Weird, Triangle Room - Bad Acoustics

[bassbrock]

Hi there! I've been lurking around here on gz for a while, now decided to register and post. Lots of wonderful info and helpful people around here!

I've got a weird, triangle shaped room that I need to figure out two things with:

1. What is the best listening position?

I know that in a square/rectangle shaped room, I should put my speakers on the shortest wall, make my listening position about 38% into the depth of the room, and fire the speakers into the length of the room. Problem is, this room is a triangle. I can't figure it out.

2. What appropriate bass trap/broadband absorption treatments should I look into?

I've experimented with some DIY bass traps (4" rock wool, 8 lb cu ft density), and the acoustics have improved but not by much.

Basically the way things are, my stereo image is really off, and I feel like the room is just one big corner, so I feel like it will be forever boomy.

Here is a sketch of the room dimensions. The ceilings are 10 ft tall, the floor is commercial carpet with no pad and the walls are sheetrock. My current listening postion is my face looking at the 16 ft wall and the odd shaped wall to my back:



The walls are blue, the doors are red, and the black lines are the wall dimensions.

Any and all help is greatly appreciated in advance. Thank you!

Oh, and the purpose of this room is a studio control room. I've got a mac with logic pro, a motu interface, mics, etc. Mackie HR824 monitors and sub, Yamaha 5" NS10 look alikes for additional reference.

[Berolzheimer]

For symetricality I'd say close up that middle door & face away from the corner or if you can't, face into the corner- but with some serious absorpsion in that and all corners, maybe all around-& especially on whatever wall ends up behind you. Not an ideal shape to be stuck with but at least it's not tiny.

[tINY]

I've used corner set-ups before. I have one now in my living room.

They arent' bad in a big enough room.

Draw a line 45 degrees from one wall (bisect the corner). This is your center line. Sit at least 6 feet out from the corner. Make sure that your speaker are toed in a bit and 1-2 feet from the wall.

Deaden the shit out of that back wall - it's pretty close. Might as well make a deep base trap in the corner too. It'll be hard to use for anything else.

That's a really tiny room - is there another place?




-tINY

[bpape]

Agree with what Tiny said. I'll just add that you're going to HAVE to do something in the other 2 corners too. I know it's not idea with the doors there but you can buy/make something that you can roll/slide out of the way.

Bryan

[Scott R. Foster]

Perhaps something like the sketch below... mega panels in the front corner or maybe a SuperChunk.. add a few symmetric ceiling/wall or floor/wall panels where practical.

[Ethan Winer]

Originally Posted by bassbrock

I've got a weird, triangle shaped room that I need to figure out two things with

Everything tINY and Bryan (bpape) said, especially about having as much bass trapping as possible. Also...

What is the best listening position?

The only way to know for sure is to experiment with different loudspeaker and listener locations, measuring the response at each combination to find the places that are closest to flat. The 38 percent rule probably does not apply here. You'll never get perfectly flat, but with enough trapping and careful positioning you can definitely get good results.

I've experimented with some DIY bass traps ... my stereo image is really off

The more bass traps you have, the better. Always. A room like that needs a lot of bass traps. Heck, all rooms need a lot of bass traps.

Imaging is a function of absorbing all early reflections. In this case "early" means any echo that arrives at your ears within about 20 milliseconds of the direct sound from the speakers. This includes the wall behind you, as well as the ceiling. The one saving grace of facing the corner is the side walls reflect sound away from your ears toward the back of the room. But then you need to absorb them back there.

Much more info is here:

Acoustic Treatment and Design for Recording Studios and Listening Rooms

--Ethan

[bassbrock]

Thanks everyone so far for such great information!

I'm a little more encouraged now than I was when I got started. It's amazing how easy it is to get discouraged jumping into purchasing equipment, etc, then all of a sudden realizing your room has horrible acoustics AFTER you've got it all installed. Having difficulty getting your mixes to translate can be a downer. I've recently been experimenting with a 31 band EQ, even after reading around here that I should avoid it all costs! And nope it hasn't worked too well.

And tINY - Man do I ever wish there was another room for me to setup in, but NOPE. I'm lucky that I even get a room if you know what I'm sayin'.

The gist of what I get from everyone so far is that with enough absorption, I can even place the listening position in the corner of the 16' and 14' wall. That's not too bad, and seems symmetrical. I had avoided facing into the corner because I thought that would make my bass issues worse.

A couple of additional questions:

1. I like the SuperChunk idea - will I have to place the SuperChunk against the wall, touching it, or should I also give some air space, 4 inches or so, around it?

2. Am I gain additional bass absorption using a 8 lb's per cubic foot density of mineral wool? Or is anything over 3 lb's just wasted density?

3. If I face the corner, should I avoid placing my Mackie sub in the center, and instead place it along one of the side walls adjacent to the listening location?

And Scott - thanks for that drawing, that gives me some ideas! I assume that is a bass trap/broadband absorption "cloud" that you've drawn over the listening area?

Ethan - thanks man for that link, I had lost it! Some AWESOME info there. I figure now with my room shape there is no point sweating the 38% rule of thumb. Thanks for clearing that up. And the ideas about early reflections - I probably need more absorption directly behind and to the sides of my listening location if I understand you correctly.

You all have given me a great start and I appreciate. I'll report back after further experimenting. Thanks!

[bassbrock]

Originally Posted by bpape

Agree with what Tiny said. I'll just add that you're going to HAVE to do something in the other 2 corners too. I know it's not idea with the doors there but you can buy/make something that you can roll/slide out of the way.

Bryan

Good point and I think those corners are giving me trouble right now. I haven't experimented with any trapping in those because of the doors. I wonder how difficult it would be to make two, 6 foot tall SuperChunks and put them on those big slick furniture gliders? Then I could just slide them out of the way. I wonder how heavy a 6 foot tall uperchunk is anyway?

[bassbrock]

Originally Posted by Berolzheimer

For symetricality I'd say close up that middle door & face away from the corner or if you can't, face into the corner- but with some serious absorpsion in that and all corners, maybe all around-& especially on whatever wall ends up behind you. Not an ideal shape to be stuck with but at least it's not tiny.

Yeah, good idea but I can't close up that middle door - in fact, that is the only door really used. I could pretty much close up the other doors, or at least block them with something movable.

[tINY]

opening the doors in the corners will help.

More density is better - I think you start loosing effect around 5pcf if I remember from looking at charts. Even 1.5pcf works if they are thick enough.




-tINY

[Scott R. Foster]

BB:

Shoot boy wow... you're in Hooterville too aintcha!

Drop me a line maybe we can hook up, I wouldn't mind seeing that roon when you get to work on it.

For making a full size SuperChunk a material with more gas flow resistance than 703 would be both a waste of money and a path to a less effective device. Keep in mind however that Rockwool has a lower GFR per unit density, so while 3 lbs. pcf is OK for semi-rigid fiberglass [or perhaps something of lower density if available] for rockwool you might be OK with a slightly higher GFR/density than 3 lbs. pcf in a full size SuperChunk.

Don't make gaps or spaces... just fill the corner with pizza slices of mineral fiber from floor to ceiling and upholster.

More here:

Acoustics :: View topic - Studiotips Corner Absorber

=================================

Tiny:

Optimizing a porous absotber is about matching the gas flow and entrance impeadance of your absorber material to the job. These properties are proportional to density for a given material [materials made of the same stuff rise in density, they exhibit a greater resistance to the pressure wave of a sound flowing through them and entering the material]. Too little and the pressure wave enters and leaves without being fully absorbed... too much and the waves bounces off instead of entering and traveling all the way through the material.

For broadband absorption the optimal density will go lower [lower than you suppose] as thickness rises - on the other hand increasing the density [and thereby the gas flow / entrance resistance] can only be beneficial if the absorber is so thin and the material so low in density that the pressure wave has to be choked off quicker to maximize effectiveness.

For a thin panel - on the order of 2" thick - raising density higher than the medium density stuff like 703 that is typically used, can bring modest improvement per panel [key on modest], but it makes little sense acoustically once you factor in per unit cost unless you also consider mechanical / material handling properties. Given that the price per unit volume of the material is roughly proportional to density [twice as dense costs twice as much], the only thing you could save by using higher density is panel count. For the same material cost you could have two panels of the lower density material - which is a much more efficient solution if space allows.

On thicker panels [best design practice for for broadband applications is use 4" thick minimum] higher densities [such 6 lbs. fiberglass - 705] never make sense versus a less dense, less expensive material [such as 3 lbs. 703] as the additional desnity is not going to provide any benefit.

If you want to improve a 4" thick 703 panel, make it thicker. Once you get to about 10" inches or so of thickness, you can continue to improve it by making it thicker still, but you need to use a material that is LESS DENSE.

Don't think you can improve a 4" thick 703 absorber by leaving it the same size and making it heavier and less porous by switching to a higher density material. You are just choking off the pressure wave from access to the material [the panel will start to bounce sound off the face before it hits bottom].

More density means fewer interstices [holes] in the material, and porous absorpers are all about the holes... the greater the density the fewer the holes and the the less porous the material.

Consider this fact... if density where defining for absorption, concrete would be the cat's pajamas.

More here:

Acoustics :: View topic - Density question from another forum

[Ethan Winer]

Originally Posted by Scott R. Foster

increasing the density [and thereby the gas flow / entrance resistance] can only be beneficial if the absorber is so thin and the material so low in density that the pressure wave has to be choked off quicker to maximize effectiveness.

That makes no sense. If dense material "choked off quicker" than less dense material when the panel is thin, by extension it will do the same thing when thicker too.

On thicker panels ... higher densities ... never make sense ... you need to use a material that is LESS DENSE.

Again, this is not always the case. When using FRK rigid fiberglass, higher densities are better even when six inches thick. Look at the three graphs in the right-hand column just above the ANALYSIS heading in my Density Report:

Rigid fiberglass density tests

This series of tests compared 701 (least dense), 703 (middle), and 705 (most dense) with the FRK facing. The improvement with each increase in density is clear in the peak just above 92 Hz and the null just below 164 Hz. The change is not huge because we had only six panels total in a medium size room. If we had more panels the measured difference would be even larger. But the value of higher density is still quite clear at the peak and null frequencies I mentioned.

In no cases is using dense material worse than less dense.

Also, I once asked pro studio designer Wes Lachot about 705 versus 703, and he said he always uses 705 because it's a lot easier to work with and also holds its shape better. As Wes explained, the small difference in material cost is more than offset by the lower labor costs to make good looking clouds and panels quickly.

--Ethan

[tINY]

I never even considered any impedance issues here. Seems to me that no mineral/glass wool would be dense enough to cause impedance mismatch issues where the return loss at the surface would become and issue. But I have no data or graduate classes to back up that thought.

In the "bang for the buck" camp. I can see where a DYI'er would want to buy more lower density material if he had the space to double up on thinckness. For the one vertical coner and the horizontal corners in this room, the 703 probably makes sense.

For the back wall that will be 2 feet behind the mix positin, I'd go with the 705 at 8" for the whole wall. I'd also open the doors if possible when mixing.



-tINY

[Ethan Winer]

Originally Posted by tINY

I never even considered any impedance issues here.

Just as with electricity, where the maximum power is transferred when the source and load impedances are matched appropriately, the same applies with an acoustic wave and an acoustic "load" in the form of fiberglass. In this case the ideal impedance is also the most efficient for converting the acoustic wave energy to heat.

--Ethan

[Scott R. Foster]

Tiny:

The impedance of a porous absorber in the context of this discussion [average resistance to pressure at a particular frequency] varies from the concept of simple gas flow resistance in that the terms GFR relate to resistance against a steady pressure gradient, whereas impedance refers to resistance to an oscillating pressure gradient. At variance to the air volume of a room, wherein the flow resistance and the impedance are the same, a porous absorber exhibits properties where these two elements differ.

In a porous absorber there is an impedance jump when the sound's pressure wave transitions from traveling through air to traveling to through the absorber... but after making that transition the intrinsic gas flow resistance of the medium comes into play. These two elements [impedance and GFR] combine to determine the material’s absorption properties*.

You can model a porous absorber and predict certain elements of its behavior using this tool:

http://forum.studiotips.com/viewtopic.php?t=1639

* see the “Flow Resistivity” sheet of the workbook for a detail explanation with formulae

In a real world application one encounters a number of elements not present in the model, such as the special case of corner mounting – nonetheless the model is illustrative of the general cases and helpful to understanding the underlying concepts.

To that end, let’s explore an extreme example. Assume you plan to build a wood frame along the full length of a wall and then fill it with mineral fiber and cover it with upholstery.

You want a superlative absorber, and you have plenty of floor space to spare… so you plan to make the depth of the frame 12” [305mm]. Having done some research on the net you find much discussion of 703 as being a good absorber… so you consider filling the entire 12” depth with this material. But before spending all that money on a pallet of 703 you model a 12” thick 703 wall using Whealy’s spiffy spreadsheet linked above [703 is about 22,0000 Rayls] and you find your proposed construct is predicted to have the following properties at normal incidence.


[SEE GRAPH #1 BELOW: 703-12" Thick]

Not too bad, but you expected more for all that expense, labor, and lost floor space… so in an attempt to improve the absorber you consider doubling the thickness to 24” [610mm] – but lo and behold you find no improvement! This is because at 12” thickness you have already exceeded the capacity for absorption defined by the flow resistance/impedance of 703… more thickness does you no good. At 24” thick, you have doubled your material expense and doubled the lost floor space, but obtained no improvement.

[SEE GRAPH #2 BELOW: 703-24" Thick]

Well… back to the drawing board… you do some more reading on the internet and gleaning “wisdom” from the pontificating pundits you are taught to expect that by increasing the density of the material you will increase the absorption performance. So you find a material denser than the 703 and double the flow resistivity to 44,000 Rayls.

[SEE GRAPH #3 BELOW: Double Flow Resistivity]

Yikes - you have again doubled your material costs, but this time you have managed to actually make a less effective absorber than when you started!

What to do?

Well… if you give some thought to the physical principals at play you can see that you are working with a material that simply has too much flow resistance to do you any good at the proposed thickness. At 12” thick, you need LESS density to find improvement. So you replace the 703 with R38 roll insulation and cut your material costs to less than half the original design.

[SEE GRAPH #4 BELOW: Lower Flow Resistivity]

And bingo, your performance leaps!

The point of the above example is that you can’t build a better bass trap focusing on one element of the design [flow resistance / density] especially if you get the basic physics wrong. To succeed you must take a grasp on the underlying physics and take a number of other factors into consideration so you can balance elements of the underlying physics, material costs, lost floor space, material handling properties, into a comprehensive solution.

The StudioTips Corner Absorber is such a design:

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

This design goes beyond the matters of gas flow properties discussed in this thread and addresses matters of efficiency in floor space consumption, material cost, the importance of mounting methodology [gaps and corners!], and takes into consideration aesthetic issues. These units are not just corner absorbers… the SCA is also an excellent wall mounted absorber, especially if mounted with a 3” to 6” gap behind the panel [more gap makes for better low frequency performance]. The SCA is a highly efficient alternative to the examples explored above. The DIY Bass Trap Build Tutorial linked at the bottom of this page has detailed instructions on how to self-assemble units based on the SCA design with lots of pics.

Happy Bass Trap Building!

[tINY]

So all the fancy math there is pointing out that , when the impedance changes drastically from that of free-air, energy is reflected. That makes a certain amount of sense. It's simpler than high speed electronic signals in that there is only resistivity, and no complex impedance to deal with (at least in your analysis).

The other thing that occurs to me with the graphs: Denser material typically has a higher acoustic propagation speed. Could the better absorbtion at lower frequencies be due more to the time that sound takes to move through the medium? That would lower the frequency where a 1/4 wavelength is contained in the aborbtive meduim.

I still think there are some significant membrane properties involved with the corner trap. I have not seen a good treatise in how they work.....




-tINY

[bpape]

I won't say that when going thicker you SHOULD go less dense, but to save money you CAN and still get approx the same benefit provided it is an unfaced absorber.

Now, when you use a facing, then the inherent difference in stiffness of the material behind when using a denser material will in fact drop the resonant frequency of the membrane action - very similarly to building a wooden pressure absorber where you tune based on the cavity depth and front panel (membrane)density.

Bryan

[tINY]

Then, if you want the best absorbtion and you have 12" of depth, a 2" panel of 705 and 10" of air might be just the ticket?



-tINY

[Scott R. Foster]

I still think there are some significant membrane properties involved with the corner trap. I have not seen a good treatise in how they work.....

Tiny:

You are quite correct.

In a corner mounting where the back sides of the panel are touching two room boundaries [as opposed to gapped off these surfaces] the triangular cross-sectioned wedge of air trapped behind the panel together with the panel itself form a very broad Q low frequency resonating system that significantly boosts absorptive performance between +/- 60 and 120 Hz.

IMO the phenomena is better described as a non-rigid panel over a trapped air volume rather than as a simple "membrane" resonance, but that is a semantic quibble... only important to the extent a less precise/exhaustive description might leave folks confused about what you are talking about [likely IMO, thus my objection].

In any event, your surmise is borne out in the lab. There is a significant resonance component to the special case of corner mounting porous absorber panels.

Also, the act of upholstering panels, as is typical with mineral fiber panels, changes the nature of the impedance jump - which can be looked at as beneficial – though it arises at the cost of trading smoothness in the absorption line for higher low frequency absorption performance. All in all, it’s a good trade off for most applications provided the increase is broad Q - not too "peaky". This affect is often erroneously referred to as a "membrane" action, but in fact it more about impedance than related to resonance.

When building your own panels, choosing a fairly dense fabric with a bit of elasticity such as we use on our Ready Acoustics bass traps will help assure that this effect is broad Q… alternatively you can choose a very light, highly breathable fabric if you want to minimize this effect.

PS: If you seek more detail on acoustic impedance the references to sources in the Whealy spreadsheet I linked you too are be excellent. You might also look to a standard introductory text such as:

An Introduction to Acoustics

S.W. Rienstra & A. Hirschberg
Eindhoven University of Technology

March 2006

Section 3.2 - Acoustic impedance

[Glenn Kuras]

Originally Posted by tINY

Then, if you want the best absorbtion and you have 12" of depth, a 2" panel of 705 and 10" of air might be just the ticket?



-tINY

Hands down filling the whole gap will work better, but cost you a arm and leg.

Take a read over this part of Ethan's FAQ.

Acoustic Treatment and Design for Recording Studios and Listening Rooms

Glenn

[Ethan Winer]

Originally Posted by Scott R. Foster

You can model a porous absorber and predict certain elements of its behavior using this tool:

Scott, of course those are simulations, not reality, as explained clearly on the graphs themselves. So it's unwise and irresponsible to rely on those graphs as proof of anything. Here's my better proposal:

As you know, science advances when someone tests a hypothesis, then other scientists try to duplicate the same results independently. I already did my tests using the ETF software, so now it's your turn. If you do the same tests in your bedroom or wherever you test stuff, we'll have a solid basis for a legitimate comparison.

Your company sells 703 and 705, so you're most of the way there. (As an aside, if you believe so strongly that 705 is a poor value, why do you sell it? This is a serious question!) It's important to also test a low density material such as 701, to better identify a trend if there is one. If you don't have easy access to 701, you can simulate that by gently compacting fluffy fiberglass into your MiniTrap knock-off frames. If you aim for the same 1.5 pcf density as 701, that should be close enough to see a trend.

You'll need a loudspeaker that gets down to at least 40 Hz, but I assume you have that, yes? You'll also need to test at least half a dozen panels in a bedroom size room, but if you can muster more panels the results will be even more compelling. I don't recall if you have ETF, but I know you have the Room EQ Wizard program, and that's certainly up to the task.

I look forward to you and I finally resolving this long-standing question of density versus effectiveness! Then we'll both know we're giving folks the very best advice based on solid science and legitimate measurements, versus relying on opinions or simulations.

BTW, I have some other comments about the StudioTips page you linked to, but I'll address those in a subsequent post.

--Ethan

[bpape]

Tiny

4" is about minimal to be effective deeper - 6" is better - BUT by the time you get enough to do 6" straddling, you can make solid chunks of the same size and height with the same materail.

Scott,

As for the trapped air cavity theory, it's interesting and there have been some experiments to suggest that this may have some merit. My question is:

When you go solid, do you feel that you are defeating this effect or do you consider the air still trapped behind but highly damped since it's all IN the fiberglass instead of behind it?

Bryan