what is and is not a bass trap ....

[fatgaz]

right ho,

I want to make some DIY acoustic treatment to damp the bass modes of my too-small listening room. i have very little money for this so it's got to be DIY and no consultant acousticiants (sp?).

One problem that I read here and elsewhere of different ideas as to what a bass trap is !

I consider bass to be about 40 Hz to 160 Hz -ish, basically two octaves, Lower and we're sub-bass sub-woofer area which i'm ignoring. Higher and it's lower midrange / upper bass really. ALSO, I'm putting these traps in the corners of the room where the acoustic pressure is high but the air velocity is relatively low for bass room modes.

it seems to me that hanging a flat panel of rockwool / fibreglass next to a wall isn't really going to have a big effect on these frequencies, because they rely on air movement (velocity) not pressure for absorption. (Tell me if i'm wrong here !) ie some of the things that people do call bass traps aren't really.

SO should I be trying to make devices based on closed/restricted air chambers filled with absorber material such as helmhotz panels with slots / pegboard ? or damped membrane resonators ?

if so, I'm thinking of making triangular boxes (to be stacked in corners) full of fibreglass with a thin plywood front as a membrane ? should i try different membranes - ie some with thicker ply and some with rubber sheeting or roofing felt ? Since this a bodged solution I wonder if I should try and cover my 40-160 Hz range better by using many different types of membrane ?

OR am I deluded and talking out of my arse

hpe you all can help,

Gareth.

NB I will try to have a couple of absorbers for low-mid range onwards to cover 160-500 ish range at least- not sure if these need to be just sheets of rockwool etc or open boxes with a fibreglass front and air cavity behind ! may put some absorbant material in the ceiling corners too.

[J.S.Vega III]

Have you tried searching "bass traps" on this site, the answers are really not hard to find.

I'm no expert but I have read probably the entire internet's resources on this subject as I built some traps myself recently.

I would say avoid any kind of box construction (apart from frames), or hemholtz resonator type constructions unless you really know what you are doing - even then, some people think they (HR's) are a very inefficient way of "trapping" sound as they are quite frequency specific I believe. Also they are very technical to build (and position I would imagine) compared to a simple panel absorber covered in fabric.

As any kind of wooden panel will resonate I would be trying to avoid this at all costs.

The overall consensus seems to be rigid fiberglass, away from the wall as far as possible, especially in corners as you say, and the more mass the better.

As for what the definition of bass is, I worked on the concept that the lower you go the better, as a 2" will absorb just as much HF as a 6" panel, but the 6" will take everything else too.

[macmod]

while waiting for an answer check this very informative site Gaz:
http://www.ethanwiner.com/acoustics.html

cheers.
Macmod

[J.S.Vega III]

Originally Posted by macmod

while waiting for an answer check this very informative site Gaz:
http://www.ethanwiner.com/acoustics.html

cheers.
Macmod

Are you saying my reply is not an answer? I appreciate it's not complete or even anything the poster should definitely act upon, but your post comes across as kind of rude... sorry, it's late here but that's how it looks.

I think a search would bring up ethan's stuff pretty much immediately, and my other point was that if you're trapping all frequencies then you don't need to worry about the numbers, do you?

[Disjointed]

ethans site is the best link... hes pretty much the 'bass trap' guy..

most ppl do what J.S vega was suggesting... use oc703 (owens corning rigid fiberglass) ... or rockwool (rockwool is cheaper but fiberglass holds its form better)

available here :
http://www.acoustimac.com/products.asp
-or-
http://www.atsacoustics.com/cat--Raw...ials--102.html

build a little frame out of 1xsomethings (depending on your desired thickness) and fill the middle with sheets of rigid.. then wrap the whole thing in fabric...

now if you keep the whole thing open (just fabric and insulation) it will be a broadband absorber....
if you put a thin piece of plywood on the front (insulation pressed against the back- to stop resonation) it will bounce the HF and be more of a bass trap...

broadband is more recommended.. unless youve done accurate testing and find that your room is perfect, besides the bass (i doubt it)

then you usually start with the corners (more bass) (dont forget ceiling wall corner too)..
then add wall absorbers as needed..

gl

[fatgaz]

Hi,

thanks for the replies. So the idea is just to shove some dense fiber glass sheets on the walls with a few inches gap behind ? Mainly because 'broadband absorption' like this is less likely to make the acoustics worse than more specific but ill thought out devices ?

I can see that these sheets will absorb low-mids and up ( i intend to have something similar on a couple of places to breakup areas of blank hard walls) , but I'm not sure that this will work that well with the low bass (40-160) frequencies ? doesn't the absorption drop off at low frequencies as these go near the wall ?

don't those bass tube traps work by having a cavity inside so that a pressure differential creates air velocity through the tube walls ? or would a solid cylinder of fiberglass work just as well ? (but be heavier !)

still a bit confused to be honest !

Doesn't ethan winner show these panel things described as bass traps that are sealed boxes with a plywood front membrane ?

cheers !

Gareth.

[Acoustic Cloud]

yes

[tINY]

Limp mass membranes are probably better for corner traps, but they are pricey....


-tINY

[whoriental]

Originally Posted by macmod

while waiting for an answer check this very informative site Gaz:
http://www.ethanwiner.com/acoustics.html

cheers.
Macmod

Wow, that guide is super informative.

[macmod]

Originally Posted by J.S.Vega III

Are you saying my reply is not an answer? I appreciate it's not complete or even anything the poster should definitely act upon, but your post comes across as kind of rude... sorry, it's late here but that's how it looks.

I think a search would bring up ethan's stuff pretty much immediately, and my other point was that if you're trapping all frequencies then you don't need to worry about the numbers, do you?

Maybe should have said...''waiting for more answers''..''waiting for Ethan or Glenn to chime in'' or is that rude too?

How's this for rude:
''Have you tried searching "bass traps" on this site, the answers are really not hard to find.''

Whatever

Cheers,
Macmod ( Extremely rude poster. Worst kind)

[Bassace]

hi
if you scroll this very page there is an excellent post called "how i built my own bass traps"
detailed info for cheap, easy, in one weekend, to make traps .
keep up the do it yourself
cheers

[macmod]

Originally Posted by J.S.Vega III

Are you saying my reply is not an answer? I appreciate it's not complete or even anything the poster should definitely act upon, but your post comes across as kind of rude... sorry, it's late here but that's how it looks.

I think a search would bring up ethan's stuff pretty much immediately, and my other point was that if you're trapping all frequencies then you don't need to worry about the numbers, do you?

It's all nonsensical cr*p you wrote dfegad

Macmod

[nathanvacha]

okay, so...

fiberglass/rockwool panels are broadband absorption. they affect highs, mids, and lows, but differently depending on depth/density... but by using thick panels with or without the frk facing placed correctly, you can keep a relatively clear sounding room while taming the low end effectively.

in a panel trap, you do not want anything soft touching the resonating front because it will stop it's motion and render the trap useless. you can put fiberglass/rockwool suspended inside, like a broadband trap sealed inside of a sealed panel trap and it will, if i'm not mistaken, widen the traps bandwidth or increase its efficiency.

contrary to what vega said, the sealed panel with a resonating front (which ethan winer has also written an article about) is, in fact, a valid bass trapping device. read his article.



vega, your reply wouldn't have shown up yet when macmod loaded the page... his post came up only a couple minutes after yours. geez guys, lets be grownups.


Garath, spend some time searching the subject here, you'll find plenty of good info and you'll understand it better the more you read.

[Glenn Kuras]

>if so, I'm thinking of making triangular boxes (to be stacked in corners) full of fibreglass with a thin plywood front as a membrane ? should i try different membranes - ie some with thicker ply and some with rubber sheeting or roofing felt ? Since this a bodged solution I wonder if I should try and cover my 40-160 Hz range better by using many different types of membrane ?<

I don't think you should do it that way (if I understand you right ). If you are making panel traps with a plywood front, those would go right on the wall in the corners. I think what you are talking about making is called a super chunk. All that is, is cutting rigid fiberglass to triangles and stacking them from floor to ceiling (no wood fronts to them). These will be the best for bass trapping if you have the money for them. If you want more of a limp membrane trap then you would us FRK on the fronts of them or build 4" or 6" panels and put them straddling the corners with FRK. I myself recommend using FRK when you can and non FRK for the panels that would go on the first reflections in the room.

Hope that helps

Glenn

[Nosound]

Excuse my ignorance but WTF is FRK? just a side line watcher here on this form.

[Scott R. Foster]

Originally Posted by fatgaz

right ho,

I want to make some DIY acoustic treatment to damp the bass modes of my too-small listening room.

snip

I consider bass to be about 40 Hz to 160 Hz -ish, basically two octaves,

snip

SO should I be trying to make devices based on closed/restricted air chambers filled with absorber material such as helmhotz panels with slots / pegboard ? or damped membrane resonators ?

Hey FG:

You can do the treatment you want without resort to resonating devices - some of the shortfalls of which are mentioned above.

The device described in the link below will work great for the range you are concerned with [though if your room is tiny you may well be braggin' to worry about 40 Hz ]:

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

punch in your dimensions here to find what the modal range of the room is:

http://www.bobgolds.com/Mode/RoomModes.htm

These simpler devices also will do the job and are a lot more efficient material cost wise than a SuperChunk... but won't work quite as well [you'll need to add a about 20 to 30% more linear feet of corner treatment with a 24' wide SCA to be equivalent to using 34" face width SuperChunks - utilize floor/wall and/or ceiling/wall corners as described above to place the additional treatment].

If you use the StudioTips Corner Absorber design, you can get a large boost in performance from +/- 60 to 120 Hz if you make sure you mount them so the rear edges touch both walls.

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

As explained correctly above, there is no need for boxes, frames, or panels. In fact all such additions will detract from the broadband characteristics. If you resist the urge to add such things you can use one of these designs together with wall and ceiling mounted first reflection point panels to do all your treatment, [low mids and highs]. In a small room - typically you will benefit from using all 4" thick panels [no need to focus on high frequencies - you'll benefit from the addition LF absorption and the additional cost to use all thick panels is small].

Just build a light weight rectangular wood frame the size of you mineral wool panels [maybe a cross brace or two] add at least 4" of mineral fiber [3 to 6 lbs fiberglass - or 5 to 8 lbs. rockwool are good choices] and upholster with a breathable fabric [literally something you can breath through] stapled to the frame on the backside. More on choosing mineral fiber and fabric here:

http://forum.studiotips.com/viewtopic.php?t=48

A nice list of various types of mineral wool and analogs that'll help you shop the local market:

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

Good Luck!

[fatgaz]

Originally Posted by Scott R. Foster

Hey FG:

You can do the treatment you want without resort to resonating devices - some of the shortfalls of which are mentioned above.

Good Luck!

Scott, thanks for the info and the links.

It seems that in practice a fat wedge of fiberglass/rockwool will do a reasonable job for mid to low-ish bass if it straddles a corner irrespective of whether the air cavity behind is 'sealed' or not.

I wonder if putting the absorber essentially 2 feet from the apex of the corner puts the absorber more in the air-flow area rather than the air-pressure area, and that a sufficiently large obstruction can further cause an increased in absorption, possibly due to pressure differentials either side of the obstruction ??

It's a dry day here, so I'm going to buy some pits of wood and some fiberglass panels. I can assemble some panels in the back garden.

I noticed some 'pond-liner' in a local DIY shop that is 1/4" poly batting - it looks ideal for wrapping the panels to stop the fibers sheding, while (hopefully) not being too reflective at higher frequencies.

I think the answer to my initial question is that, in practice, any absorber big enough will make-do as a bass trap, and to leave the panel/helmholtz absorbers to the people who are building studios etc !

Gareth.

[bpape]

Agreed. In smaller rooms, there is rarely the luxury nor the need for specialized, more tuned absorbers. In most cases, a more broadband approach is preferable. You just don't have the space for specialized devices for each part of the range.

Bryan

[Ethan Winer]

Scott Foster,

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

With all due respect, the graph on that page is probably the lamest thing I've ever seen you link to. To imply that the only important feature of a bass trap is its cost per sabin is disingenuous, dishonest, and - okay, I'll say it - incompetent coming from someone who wants to be a pro in the acoustics business.

To recommend buying room treatment products on price alone is exactly the same as telling people to eat only at McDonald's or Burger King where a quarter pounder costs much less per pound than an excellent fillet mignon at a fine steak house. You want cheap calories? You can't beat Wendy's or Pizza Hut or Taco Bell!

But words alone don't tell the whole story, so let's take a closer look at "value" in several popular bass trap products. The graph below shows the cost per square foot for a number of popular traps. For now let's assume all perform similarly, so cost per square foot is not an unreasonable assessment. Except maybe for the Echo Busters stuff.

Viewed in this light it's obvious that the very best value by far is Certainteed's R38 fiberglass insulation. At 12 inches thick this product is at least twice as effective as the 703 your company sells, yet costs only 1/4 as much. My goodness, that's nearly an 8 to 1 better value! Since I have now identified the very best value in bass trapping I could stop here and have made my point. But I won't stop here.

Now let's look at finished bass traps. The best value I see is GIK's 244 panel which costs only 3/4 as much as your RT424. GIK's Monster Bass Trap also costs less and performs better than your Super Sub Bass trap. On that basis I heartily endorse GIK for offering the very best bang for the buck. Can there be any dispute about that? I think not! Let's hear it for Glenn and Bryan!

Then we get to the RealTraps MiniTrap and Auralex MegaLENRD. Both of these cost more per square foot than what GIK sells, but they do seem to be a much better deal than anything sold by ASC or RPG or EchoBusters. Given this new perspective, why would anyone in their right mind ever buy anything from RPG or ASC? How can they even still be in business?! I wonder if it's related to something other than raw cost per sabin?

Scott, to lump together the cost of DIY materials with the cost of finished products is deceptive and silly. What you have done is ignore the value of someone's time to research materials, find a fiberglass vendor and lumber yard and fabric store, drive through three towns to buy them, plan the construction, and actually do the work. Nor does it take into account the quality - both performance and appearance - of the DIY version. By your logic nobody should buy high-end mic pres for $2k+ either when a couple of 5534 op-amps and a power supply cost all of $50 or even less.

Just as silly is that your chart ignores the value of a treatment vendor's advice. Oh, so that's why RealTraps and Auralex and ASC and RPG charge more than Ready Acoustics! Where's that light bulb smiley when you need it? I routinely advise my customers on all things related to audio, not just "what panels to buy and where to put them." I value my time and expertise pretty highly and, even more relevant, so do my customers.

Just as important is product appearance and durability, overall appearance and professionalism, ease and versatility of installation, etc. Of course, performance is very important too! The best bass trap is not necessarily the one that appears to have the highest absorption at 80 Hz or whatever. In fact, it most certainly is not the trap that appears to have the highest numbers at 80 Hz! The key word here is "appears" because you know quite well that the data in your graph is deceiving, yet you intentionally deceive anyway.

This is hardly the first time you've been intentionally deceptive and had it pointed out to you, yet you continue anyway? You should be ashamed. It's obvious that your intent is to discredit me and RealTraps, but you do a larger disservice to the entire acoustics community. Especially to companies like Auralex and RPG and others that, like RealTraps, are staffed by true acoustics professionals and offer far more than just a box of bass traps.

There's a reason the RealTraps site features a huge list of very high-profile clients. Here's a clue: It's not because RealTraps charges less per sabin than anyone else.

--Ethan

Click HERE to see the cost per sabin comparison.

[Scott R. Foster]

Ethan:

You should take the matter up with Bob Golds... who authored the graph you dislike... he may be interested in your thoughts on the matter.

[tINY]

That sounds like a dodge to me. You quoted it.



-tINY

[Brent]

*Makes popcorn*

[Scott R. Foster]

Originally Posted by fatgaz

Scott, thanks for the info and the links.

It seems that in practice a fat wedge of fiberglass/rockwool will do a reasonable job for mid to low-ish bass if it straddles a corner irrespective of whether the air cavity behind is 'sealed' or not.

I wonder if putting the absorber essentially 2 feet from the apex of the corner puts the absorber more in the air-flow area rather than the air-pressure area, and that a sufficiently large obstruction can further cause an increased in absorption, possibly due to pressure differentials either side of the obstruction ??.

FG:

I think you are correct, a simple 4" panel of mineral fiber is the best answer for your circumstances. Acoustic engineers have been using this sort of device for many years - when the need matches their abilities, these devices are world class.

As to panel gapping - once you get into a corner mounting the pressure/velocity model falls apart to some degree, there are other things going on. In a corner straddle mount, if the backside vertical edges touch both of the walls, you get a large boost of absorption in your target range. Even a small gap between the walls and the edges of the panel cause this to collapse. Below is an example of one of our 4" broadband bass traps mounted in a corner with edges touching the walls versus the same panel gapped a couple of inches off the walls.

Tight to the corner is the way to go.

It's a dry day here, so I'm going to buy some pits of wood and some fiberglass panels. I can assemble some panels in the back garden.

I noticed some 'pond-liner' in a local DIY shop that is 1/4" poly batting - it looks ideal for wrapping the panels to stop the fibers sheding, while (hopefully) not being too reflective at higher frequencies.

I think the answer to my initial question is that, in practice, any absorber big enough will make-do as a bass trap, and to leave the panel/helmholtz absorbers to the people who are building studios etc !

Gareth.

I am not familiar with poly batting pond liner.. but anything you can breath through will work.

Many a great studio was built with nary a panel absorber in sight - or any other resonating absorber for that matter. So get out in the garden, whip yourself up some upholstered mineral fiber panels, and get ready for the creamy goodness of a room with low frequency resonances under control.

[bpape]

IF the room requires the hump in absorbtion at 100Hz then 'hugging' the corners may be desirable. For instance, when one double drywalls a room, the resonant frequency of the wall drops and the efficiency of absorbtion at around 100Hz also drops. In this case, it's preferable usually.

However, if you have a single drywall structure with insulated cavities then you already have a substantial amount of of absorbtion around this hump and the extra may or may not be desirable depending on what else is going on in the room.

Unfortunately, there isn't an 'always best' recommendation. It all depends on many factors in the room including seating position, monitor position, room construction, usage, other treatments, etc.

Bryan

[fatgaz]

Originally Posted by Scott R. Foster

FG:

Tight to the corner is the way to go.

Great - that graph really shows that for low bass control I need to have these panels tight to the walls. I have STONE walls, so they're vey reflective.

OK, seems that I'll need to consider carefully how I form the edges of the panels and how i mount the panels so that they bridge the corner properly without any noticable gaps. This would be easy if i was doing a permanent fixture, but i'm not sure i want to do that in this room. (ie i'm not sure I should fix batons to the walls and drill holes !)

Do you think that a triangular right angled box with rockwool across the long edge might also work as well ? this would mean i could construct a good seal of the edges of the rock wool to the box walls and have a non-permanent movable bass trap ?

I was thinking of a box with thin ply sides to reduce wieght. OR are these back panels (which will be nex to the walls) likely to vibrate and mess things up ?

OR how about corrigated cardboard box walls - just to seal an open air cavity ?

Sorry I'm going a bit off topic now.

thanks for the help

Gareth

[Ethan Winer]

tINY,

> That sounds like a dodge to me. You quoted it. <

Indeed, and Scott Foster also placed the graph there. Scott Foster is the moderator at the StudioTips site where the graph is hosted. So for Scott Foster to deny responsibility is yet another instance of his dishonesty. Had that graph included a more comprehensive sampling of products - both lower cost (R38) and higher (ASC, RPG), Scott Foster could at least pretend it's a fair assessment. If you or anyone else wants to know more about the history of that graph I'll be glad to elaborate. Suffice it to say the intent was to discredit RealTraps when it was posted a few years ago.

--Ethan

[tINY]

The problem is that acoustics are complicated and everyone wants simple answers. Those tube traps looked really cool and useful - then I saw the price...

Still, for a large studio space with various recording projects goin on, having a couple dozen around might make a lot of sense. The right tool for the job.

As far as removeable bass trapping in stone-walled listening room - building corner traps with plywood against the walls doesn't sound like a bad idea. I'd be looking at 3 - 4 foot faces if possible.....




-tINY

[Scott R. Foster]

Originally Posted by fatgaz

snip

OK, seems that I'll need to consider carefully how I form the edges of the panels and how i mount the panels so that they bridge the corner properly without any noticable gaps. This would be easy if i was doing a permanent fixture, but i'm not sure i want to do that in this room. (ie i'm not sure I should fix batons to the walls and drill holes !)

snip

No need to keep crazy about it - what you are referring to as the "sealed" panel in the graph above isn't sealed at all, its just resting against to flat smooth surfaces. There is only the weihght of the panel holding it in place.

As long as the surface irregularities of the wall you are mounting against is not quite large - on the order of a few points poking out an inch or so that the panel edges would rest on the points - leaving the rest of the edges with a noticable gap - I think you will you will be fine.

Maybe you can make the frame on the backside a bit smaller than the mineral fiber panels so that the soft edges of the MF can conform to the irregularities?

If the stone walls are quite irregular and you fear a collapse of the the LF boost you wish to keep, could you mount a couple of thin boards flat to the wall - square and plumb - and caulk the gap - so the edges of the panel have something smooth and flat to rest against? If so, that would fix things perfectly.

Also, post pics of your panels and mounting methods to share with others please.

[midigod]

I would not recommend using Pool Liner as a wrapper, unless you want a ton of high-frequency reflection. If you're looking for something that will not reflect, get something breathable, like Craft felt.

Though a lot of good advice has already been given, I'll post the plans to my broadband absorbers anyway:

http://www.pmerecords.com/broadband.cfm

-Craig

Originally Posted by Ethan Winer

I routinely advise my customers on all things related to audio, not just "what panels to buy and where to put them." I value my time and expertise pretty highly and, even more relevant, so do my customers.

Next time I need to know what pre to track a rapper with, I'll be sure to hit u up.


I think you and Scott should post your backgrounds/qualifications in acoustics so we'll have a better idea of who's opinion to trust.

As consumers and DIY's we really don't know WHAT to believe on acoustics.