I asked whether it'd be better to go for the standard or HF panels and I was advised that in normal circumstances, HF panels were not required.

From the RealTraps website:

...the HF type [of trap] trades slightly less low frequency performance for maximum absorption above 250 Hz.

Given that you're after bass traps, I'd suggest that reducing low frequency absorption by making your traps HF is the opposite to what you're trying to achieve.

The board would be thin. You mention paper. I''m not sure what you are talking about. Perhaps you could explain.

Refer to thumbnail. As far as I am aware, the silver backing paper is a reflector of high frequencies. Could be wrong though. Wouldn't be the first time. Wont be the last.

I was thinking about laminating two 50 mil thick boards together for a 100 mill thickness. So you are saying that this is not the correct way to go ?

Not at all. If the panels are thicker, they'll absorb to a greater extent without a doubt. All I'm saying is that there'd normally be no need to laminate/glue/whatever panels together if they were held firmly in a frame. Since you're planning such a tall trap however, this may not apply.

 

[Vaughan Odendaa]

Calm down. Some of us have lives outside of Audioholics.

I am calm.

If you have a frame, it wont do anything apart from frame the traps. Are you meaning 'frame' or 'panel'. The former borders something whereas the latter is infilled.

I guess what I am unsure of is the paper backing. Some people use foil but I'm not sure where to apply it or how much to use.

Given that you're after bass traps, I'd suggest that reducing low frequency absorption by making your traps HF is the opposite to what you're trying to achieve.

But if I stack these large bass traps in my room corners, won't they also absorb a lot of high frequencies as well ? Unless I do something to prevent the traps from doing so and creating a very dead room. This is something that I don't want. I want a relatively lively atmostphere.

As far as I am aware, the silver backing paper is a reflector of high frequencies. Could be wrong though. Wouldn't be the first time. Wont be the last.

Silver backing paper ? What exactly is it ? I appreciate the time taken to help me with some of these issues. Thanks a lot.

--Sincerely,

 

[Buckle-meister]

I guess what I am unsure of is the paper backing. Some people use foil but I'm not sure where to apply it or how much to use.

If the paper backing on the thumbnail is the means of reflecting high frequencies then as far as I am aware this comes already attached to the rigid fibreglass when you buy it. Send a PM to Sploo as he'll know for sure.

...if I stack these large bass traps in my room corners, won't they also absorb a lot of high frequencies as well ? Unless I do something to prevent the traps from doing so and creating a very dead room. This is something that I don't want. I want a relatively lively atmostphere.

I do not believe you are in any danger of creating too dead sounding a room from two bass traps, even though they be from floor to ceiling.

You could try emailing Ethan Winer. He has always been very helpful in answering questions, be they customers or not.

 

[Scott R. Foster]

You don't need to laminate anything, just build a light weight timber frame, lay down your mineral fiber, and cover with a fabric you can breath through and staple to the backside.

You could make them ceiling height but most folks find +/- 2x4' units more handy.

Make them at least 100mm thick.

Shop for price - 3 lbs. semi-rigid fiberglass is your best bang for the buck in fiberglass... in rockwool get something about 50-100% denser [but rockwool up to about 140 kg/m3 works fine - the panels just get awful heavy]. Here's a link with a number of materials and absorption coefficients that while help you pick something from the local market:

http://www.bobgolds.com/AbsorptionCoefficients.htm

Don't get faced/foiled/papered/scrimmed panels and don't add backer board or glue the panels together - just plain old cloth covered mineral wool will work great and cost less.

Don' just do the corners of the room, treat the ceiling and wall reflection points also. Here's an explanation of early reflection treatment:

http://readyacoustics.com/index.php?go=home.early-reflections

Here's a description of such panels with links to acoustic measurements:

http://forum.studiotips.com/viewtopic.php?t=534

Here's a full corner wedge design... has better performance than a 4" panel but you use a lot more mineral fiber per Sabine:

http://forum.studiotips.com/viewtopic.php?t=535

 

[Vaughan Odendaa]

Don't get faced/foiled/papered/scrimmed panels and don't add backer board or glue the panels together

Could you please elaborate on this ? Why should I not go for faced/foiled etc ? I'm not sure.

Thanks.

--Sincerely,

 

[Scott R. Foster]

Don't get faced/foiled/papered/scrimmed panels and don't add backer board or glue the panels together

Could you please elaborate on this ? Why should I not go for faced/foiled etc ? I'm not sure.

Thanks.

--Sincerely,

Vaughan:

Several reasons:

1) scrimmed/foiled panels generally cost more then naked panels [not much but it adds up];

2) it is often difficult to get panels in the thickness desired - with naked panels you can simply stack thin ones to make thick panels - if you try this with scrimmed panels you either make a funky sandwich of scrims and absorbent, or you have to spend a lot of time skinning the scrim/foil off.

3) the broadband approach to acoustic treatment teaches that by using devices that are absorptive to all frequencies and leaving the room surfaces hard [no thin carpet] then such treatment will address all ills. Scrims and foils will not only reject HF absorption but exhibit resonant behavior and/or cause entrance impedance jumps in the gas flow properties of the panels - both of which cause absorptive peaks and dips in the performance of the devices. This is antithetical to the broadband approach.

4) Most factory applied scrims and foils are adhered all the way across the face of the panel - sometimes it is both glued and stitched - if you need to reject HF but want to still absorb LF - you can do so in a more predictable fashion by adding your own scrim [6 mil poly film or heavy duty aluminum foil will work]. In that case best practice is to use adhesive only on the edges of the panel.. leave the bulk of the scrim loose and free to vibrate. This will increase the acoustic mass of the scrim [the sound wave "sees" the mass of the scrim plus the air spring pocket between the scrim and the panel]. The result is a panel that rejects HF but still works quite well on LF and the resonate peak is and smooth and fat, as opposed to a somewhat more unpredictable and narrow peak.

5) Porous absorbers work because they have interstices - little holes - if you smear glue across a panel of mineral fiber you cover up all the little holes and vastly modify its absorptive properties. The same occurs if you add acoustically opaque backer panels [such as plywood or MDF] - the formerly absorptive panel ceases to be a useful for broadband absorptive treatment.

In summary - generally, these elements are not acoustically beneficial, and to the extent they might be in some circumstances, the factory made versions are not optimized for acoustic uses. Additionally, they all have costs in time/money to add to the panel - for most folks, in most applications, they are the equivalent of walking to town to buy gourmet sugar to pour in your car's gas tank.

 

[Vaughan Odendaa]

Readyacoustics ? I just looked at the link you provided. I also looked at the panels and I must say that they look very nice. But how would these panels differ from Ethan Winers, or the GIK panels ?

I ask because I'm not sure it's even worth the effort to build the traps if I can get a far better looking panel with excellent absorbing properties at slightly higher price. I mean, the price of these panels are pretty amazing. 3 4" broadband bass traps for like $250 or so.

That's nice.

--Sincerely,

 

[HiFi Jake]

3) the broadband approach to acoustic treatment teaches that by using devices that are absorptive to all frequencies and leaving the room surfaces hard [no thin carpet] then such treatment will address all ills.

Yeah, but most rooms need more LF absorption than HF and MF absorption.

Scrims and foils will not only reject HF absorption but exhibit resonant behavior and/or cause entrance impedance jumps in the gas flow properties of the panels - both of which cause absorptive peaks and dips in the performance of the devices. This is antithetical to the broadband approach.

But what I think you are forgetting is that the realtraps absorb HF and MF from the back of the panels, as it reflects off the walls. Doesn't seem to me that would change any entrance impedance on those, would it? And I seem to remember Ethan explaining somewhere that the membrane on the fornt of the traps makes the whole panel act as a damped membrane or something. Is there a reason that this behavior couldn't be acting to a certain degree together with the porous absorber properties of the fiberglass when you mount the traps in a corner, or with some space between the walls?

 

[HiFi Jake]

Readyacoustics ? I just looked at the link you provided. I also looked at the panels and I must say that they look very nice. But how would these panels differ from Ethan Winers, or the GIK panels ?

A buddy of mine put a set of ready traps in his home theater. He bought the bags and got the 703 locally. They were a bit if a pain in the rear to stuff the 703 in the bags, but they did help quite a bit. But they didn't look that great, and his wife hated them. So he ended up tossing them out and replacing them all with Minitraps to accomodate the WAF. The Minitraps looked so much better, and I was pretty surprised when I went over there one night that they actually sounded better than the readytraps! They really are a much more professional looking and sounding product. Everything in the low end just sounds tight, the imaging is really focused, and the room has a nice, open sound at the same time. Just sounds natural in there. I guess its kinda hard to explain.

So I ended up buying a set of realtraps myself. They really make a HUGE improvemtn. TOTALLY worth the investment, and it gave me a bigger improvement than any upgrade I've ever made to my system. Ethan was really helpful too! I'm definitely a really happy customer! Totally sold on the product. If you really don't have much money to spend, then the readytraps will help a lot. But IMO acoustic treatment is the LAST thing you should cheap out on. Doesn't make sense to spend a lot of money putting together a really high quality system and then don't fix your acoustics so you can actually hear the benefits of the money you spent on your electronics!

 

[Vaughan Odendaa]

Good points. The only thing that concerns me is that I could get three Readytrap bass traps for the price of just one Realtrap. Are you trying to tell me that a realtrap can absorb more than three readytrap broadband bass traps ?

About the look of the bags. I was actually considering this as well but now you say that they don't look good ? I saw the video on the site and the bags actually looked very nice. So I don't know.

And if I decide to use 4" (100 mill) or more rigid fiberglass in each bag, I think that would be a lot of absorbtion. Unless I'm wrong.

--Sincerely,

 

[Scott R. Foster]

Yeah, but most rooms need more LF absorption than HF and MF absorption

Most rooms need both... and a good bit of it... are you suggesting that broadband absorbers are not appropriate for most rooms?

If so, that is a novel suggestion indeed, as such devices have been succesfully used for acoustic treatment for many years by professionals in all manner of locations.

But what I think you are forgetting is that the realtraps absorb HF and MF from the back of the panels, as it reflects off the walls.

No... I know exactly what a Mini-Trap will do and how it compares in performance to an upholstered panel of mineral fiber... FWIW I also know how RPG, Aualex, Ready Acoustics and GIK absorber products perform... and could discuss that matter at length... but I fail to see what this has to do with the topic [namely - how to make home made porous absorbers].

Any porous absorber panel open on all sides - such as the design described above -will present an absorptive edge along its backside to the wall. And if you don't close the sides in with a boxed frame [as is done on the Mini-Trap] then these edges will also present an absorptive surface to the room's air volume.

As explained above, best practice is to NOT cover any side of the mineral fiber with an acoustically opaque panel. You paid for the entire panel.. use all 6 sides. it is illogical to do otherwise.

Doesn't seem to me that would change any entrance impedance on those, would it? And I seem to remember Ethan explaining somewhere that the membrane on the fornt of the traps makes the whole panel act as a damped membrane or something. Is there a reason that this behavior couldn't be acting to a certain degree together with the porous absorber properties of the fiberglass when you mount the traps in a corner, or with some space between the walls?

I am not sure what you are asking/saying here - but if the question/point is about whether covering mineral fiber with other materials really does change gas flow properties - then the answer is YES... just as common sense would suggest.

Even a breathable fabric added to a mineral wool panel will cause some impedance jump. If you want to call that a "constrained membrane", you can correctly do so, but you are likely to cause more confusion than understanding. Better to try and focus on the causes and results than wallow in the semantics IMO.

Here is a graph of absorption measurements showing a corner mounted "naked" 4" 703 panel versus the same panel with a Ready Bag as upholstery - note the impedance jump around 100 Hz.

As you can see - even a naked panel already has more LF absorption than HF. Adding elements which cause a greater imedance jump than a breathable fabric layer will cause a more radical peak - if you want a sharper peak and HF rejection, then causing such is trivial. But, radical suppression of Hf absorption is not a desirable attribute for absorptive treatment for a crtitcal listening environment in a typical small room [less than about 5,000 cubic feet].

It is possible, in the event of significant lop sided absorptive properties in the room's fixtures and finishes, that such could be desirable, but you would be well served to confirm that need through measurement [or at the very least a comprehensive model of the absorptive qualities of the room's contents and surfaces] before embarking on an installation armed only with your presumptions.

In any event, if that result is desired, the methods described above will accomplish HF rejection, and for the reasons explained above you'd be better off not using panels which do have factory applied scrims [in short, they are not optimized for acoustic purposes].

 

[Scott R. Foster]

Good points. The only thing that concerns me is that I could get three Readytrap bass traps for the price of just one Realtrap.

Yes, that is the approximate price differential... depends on your location... Ready Traps are priced with "free shipping" [shipping is included].

Are you trying to tell me that a realtrap can absorb more than three readytrap broadband bass traps ?

If that is what Jake is saying, he is wrong.

In fact the ratio works in the other direction - per the manufacturers' published specs the difference is approximately 1.5 to 1 in favor of the 4" thick RT424 [primarily becaue a Mini-trap is only 3" thick].

As to the issues of HF rejection previsously discussed - the absorption curves for the two devices in the reccomended corner mounting overlay with significant uniformity once adjusted for the specified performance disparity [though the MT is a bit less absorptive at 10k Hz, and a bit more "peaky" at 100 Hz]:

sources:

http://readyacoustics.com/index.php?go=home.acoustic-data

http://www.realtraps.com/data.htm

About the look of the bags. I was actually considering this as well but now you say that they don't look good ? I saw the video on the site and the bags actually looked very nice. So I don't know.

Well I do! I think they look great and I question Jake's taste in who he takes on as a "buddy" - and i further question that guy's taste in women.

Jake:

But seriously, what's your buddies name? I'd like to contact him to discuss the matter. We have sold thousands of Ready Traps and have never gotten a return from anyone who disliked the looked. Nor has anyone ever reported an inability to assemble panels using our Ready Bags. If this "buddy" of yours actually had the experience you describe, we never heard from him, and I would like to.

Vaughan:

If you can make your own bed, and follow the below linked instructions you can make your own panels using Ready Bags:

http://readyacoustics.com/index.php?go=support.bag-installation-video

or if you prefer, we sell finished panels... here are a number of pictures of these panels in place:

http://readyacoustics.com/index.php?go=artists.home

And if I decide to use 4" (100 mill) or more rigid fiberglass in each bag, I think that would be a lot of absorbtion. Unless I'm wrong.

You are correct - whether you buy factory-made or build your own, whether you use fiberglass or rockwool, you can count on getting significant absorptiion from 4" thick corner mounted mineral fiber panels.

Upholstered mineral fiber broadband absorbtion panels work. Avoid the pitfalls of funky scrims and framing in the back and sides of the panels and you will get great results.

 

[Vaughan Odendaa]

I've sent an email to the guys at ready acoustics for further information. Thanks Scott. Excellent information all round.

More questions will follow soon. Again, thanks for the info and the graphs.

--Sincerely,

 

[Vaughan Odendaa]

If I decide to use more than 4" thick rigid fiberglass, like, for example, 6" thick, could the bags support that ? How thick could I make the panels to use with your bags ? Must I use standard sized cuts or are there specific cuts that I need to make in order to fit in the bags ? Sorry for all these ridiculous questions.

I live in South Africa. So I don't think that shipping will apply to me.

--Sincerely,

 

[Scott R. Foster]

Vaughan:

We have 6" thick [150 mm] traps and bags - they are 24x48" - contact us and I'll work with you to use locally available mineral fiber. Or, just do as described above and build light weight rectangtular frames and upholster with a breathable fabric.