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Quick questions on bass traps

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Quick questions on bass traps

Postby new sonic arts » Wed Jan 05, 2005 1:42 pm

Hi again everybody

Some of you may have read my previous posts about soundproofing a dedicated studio room. Ive regrettably come to the conclusion that basically this is much much more trouble than its worth, given that the room faces onto a busy street, with big bay windows, and I cant do any alterations to the flat as its rented.

So instead, I have decided to port my studio to my bedroom. Ill give up my living space... but at least its quiet, and isolated from the busy road at the front of the flat. Its a smaller room, 3.65m x 2.62m.

So my focus now is acoustic treatment. It currently has very bare walls with nasty boing, which I will treat by covering 80% of the walls with thick duvets. But I also plan to put in some absorber-type bass traps (made from mineral wool slabs).

I've read a lot of stuff on bass traps, but there seems to be a bit of a lack of consensus, so I have some questions.

1) front 'membrane'?
How important is it to have a front membrame/skin, seperated from the actual mineral wool slabs? most of the stuff ive read has mentioned having an outer skin, but then some diy projects ive seen online are essentially slabs wrapped in fabric - no distinct membrame. From my understanding (not neccesarily from what ive read) the purpose of a panel would be to:
a) target a specific frequency, (depending on the panel size)
b) stop reflections, and promote energy to pass into teh trap.

im not interested in (a), i want to make a broadband to cover as much low and frequency as possible, but obviously i want to do (b).

i was thinking of just wrapping the rockwool slabs in soft cotton sheets, i cant imagine that this is going to reflect back into the room? but to be safe, perhaps i could put 1" thick wood strips around the outer edge of the slab, to cause a gap of about an 1" between the cotton sheet and the slab. is this sensible? also i guess i should avoid pulling the sheet too tight as this may promote reflection of lower frequencies???

2) panel thickness and width
the slabs i will buy are 1200mm x 455mm. depth either 100mm or 80mm. is 455mm wide enough? or do i need 2 next to each other (ie 910mm)? what depth is good? i was thinking using 2 x 100mm to create a 200mm depth. or would 2 x 80mm be sufficient? or what about using only one 100mm???

3) how many panels?
i was planning at least 1 trap in each corner or the room, and then possibly another 2 on the wall directly in front of each speaker, as that is where the initial energy will be focussed. is this overkill? or not enough..?

as ever i look forward to reading your helpful replies,

many thanks

james

ps ok so that wasnt so 'quick' was it :)
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Re: Quick questions on bass traps

Postby Martin Walker » Wed Jan 05, 2005 3:31 pm

1. Wrapping the slabs in cotton sheets won't cause reflections, and you won't need to space the cotton from the slabs - it's primarily there to prevent particles from the slab getting into the air and being breathed (they are an irritant, and you should use a breathing mask and gloves, or even a disposable boiler suit when working with the raw slabs).

2. 100mm is a typical depth used since it doesn't take up too much of your room, but if you've got space to have a deeper assembly then it should absorb down to lower frequencies (if you're placing some slabs in the most recommended place across the corners or wall/ceiling corners then this is the place where you can add more depth without intruding further into the room). Spacing thinner slabs away from the side walls by a few inches also helps in this respect.

3. The four corners positions are a good start, but you'll also benefit from side and ceiling panels at the 'mirror' points' (sit yourself at your normal listening position, and get a friend to move a mirror down each side wall and on the ceiling in front of your head - where you see reflections of your speakers are the mirror points where sound bounces off and arrives at your ears slightly later than the direct sound). More on the back walls will help control front/back reflections, and generally, the more slabs the better, as this should further flatten your frequency response at the low end. However, if you fit too many you may find you've deadened the life out of the mid and high frequencies, which is why side panels are sometimes fitted with membranes to reflect back frequencies over a few kilohertz.


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Re: Quick questions on bass traps

Postby new sonic arts » Wed Jan 05, 2005 5:11 pm

Thank you Martin that is really really useful, and has cleared up a lot of confusion for me.

However, if you fit too many you may find you've deadened the life out of the mid and high frequencies, which is why side panels are sometimes fitted with membranes to reflect back frequencies over a few kilohertz.


Thats an interesting comment, as i hadnt really given any thought to the possibility of over kill. i expect i might totally kill the high frequencies anyway by covering the walls in duvets. so is a dead room always bad? or is the live/dead thing more an issue of personal taste? and is it the case that a dead room is only undesirable for recording (where i can appreciate a bit of natural reverb is desirable) or would you argue is it undesirable for mixing too? please explain!

anyway, many thanks again
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Re: Quick questions on bass traps

Postby Scott R. Foster » Wed Jan 05, 2005 8:53 pm

Generally, the smaller the room the less reverb you want... too much reverb and the rooms rings out and "colors" the music... small rooms are more prone to this fault. Your average bedroom is so small it will need a lot of absorption to avoid this problem.

You can model the Sabin content of a room's finishes and fixtures and design same to provide for a smooth capture of all frequencies. For example avoiding wall to wall carpet is generally a good idea as it will suck up your highs and leave the troublesome LF resonances standing out even more.

Tuning absorbers is tricky [with membranes or otherwise] and can get both frustrating and expensive. An easier path to Buddha is to avoid narrow band absorptive finishes and treatments altogether... so that such absorption as is added works at all frequencies.

Also, capturing HF is trivial.. LF is not... and the corners is the most efficient place to catch the LF stuff... so most folks start there and work there way out to the mirror points [to diminish early reflections]... and then put some diffusion on the back wall... and then give it a listen... need still need more? Then add more broadband absorption until the room has a nice tight sound.

If you overdo the absorption your room gets more dead than need be... but, in a room of about 1,000 cubic feet this is not likely. For an example of a small room that went a bit to far and had to be brought back to life a tad with some reflective panels see:

http://forum.studiotips.com/viewtopic.php?t=456&start=0

You won't get that far by mistake... but I gather the results at the above linked project are a small room that works at very high SPL's, uncolored, sweet sounding and precise.... not trivial.
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Re: Quick questions on bass traps

Postby new sonic arts » Wed Jan 05, 2005 11:59 pm

thanks foz... its comforting to read your thread as that confirms my own intuitions, ie: 1) in a small room i should worry about getting rid of reflections far more than preserving them; 2) tuned absorbers are a non-starter for a diy project with crude materials and no measurement devices.

i skimmed/waded through that huge thread for about half an hour, and i now feel partly depressed given the huge amount of effort that guy went to, but also partly encouraged, as i would never have made the mistake of putting down a laminate floor. so i must have learnt something from all the stuff ive been reading over the last few weeks... :)

one more question: in a room of my dimensions, would it be total madness to put the speakers against the long wall, that is, so that they are facing a wall only 2.6m away? is that gonna be much worse than a distance of only 3.6m?
thing is it makes my life much much easier (given the position of door, built-in cupboard etc). but then room is so small that maybe the reflections will be all over the place anyway, so maybe it wont make much difference?
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Re: Quick questions on bass traps

Postby Scott R. Foster » Thu Jan 06, 2005 1:08 am

You pose an interesting question... consider that it aint the depth of the room in the two dimensions that matters.

What will the distance from your head to the "rear" wall be in the two configurations?

I think the difference of those two distances [head to wall] is the critical fact.

Also, could a giant horizontal poly on the rear wall help in thise case? Maybe... worth a try IMO.
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Re: Quick questions on bass traps

Postby new sonic arts » Thu Jan 06, 2005 10:29 am

well the distance between my head and the rear wall would be greater if i put the speakers along the short wall, so that they aim down the longer walls. its just about workable in practical terms too...

um.. excuse my ignorance, but whats a giant 'poly'? :)

more bass trap questions...
- to avoid tons of hassle making wooden frames and then covering in hessian etc, im thinking of wrapping the slabs in clingfilm. mounting them on the wall, then either dropping a thick duvet, or just a sheet of hessian in front of them. i realise cling film is a bit reflective, but its not going to reflect any mid or bass is it? (which are my anticipated problems, i can deal with HF easy)

- im still a bit confused about the importance of depth. if i mounted the either the same overall volume of 100mm slabs on the wall (ie with some gaps) or 75mm slabs on the wall (ie no gaps), would the overall absorbancy be the same? (i hope so, more room space and its quite a bit cheaper)
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Re: Quick questions on bass traps

Postby thefruitfarmer » Thu Jan 06, 2005 11:01 am

James

I would suggest you take a "suck it and see" approach. Rockwool slabs wrapped in fabric and fitted in the corners of the room will tame the bass reflections. Your room is similar in size to mine, but will probably need less treatment as my room is a cube...

What you could do is just get four Rockwool slabs and then see if they have the desired effect. Then you can add more if necessary.

Something you may wish to consider is making the bass traps portable as I understand the place you live in is rented and you will move out at some point.

Have a look at my studio pictures and the thread where there is some discussion as to the benefits and drawbacks of my DIY system. I did design it so I could take it all down and set it up somewhere else without too much hassle.

Oh yeah, give some thought to the colour of the fabric. Black is cool but hard to keep clean and it makes my room a bit dark and dungeon-like without some clever lighting.
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Re: Quick questions on bass traps

Postby new sonic arts » Thu Jan 06, 2005 12:20 pm

hi

yeah im definitely gonna make them portable, as i may well be moving out in may. ive been toying with the idea of making the corner traps in a wooden frame so that they can stand without any attachmetn to the wall. and stackable.

btw what thickness did you use on your slabs?

and also, where did you buy your fabric - that grey stuff of yours is pretty nice! anybody have any cheap recommendations for fabric??
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Re: Quick questions on bass traps

Postby Ethan Winer » Thu Jan 06, 2005 2:59 pm

James,

> would it be total madness to put the speakers against the long wall, that is, so that they are facing a wall only 2.6m away? is that gonna be much worse than a distance of only 3.6m? <

In a room that size you definitely want the speakers firing the long way down the room. In his REVIEW of my company's MiniTraps, Martin Walker actually tried it both ways, measuring each, and posted the response curves. Those graphs are shown in the sidebar The Long & The Short Of It.

The main issue, as Scott pointed out, is the distance between your ears and the rigid boundary behind you. Comb filtering is worse off the rear wall than off other boudaries, so you get a series of peaks and deep nulls at quarter wavelength-related distances. This is a big problem in all small rooms, where there's typically a huge null somewhere around 80-120 Hz due to listening 1/4 or 3/4 wavelength from the rear wall.

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Re: Quick questions on bass traps

Postby thefruitfarmer » Thu Jan 06, 2005 3:41 pm

Making the traps in a wooden frame sounds good.....

don't forget to post the pix when you're done.

You need 100mm RW slabs to trap the bass. I used RW6 density but people use RW3 (which is easier to get) with success.

Fabric you can get from dress making supply shops (full of lonely housewives) or upholsters. I used a grey denim, a blue stretch denim for a tight finish and black upholstery cloth for a neutral shade. You need cloth that is close weave enough to keep the RW fibres sealed but not so close weave as to reflect rather than absorb the high frequencies. I'm no interior designer but I reckon it's worth taking time to choose the colours (or finding a mate with a talent and an opinion) as it has considerable effect on the "feel" of the room.
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Re: Quick questions on bass traps

Postby new sonic arts » Thu Jan 06, 2005 4:32 pm

actually my girlfriend is an interior designer... god knows what she is going to make of this monstrosity once its finished :)

i keep toying with the idea of making a bass trap out of 3 pieces of 75mm rockwool, to form a right-angled triangle that fits into the corner. this could pretty much support itself if i made an appropriate top and bottom and then wrapped it in cloth. obviously there would be a gap in the centre of the triangle, and im not sure (please someone tell me) if i would lose something by not having the slabs face to face to get more solid depth...

to see what i mean, look at A vs B in... http://jameswalkerhall.netfirms.com/corners.htm

btw any comments on the cling-film idea?

ethan, thanks for your post - since reading that article my decision is simple!

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Re: Quick questions on bass traps

Postby Ethan Winer » Thu Jan 06, 2005 4:42 pm

James,

> to see what i mean, look at A vs B <

People ask "which is better" questions like this all the time, but the only way to know is to measure each way. If you have the ETF software you can stack the boards in all four corners one way, measure, then try it the other way and measure again. I don't know any other way to know for sure. My guess is that "B" will be slighty better because it seems to have a little more total surface area exposed.

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Re: Quick questions on bass traps

Postby Martin Walker » Thu Jan 06, 2005 5:40 pm

James - 'A' would be easier to hang like Ethan's Minitraps from a couple of screws in the wall (this is the way I've mounted my Minitraps), whereas 'B' is exactly the same as the DIY Studiotips Corner Absorber http://forum.studiotips.com/viewtopic.php?t=534, which as you say could be self-standing for easily removeable trapping in rented premises.

Yet another approach is to fill the void as with the DIY Studiotips Superchunk (let's call it 'C') http://forum.studiotips.com/viewtopic.php?t=535

I don't think there's any 'best way', although the deeper the material used the more effect it should have. However, personally I would have thought that 'B' would be more effective than 'A', while 'C' would be better than both, simply because there's more absorbing material being used.

BTW, I think clingfilm has been mentioned here before somewhere - the best idea is to have a covering that can let the air through (like cotton or hessian).


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Re: Quick questions on bass traps

Postby Scott R. Foster » Thu Jan 06, 2005 6:50 pm

Poly

A Poly is short for polycylindrical - an easy to make diffuser. ”Polys” are basically a sheet of plywood bent into an arc - but not a pure circle section - rather the sum of an infinite number of curves (thus polycylindrical or "many cylinders").

Image

Making a poly is simply a matter of building a frame that squeezes a panel of something like plywood into an curve. Squeezing the edges of the plywood cause it to bow out in a curve, but not a pure arc section [part of a perfect circle], but rather into a curve that is a part of a broad ellipse shape, and thus a concatenation of an infinite number of discreet pure arc sections. A pure cylinder would present a single curved surface to the sound waves that hit it - polycylindricals present many curves in one object and thus are more diffusive than pure cylinders.

http://mathworld.wolfram.com/Ellipse.html

The size of the device determines the lower limit of the frequencies which it will diffuse. A simple design for a poly is to take a full 4’x8’ sheet of 1/4” plywood bent along its long axis and stuffed into a rectangular frame built of 1x4’s. Heavier plywood could be used but will be very hard to bend (you might need to make kerfing cuts to get the bastard in place). The interior dimensions of the 1x4 frame should be less than the width of the panel, for a 4’x8’ panel try 47” wide (and 8’-2” tall - 2” longer than the panel - if you want to leave the ends free to wiggle). Put a pair of horizontal cross braces at the back of the panel and then squeeze the panel into the frame so that it bows out. To install just hang the panel from the ceiling or mount to the wall with the bowed side facing out.

For your situation I would flip the axis and try and make a poly with the bend through the short axis of the panel and then hang the long axis parallel to the floor and center the poly on the rear wall behind your head... to try and divert the rear wall reflection to the side and minimize the direct flat panel reflections back to your head off the rear wall and increase the ITD.

As why the long dimension, it aint that your absorption works better that way, its that the reflections off the rear wall are less critical the longer it takes em to get back to your head.

The goal is to control early reflections of sound so that the loudness of the direct sound from the monitors overwhelms the energy content of any reflected source arriving at the mix position during a critical time gap.

In the first few milliseconds of an impulse the ear and mind form enduring impressions of a sound’s character and location. This critical time gap during which the arrival of sound at the ear from reflected sources should be controlled is called the initial time delay or “ITD”. When multiple copies of the first impulse arrive at the ear during the ITD, the multiple sources “smear” the sound, making it indistinct, fouls up locational cues needed to place sounds in the stereo image, and generally renders the task of critical listening more difficult.

The direct sound from the monitors always arrives first because it has the shortest path. Reflected sound with a total path length of 22’ will show up at the listening position just about .020 seconds later. Sound arriving .020 seconds after the initial impulse, or later, is not perceived as part of the initial impulse. Too much sound arriving earlier than .020 seconds can cause problems especially if there is too much of it in the mid band (critical for taking locational cues).

The closer the rear wall, the tougher it is to keep the loudness of reflections during the ITD gap below troublesome levels. Bottom line... rear walls should be at least 11' behind your head... but you can't do this given your room size... but still, further away is better so try and get all you can. And a poly that throws a lot of the rear wall reflections to the sides as opposed to straight back at your head might help.
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