Questions on bass traps optimal density, edge effect, layered construction, etc.

[azzzy]

As so many before me I am thinking of building my own bass traps. My head is about to expload from the amount of info I've read in the past few weeks. Here are some questions I haven't found a definite answer for:

1. I am thinking of using Roxul Rockboard. It comes in various densities from 4 pcf to 8 pcf. From what I read I understand the denser the better. Is there such thing as too dense? What would be the optimum density for the absorber material for low freq absorber?

2. The edge effect. Since exposed edges of the absorber contribute to more absorption would it make sense to place two 2'x2'x6" squares (one above the other) with an air gap vs. one solid 2'x4'x6" absorber? What would be the optimal air gap? And if the above is true why not split it into 4 1'x2 pieces and place them one above the other with gaps between them? Anybody done that?

3. Placement. I read that the common space between the wall and the absorber is 16". Does the effectiveness of the low freq absorber diminish once it's moved farther from the wall? If not, would it make sense to place a couple of low freqs absorbers somewhere in the middle of the room (in addition to the absorbers in the corners, of course)? Or the 16" gap is the fartherst the absorber should be away from the wall?

4. If I have a bass trap in the corner does it have to touch the walls or is it better when it's farther out into the room? Can I place something relatively thin behind the bass trap (like a PA speaker stand - a 3" thick tripod)?

5. Different densities are optimal for absorbing different frequencies. Does it make sense to make an absorber layered with different densities? For example, using Roxul:

a. 4" Rockboard 80/4" Rockboard 60/4" Rockboard 40. The densest material is facing the room and is farther away from the wall, which is optimal for absorbing low freqs. The least dense is the closest to the wall but it's OK since the higher freqs have shorter wave lengths. Would this work or would the higher freqs not pass through the denser material to the less dense one?

b. 4" Rockboard 40/4" Rockboard 60/4" Rockboard 80. The least dense material is in the front. It absorbs the higher freqs and lets the lower freqs through towards the more dense material where they get absorbed. The higher density is closer to the wall though. Would this work better than the a. version above?

c. 2" Rockboard 40/2" Rockboard 60/4" Rockboard 80/2" Rockboard 60/2" Rockboard 40. The densest material is in the middle between the less dense ones. This allows it to be resonably far from the wall and at the same time not reflect the higher freqs before they get absorbed by the lower density materials. Would this work?


***OK, I understand this might be a lot of questions for one post. Perhaps I should have made each of these into a separate thread. Please let me know if that's your preference.

[PaulP]

Originally Posted by azzzy

As so many before me I am thinking of building my own bass traps. My head is about to expload from the amount of info I've read in the past few weeks.

I know the feeling.

Here are some questions I haven't found a definite answer for:

I'd say you haven't read enough. There should be no rush involved with
treating your room if you want to do a good job. Most if not all of your
questions have been answered in the last week or two. Some will no
doubt address them but I think you'd benefit more from further study.

You have picked up a few wrong ideas like "denser is better" and "different
densities are optimal".

I think reading threads in this forum and its sister Studio-building / acoustics
would be enough but if you really want to learn some detail see this thread
or get yourself a good book or two.

Paul P

[azzzy]

Originally Posted by PaulP

I'd say you haven't read enough. There should be no rush involved with
treating your room if you want to do a good job. Most if not all of your
questions have been answered in the last week or two.

Unfortunately there is a rush. I'm way behind schedule (like 3 months behind!) and I need to start recording soon or might as well scrap the whole project and get a day job.

Originally Posted by PaulP

You have picked up a few wrong ideas like "denser is better" and "different densities are optimal".

I think reading threads in this forum and its sister Studio-building / acoustics
would be enough but if you really want to learn some detail see this thread
or get yourself a good book or two.

Paul P

I saw your thread with the links to the BBC papers. Thanks for posting it. I will read all of it (and probably understand about 10% if I'm lucky) but for now I'd like specific answers to my specific questions if possible. As evident from my questions I already picked up some wrong ideas and I don't want to do something based on what I think is right only to find out later I have to re-do everything. Once again, thanks.

[Bubbagump]

Originally Posted by azzzy

1. I am thinking of using Roxul Rockboard. It comes in various densities from 4 pcf to 8 pcf. From what I read I understand the denser the better. Is there such thing as too dense? What would be the optimum density for the absorber material for low freq absorber?

See here: http://bobgolds.com/AbsorptionCoefficients.htm

Looks like at a 4" thickness, 3.5-4 pcf mineral wool is the sweet spot.

2. The edge effect. Since exposed edges of the absorber contribute to more absorption would it make sense to place two 2'x2'x6" squares (one above the other) with an air gap vs. one solid 2'x4'x6" absorber? What would be the optimal air gap? And if the above is true why not split it into 4 1'x2 pieces and place them one above the other with gaps between them? Anybody done that?

I wouldn't bother as that is a side effect essentially. You want to focus on having usable surface area.

3. Placement. I read that the common space between the wall and the absorber is 16". Does the effectiveness of the low freq absorber diminish once it's moved farther from the wall? If not, would it make sense to place a couple of low freqs absorbers somewhere in the middle of the room (in addition to the absorbers in the corners, of course)? Or the 16" gap is the fartherst the absorber should be away from the wall?

I would not consider that common at all. 16" is used for measurement purposes at times. Usually a 2-4" gap is a very good place to be to have excellent performance AND still be able to fit yourself and maybe a chair in the room.

4. If I have a bass trap in the corner does it have to touch the walls or is it better when it's farther out into the room? Can I place something relatively thin behind the bass trap (like a PA speaker stand - a 3" thick tripod)?

You don't HAVE to touch the walls (though you should be in a very close proximity to the corner), but for space efficiency sake, you typically do. You could place something behind it, but generally you want to go floor to ceiling, so the space is not usable in any event.

5. Different densities are optimal for absorbing different frequencies. Does it make sense to make an absorber layered with different densities? For example, using Roxul: ....

No. First, what you describe will not do what you are thinking it will. And even if it did, you want broad band absorbers, not tuned traps.

[azzzy]

Originally Posted by Bubbagump

See here: http://bobgolds.com/AbsorptionCoefficients.htm

Looks like at a 4" thickness, 3.5-4 pcf mineral wool is the sweet spot.

A great link. Thanks a lot.

Actually Roxul Safe seems to be performing the best at 4". But the table has no data for Roxul Rockboard 80 at 4". Besides I'm thinking of making my bass traps at least 6" thick, if not more. Any recommendations for the thicker absorbers?

Originally Posted by Bubbagump

I wouldn't bother as that is a side effect essentially. You want to focus on having usable surface area.

From what I understand this side effect is significant enough to affect testing data. I keep reading that you should expose as much of your absorber as possible. Why not go further and divide it into more pieces to create even more surface?

Originally Posted by Bubbagump

I would not consider that common at all. 16" is used for measurement purposes at times. Usually a 2-4" gap is a very good place to be to have excellent performance AND still be able to fit yourself and maybe a chair in the room.

I am not too concerned with space. I just want to know the optimal distance from the wall and then see if I can make it happen.

Originally Posted by Bubbagump

No. First, what you describe will not do what you are thinking it will. And even if it did, you want broad band absorbers, not tuned traps.

Can you please explain what it will do? How is it a tuned trap? I think (and I might be wrong, which is why I'm asking these questions) a layered trap would be more broadband than a single density one. By the way, I am more interested in absorbing the lows. I will have diffusors to take care of the mids and the highs.

[PaulP]

Originally Posted by azzzy

Unfortunately there is a rush. I'm way behind schedule (like 3 months behind!) and I need to start recording soon or might as well scrap the whole project and get a day job.

That is a problem . The thing is, in my opinion, treating a room is not
something that can be rushed. I've spent many months studying this stuff
and I don't feel comfortable enough to start myself. The more you read up
on acoustics the more you realize that everyone has a different opinion.
They all make some sort of sense even though they're often completely
contradictory.

This is why I don't trust myself to answer your questions directly, even
though I have some idea of the general concensus on the answers. You
haven't provided any information on the size, shape and construction of
your space nor what you want to do in it. What treatment you need
depends heavily on this as well as your own preferences regarding what
kind of sound you like.

Any direct answers you get to the type of questions you've asked so far
may be completely wrong for your needs. You're talking absorber details
when you/we don't even know if you need any (though several people
around here will tell you to fill your room to the ceiling with absorption and
be done with it).

...As evident from my questions I already picked up some wrong ideas and I don't want to do something based on what I think is right only to find out later I have to re-do everything.

Which is why my suggestion remains that you study enough to get to the
point where you have a good idea of what you need given your requirements.
To do this you'll need a lot more knowledge on the different types of
treatment and how they're used in different situations. One good way to
get this is to read about a lot of other people's experiments. None will be
exactly like yours but you'll see the trends be able to predict something
more to your needs. You're the only one who can do this (unless you hire
a professional and bring them to your place). Your head will hurt but it
would be worth it.

In the meantime, do give us more information on your space and what you
do in it. That is of interest to all.

Paul P

[Bubbagump]

Originally Posted by azzzy

From what I understand this side effect is significant enough to affect testing data. I keep reading that you should expose as much of your absorber as possible. Why not go further and divide it into more pieces to create even more surface?

Edge effect is described in articles primarily to explain why testing data can have values greater than 1. ( NRC of 1 = complete absorption, >1 is impossible in real life). So go for surface area in the room.

I am not too concerned with space. I just want to know the optimal distance from the wall and then see if I can make it happen.

Old rule of thumb for broad band traps is that the optimal gap is equal to the panel's thickness when mounting the traps flat to the wall.

Actually Roxul Safe seems to be performing the best at 4". But the table has no data for Roxul Rockboard 80 at 4". Besides I'm thinking of making my bass traps at least 6" thick, if not more. Any recommendations for the thicker absorbers?

The thicker the trap the less density you want to have. However, at 6", you will still be fine in that 3-4pcf area. If you do SuperChunks you may want to use even less dense like 2.5 pcf... but we're talking getting the last drop of blood out of the turnip here. Use whatever it is you can find locally that is in that density range.

Can you please explain what it will do? How is it a tuned trap? I think (and I might be wrong, which is why I'm asking these questions) a layered trap would be more broadband than a single density one. By the way, I am more interested in absorbing the lows. I will have diffusors to take care of the mids and the highs.

It won't be a tuned trap. Panels as you described are broad band absorbers. What it will do is cause you to buy 6 different materials and see no acoustic advantage. Look up posts from Avare and where he describes gas flow resistance. Again back to Bob Gold's.... A 4" thck panel of RHT40 has a NRC of 1 across the measured spectrum. Why change up the materials when you are already absorbing everything? There is no advantage.

You don't mention the size of your room, but if I had to venture a guess based on past experiences with small rooms, you will likely be better off with more trapping and no diffusion especially for someone with limited experience.

[avare]

Originally Posted by Bubbagump

Old rule of thumb for broad band traps is that the optimal gap is equal to the panel's thickness when mounting the traps flat to the wall.

There is no validity to that myth. Ratios up to 2:1 are used in critical designed spaces. Ratios up to 20:1 are used in office spaces. It is also dependent on the acoustic properties of the material.

Andre

[azzzy]

Originally Posted by avare

There is no validity to that myth. Ratios up to 2:1 are used in critical designed spaces. Ratios up to 20:1 are used in office spaces. It is also dependent on the acoustic properties of the material.

Andre, nice to see you in my thread. Any answers/comments to the questions in the original post? Thanks.

[azzzy]

About my space. To be honest I felt my questions were more or less on the properties of the material used and somewhat independent of the space. I kinda wanted to get "unbiased" answers, which I could then interprete for my space. But OK. My space is about 24' x 24' x 7'. Unfortunately it has to have functions other than recording space as well. Which is why my arrangement might be quite unorthodox (asymmetrical to say the least). It's oddly shaped in both horizontal and vertical planes: there is a nook under the stairs which creates 2 extra inside corners and the ceiling drops 1' over an area of 8' x 16'. The walls and the ceiling are drywall. The floor is carpet (I know it's a no-no but that was the only practical solution for my situation). There are no windows (actually there are 2 small windows but they are covered by removable covers made of plywood). Oh... and it's painted light green.

[Bubbagump]

Originally Posted by avare

There is no validity to that myth. Ratios up to 2:1 are used in critical designed spaces. Ratios up to 20:1 are used in office spaces. It is also dependent on the acoustic properties of the material.

Andre

I was more going for a generic answer for a small room application. Absolutely an over simplification. However a 2-4" gap should do at least something to help the low end without taking up a ton of floor space. So I will defer here as without a lot more info, I can't answer with any real specifics.

Also, at 24x24 you might just have a case for diffusion on the walls depending on how things are layed out.

[PaulP]

Originally Posted by azzzy

My space is about 24' x 24' x 7'. Unfortunately it has to have functions other than recording space as well. Which is why my arrangement might be quite unorthodox (asymmetrical to say the least). It's oddly shaped in both horizontal and vertical planes: there is a nook under the stairs which creates 2 extra inside corners and the ceiling drops 1' over an area of 8' x 16'. The walls and the ceiling are drywall. The floor is carpet (I know it's a no-no but that was the only practical solution for my situation). There are no windows (actually there are 2 small windows but they are covered by removable covers made of plywood). Oh... and it's painted light green.

If the place is exactly square it could be a bit of a problem, being irregular
can help as long as you can achieve some sort of symmetry up front. The
ceiling is pretty low.

Are you just mixing at a desk or do you need to record vocals/voice or any
acoustic instruments ? Drums ? What kind of budget do you have for your
treatment ?

A word of caution regarding all the suggestions you'll get to add more and
more absorption. It may make your place sound worse. With no treatment
there are a lot of reflections that can fill in nulls and cover up problems. For
example, if you remove all the reflections you may end up with some huge
nulls that you can then do nothing about. I suggest you go slow and get
used to each treatment you add to see if it helps or not.

Can you provide some sort of plan or photo of your space ?

Paul P

[SAC]

I know it sounds simple, but to build on what PaulP said, why not measure and determine what real circumstances you are actually dealing with? And once you have determined what type of response you desire, then it becomes MUCH easier to apply appropriate types of treatment appropriately in the proper location.

You will save both money, effort, and time as you will achieve what you actually want, instead of the 'assumed necessary' treatment dictating your results.

[azzzy]

Originally Posted by SAC

I know it sounds simple, but to build on what PaulP said, why not measure and determine what real circumstances you are actually dealing with? And once you have determined what type of response you desire, then it becomes MUCH easier to apply appropriate types of treatment appropriately in the proper location.

How do I measure?

[azzzy]

Originally Posted by PaulP

If the place is exactly square it could be a bit of a problem, being irregular
can help as long as you can achieve some sort of symmetry up front. The
ceiling is pretty low.

Are you just mixing at a desk or do you need to record vocals/voice or any
acoustic instruments ? Drums ? What kind of budget do you have for your
treatment ?

As I said before this is a multifunction room. I will be tracking vocals and guitars (close-miked). The rest (bass, drums, keyboards, etc.) I either record direct or use sampled instruments. The space will be also used for band rehearsals. And I will be mixing there as well. It will not be an empty room with bare walls. It will be filled with various guitar amplifiers, a drum kit or two, racks with recording equipment, as well as several cabinets full of crap I sometimes can't even identify. I will also probably use part of the space as a weight-lifting area - put in my rack with a barbell and weights.

Originally Posted by PaulP

Can you provide some sort of plan or photo of your space ?

As I mentioned earlier the room is 24'x24'x7. Well, not exactly. It's kinda devided into three parts by a soffit housing the airducts. It creates areas of 21'x12'x7', 24'x8'x6' and 24'x4'x7'. In hindsight I should have walled off the 21x12x7 area into a separate space dedicated to music but at this point it's too late to do anything about that - I'm way over budget as it is. I will be recording/mixing in that area. It will be somewhat separated from the rest of the room by the equipment racks and I might just put up a couple of deviders when I'm mixing to make the space symmetrical (I will see if it's necessary). My thoughts as far as absorption layout are fairly generic - 4" or 6"-thick 2'x6' in the 4 vertical corners, 3 2'x4'x4" along the middle of the ceiling, 3 2'x4'x4" along the corners where the ceiling meets the wall on each side, and a couple on the sides in the reflection points. In the back and on the sides (not at the reflection points) I will have diffusors. That's pretty much it. I don't have a lot of money (I can't even give you a specific number because I'm way over what I was going to spend on the construction) but I think I could afford to buy enough material for the treatment described above (not OC).

[PaulP]

Originally Posted by azzzy

How do I measure?

There is a free software tool called Room EQ Wizard (you have to register
on Home Theater Shack to gain access to the software).

You also need a SPL meter and an optional test microphone (like the
Behringer ECM8000) plus your computer and soundcard. If you use
only an SPL meter it must have a signal out connector.

For setup and use see the REW Help Page : Room EQ Wizard Help Index

There's also a forum on Home Theater Shack dedicated to using REW : REW Forum

Paul P

[PaulP]

Originally Posted by azzzy

As I said before this is a multifunction room. I will be tracking vocals and guitars (close-miked). The rest (bass, drums, keyboards, etc.) I either record direct or use sampled instruments. The space will be also used for band rehearsals. And I will be mixing there as well. It will not be an empty room with bare walls. It will be filled with various guitar amplifiers, a drum kit or two, racks with recording equipment, as well as several cabinets full of crap I sometimes can't even identify.

Sounds a bit like dykstraster's space which you can see here and here
though he's lucky to have a separate control room.

Paul P

[SAC]

Very generally, the basic measurements that will prove most useful will be the impulse response, from which can be derived both the frequency response, cumulative spectral decay and the Envelope Time Curve.

The frequency response can also be measured as a time averaged measurement by playing pink noise over a period of time, and any transient/spurious noise sources will be 'averaged out'. You may find this method advantageous for LF room mode measurements as it will insure the proper pressurization of the room over the low frequencies. The waterfall display perspective is also available.

You will find the frequency response and waterfall below about 250-300 Hz useful for identifying room modes. And as the region where modal behavior ceases to dominate, the ETC will show you the gain and arrival times of specular reflections.

While many focus solely on eliminating first order reflections, the ETC allows you more granular information and control.

It can be used to identify and address such fine issues as cabinet diffraction, driver signal alignment and speaker positioning issues to first and higher order reflections as well as the energy content of each (a factor that is easily as important as simply the arrival time!). From the information provided, the precise paths of the various reflections can be determined and subsequently treated as appropriate for their arrival time, gain and temporal and spatial distribution.

From these measurements, and according to your desired acoustic room response model, the geometry of your room, your creativity and budget, you can then use the responses to guide your application of the various treatment tools as you manipulate the total energy response of the room via absorption, reflection, and diffusion such that you accomplish the tailored response you desire while 'seeing' exactly what is occurring spatially and temporally in the room before and with each step of the treatment.

[johndykstra]

A band rehearsal space you say?

Don't know about your area, but that's probably saving you at least $300 a month... well... that's what it's saving your band. This is something we took into consideration when budgeting for room improvements, and I don't at all think it's out of line to request band members to toss in on the action.

While I can appreciate Paul reluctance to start, until you know where you're going; there's also a lot of value to learning as you go, and in terms of porous wall panels, it's really hard to mess it up. (stay away from diffusors and tuned traps... for now) They may end up moving, they may end up being altered, but you won't be wasting $ or time. It's really easy to get distracted with all of the theories here, and forget why we are doing this stuff in the first place. AS soon as we start taking time away from making music to split hairs on marginal gains in effectiveness, we miss the point all together.

In terms of the edge effect, I wouldn't sacrifice having open space between small traps in order to maximize edge effect. There's simply a limited amount of area in corners to begin with, and I'd want to cover all of it. Edge effect, as I see it, is best implemented on the edges of a large trap, where you don't frame in the absorbent, rather, build a frame behind it... so the edges are contributing. Remember, low frequencies travel in waves, not rays. So the thickness of the trap should not only be seen in terms of depth, but in height and width as well. By breaking a vertical corner trap into sections, you are essentially making many thinner traps, as the height dimension IS thickness in a round about way.

[EDIT] Chicago? Probably saving at LEAST $600 a month. Go forth and conquer.

[DanDan]

azzy, the different areas/rooms have very different acoustic requirements.
The drum area needs to be live. A vocal area needs all the early reflections gone.

You don't have to extensively treat the recording areas. Just kill flutter. Use the rooms foibles to your advantage. e.g. Drums in a corner get a wonderful bass boost. A cloud over the kit will be very helpful.
The mixing area will very likely be highly treated. 2-4 inch panels of 6 pcf can be wrapped in fabric and should be stiff enough to be self supporting. This will be useful for controlling flutter anywhere. They will be useful for creating a Reflection Free Zone in your mix area.
A cloud over your mix area will be very important. Go for a thick one, 4 inches plus a 4 inch gap. FRK on the side facing the ceiling. No harm in having carpet or area rugs there also.

IMHO measurement is not at all necessary. Your time and effort would be a lot better spent learning the basic acoustic principles at work. Ears, handclaps, your own voice, can all be used to figure out what is going on locally. A lot of the treatment can be very certainly applied, based only on visual observation and a basic understanding of the physics/acoustics.
For DIY treatment details try studiotips - tips on studio design, acoustics, and wiring
Lots of articles on how to set up the mix room at realtraps.com and gik.com
DD

[Mr.Smith]

Without giving expert advice to your problem, I think I can give you some food for thought.

As far as people saying acoustics is a no rush matter, I completely disagree. Time and budget are a huge factor in any space and its quite obvious that you need to do something quick to complete your job. You can spend 2 years trying to come up with the best possible senario for your room and then commiting to the work. Or you could educate yourself, build, re-educate yourself, rebuild etc...

I personally think hands on experience is also a big factor. I want my studio to sound good, but also LOOK good lol. I can't speak for anyone else but who wants butchered looking panels that "sound" good.

Do as much as you can with the knowledge you have and with the timeline given, that way you can get a better sounding room (maybe not perfect) and finish your project. After that, educate yourself some more and maybe move some panels around or rebuild some panels. Your second panel will always be built better than your first.

Also dont make anything TOO permanent that way you can take things down and use them somewhere else, if necesary.

God Bless and get to work!! NOW.