Bass Traps that do just that!

[Riddler]

Hello guys,

I've been reading loads of the threads about bass traps over the last few days but it seems that most people who are DIYing these "Bass Traps" are actually building broadband absorbers. I'm interested in building some bass traps but really don't want to end up with an overly dead tracking room caused by too much HF absorption.

There has been some talk of putting cardboard or paper over the fronts, but for purely aesthetic reasons I'm wondering if 4mm plywood on the front will prevent the highs from being absorbed and look better. In this thread the idea seemed to be a tutt but why? Does this lower the efficiency of the bass trap? I can always add broadband absobers to take care of the highs after I've sorted out the bass can't I?

Ethan, Glenn, Jason, Jay your advice is greatly valued!

Tim.

[avare]

Your comment about bass traps being full range absorbers is 100% correct. To make true bass traps, you cover the front with a low pass filter. The best article I now of for this is the BBC RD report 1992-11 linked here.


http://downloads.bbc.co.uk/rd/pubs/reports/1992-11.pdf

They use 20% perforated 4 mm panel.

Enjoy!
Andre

[Slaytex]

Leave the FRK facing on the front of the traps as this will reflect the high end back into the room and will aid in absorbing more of the low end. All corners of my room have the FRK facing on the front, but all my first reflection points do not.
Hope this helps.

[bpape]

FRK/FSK is an easy and reasonably priced way to do what you want. It's still a broadband bass absorber but it pretty much cuts in half the absorbtion from 7-800Hz up.

If you want something even more targeted, then there are options for different types of membrane absorbers both damped and undamped. There are also even narrower types such as Helmholz resonators.

In most rooms, broadband bass control is the best bet IMO unless the room is really really big.

Bryan

[Demian]

Hi to all!! kind of noob here!


Guys need to do some Bass traps too. I have a rectangular room of 5Mx4Mx2.3M and I was thinking to do glasswool triangles (50Kg) to cover each corner but dont know if is really neccesary to do it from floor to ceiling.
I´d prefer to make them with 1M height and put it at the top of each corner. So what do you think ?
oHh I dont intend to steal the thread sorry

Thanks,
Demian

[Riddler]

Oi Demian! Leave my thread alone!

Thanks for the comments, I'm not sure that the FSK trick will help me because I'm in the UK and 703 doesn't seem to be available over here. They may be an alternative but I'm not sure.

Anyway, let me clarify what I think I need. I do want broadband bass absorption down to as low as I can get but I don't want absorption to extend into the higher frequencies. I'm specifically interested in what frequencies will pass through 4mm plywood. This is because I can then use loads of plywood fronted bass traps to control *only* the bass and then use a smaller quantity of mid/high absorbers. I'm concerned that if I just slap 12 or so full range absorbers up in the room then goodbye lively sound, goodbye nice RT60, hello boring dead room.

Thanks Andre for that link, I'll read over that properly later today but from first skim read it looks like I'm after somethink similar to the BBC A2 absorber. Looks like a really interesting report, gotta love the good old BBC!


Tim.

[Disjointed]

i think they recommend that around 15% of your walls to absorption (?).. if this is the case then a broadband absorber in just the corners probably wont even get that close.... (in my room the bass traps equal about 9.5% of wall surface coverage, 18x24 room 2' traps)

more of a 2 for 1 deal as i see it... why build separate absorbers when you can have them both together....

just my 2c

gl

[Ethan Winer]

Tim,

Originally Posted by Riddler

most people who are DIYing these "Bass Traps" are actually building broadband absorbers. I'm interested in building some bass traps but really don't want to end up with an overly dead tracking room caused by too much HF absorption.

Bass traps in "typical" small rooms should be broadband rather than tuned. As others suggested, cardboard or FRK paper does a good job of reflecting mids and highs while still absorbing the lows. Or use a thin perforated cover as Andre suggested. But don't use thick solid plywood.

--Ethan

[lwr]

i have 8 asc tube traps and they are absoultely amazing. you can really hear the difference if you put them near a kick drum, really tightens the sound up. you can turn these around to either reflect or absorb high freq too.

[avare]

Riddler:

In no particular order, adding and responding to posts so far:

Experts

There at least two other acoustics experts writing in this thread. There may be more, so I won't name the ones I know. They know who they are. There is lots of excellent advice in this thread.

703 in UK

703 is the Owens Corning name for 48 kg/m^3 glass fibre insulation. There is nothing about it and of itself. Mineral wool insulation with sinilar densities will work just as well. Rockwool (Roxul), Knauf and numerous other companies sell equivalent product in the UK. Look at price.

FRK is insulation with a heavy paper glued to on side. Again, look for generic equivalents from other companies and what it is known as in the UK.

High frequency cut off

One of the great things about the BBC RD reports is the vast majority of what is written is supported by test data (thank Aunt Beeb). The high frequency cutoff of the absorber in 1992-11 is shown in figure 16, with and without the covering.

A2 Absorber

The main details of the A2 design are in "Guide to Acoustic Practice" available at this link:

http://www.bbc.co.uk/rd/pubs/archive...icpractice.pdf


Good luck and let us know how things turn out!
Andre

[squeegybug]

Originally Posted by Riddler

it seems that most people who are DIYing these "Bass Traps" are actually building broadband absorbers. I'm interested in building some bass traps but really don't want to end up with an overly dead tracking room caused by too much HF absorption.

Exactly my experience with cheapie fiberglass panel 'bass traps'. Killed all the midrange and treble, did very little for the bass. Weirdest sounding room I had ever heard, it was terrible for singing and playback. I could not stand it and immediately ripped them out.

I put in RealTraps MiniTraps and ahhh
Those are the real deal. My room is such a pleasure to just walk into now, very peaceful and smooth sound, still enough life to sound 'right'. And the measurements confirm it.

Steve

[Riddler]

Hi all,

That BBC acoustic practise paper is AWESOME! It's 160 pages or something and goes into really good detail.

I'm probably going to end up building some bass traps with a 4mm plywood face that has 20% cutout with 4" fibreglass behind.

Does anyone know anything about the process of 'furring out' fiberglass? I've built several walls before and never tried it but my trusty book tells me its a good idea.

I've also seen some interesting graphs about fiberglass density, basically saying that the density makes a tiny difference in comparison to the thickness of the fiberglass and the air space behind. I'll scan and post the graphs if anyone's interested.

One last thing slightly off topic; who's here has heard of acoustic scaling? The BBC were investigating this but I've not had time to read all the research so far.

Tim.

[avare]

the density makes a tiny difference in comparison to the thickness of the fiberglass and the air space behind.

Correct.

who's here has heard of acoustic scaling? The BBC were investigating this but I've not had time to read all the research so far.

An incredibly vague question. There are several BBC RD reports on acoustic scaling. It has become somewhat reduced in use as the increasing power of computers make various analysis/tools affordable. It is still used but mostly on big budget projects.

You seem to be getting into the research alot. BBC has many RD Reports online (and free!) Rather than link one or two reports at a time, here is the link to the index. There are at least two dealing directly with acoustic scaling.

Reports 70-96

Andre

[elektroglide]

does anyone know of a cheap simple substitute for the FRK on 703? i've just received a load of rigid glassfibre for a 40m2 room (7mx5.8mx2.9m), but they have omitted to include the requested covering on 30 of the delivered 60m2 of glassfibre allocated to walls and ceiling. i'm guessing there are several alternative coverings which could be applied to the panels i have to give a similar bass absorption/hi reflection to the FRK on 703?

cardboard is mentioned above - what thickness and type works well? if corrugated cardboard as used in supermarket boxes works, then that's a free solution to put in front of normal rigid glassfibre panels.

any advice would be gratefully received!

[Ethan Winer]

You can spray glue heavy paper to the fiberglass.

--Ethan

[elektroglide]

thanks ethan! i was trying not to bother you any more - you've just answered two emails regarding corners and airgaps for me!

i really appreciate your help

[mous]

Originally Posted by Ethan Winer

Tim,



Bass traps in "typical" small rooms should be broadband rather than tuned. As others suggested, cardboard or FRK paper does a good job of reflecting mids and highs while still absorbing the lows. Or use a thin perforated cover as Andre suggested. But don't use thick solid plywood.

--Ethan

would a tight weave fabric does the job? as in not air couldn't penetrate through.

[Glenn Kuras]

Originally Posted by mous

would a tight weave fabric does the job? as in not air couldn't penetrate through.

Yea that could work.

Glenn

[elektroglide]

if thick paper works, does it have to be bonded to the glassfibre panels or could the paper - in the case of ceiling suspension with netting - simply be placed on the netting with the glassfibre sheets resting on top of it?

i'm unsure as to whether the membrane needs to be physically coupled to the absorbent/dampening material or not.

if not, then i can continue work tomorrow because i've run out of glue and the shops are shut!

also, does it make any significant difference if the paper/membrane is gloss or matt??

[Ethan Winer]

Originally Posted by elektroglide

if thick paper works, does it have to be bonded to the glassfibre panels

Yes.

does it make any significant difference if the paper/membrane is gloss or matt??

No.

--Ethan

[elektroglide]

i'm in the middle of building a corner trap with sides 3' x 3' x 4.5' (approx), floor to ceiling (around 11ft high). the drum kit will be on a plinth/riser immediately in front of this (it's a rehearsal/recording room - the less than optimal drum position is because of room layout/space usage considerations).

i'm wanting to kill lots of bass, but also avoid excessive highs/mid reflection back at the drummer's head. would it be effective in this one trap to have a combination of frk (equivalent) and plain glassfibre surface area to broaden the absorption properties?

the glassfibre is 8" thick with air void behind (see diagram), and i'm thinking of only partially covering the front surface with paper glued to the glassfibre (maybe a central vertical strip).

will this give the desired effect (broadband absorption including decent bass absorption) or is it better to just cover the whole front surface with paper? i just don't know the mechanics of partial/full membrane dampening.

i hope this is the last question i have. maybe a degree in acoustic engineering would help me... but for now, thanks in advance for any help!

[Ethan Winer]

Traps in corners benefit from a reflective surface, and traps at reflection points (and sometimes the rear wall) are better without. No need to mix materials in a single panel.

--Ethan

[bwo]

Older thread, but I'm throwing in a question here.

How will 1-3 cm (0.39-1.1") plastic compare to FRK/FSK material?

[Ethan Winer]

^^^ What are you asking? FRK is basically paper, like the brown paper used for grocery bags.

--Ethan

[bwo]

Originally Posted by Ethan Winer

^^^ What are you asking? FRK is basically paper, like the brown paper used for grocery bags.

--Ethan

Sorry. I'm wondering how 1-3 cm thick plastic used as membrane (reflecting of mid and highs) will compare to something like this:
osCommerce

[Ethan Winer]

Do you mean cm or mm? That's my entire confusion.

--Ethan

[DanDan]

For UK and EU readers.
ISOVER Duct Slabs are 48KG and have a foil membrane.
=703


DD

[bwo]

Originally Posted by Ethan Winer

Do you mean cm or mm? That's my entire confusion.

--Ethan

Edit: 1-3 mm.

[DanDan]

bwo, 3cm thick would be a very heavy stiff plastic board. FRK looks less than 1mm to me.
DD

[bwo]

Edit: When I tried to tighten plastic that was a little over 1 mm thick the way the pictures shows, the absorbent panel became a bit bulky. This time I'm planning on making a visible frame and tighten the fabric and plastic another way.

But the question is how will thick plastic that are 1-3 mm reflect and what frequencies will get a hump? My goal here is only bass absorption. It's easier to glue some FRK to the rock wool I assume, but will thick plastic reflect more?