bass trap distance from wall

[mike c]

I read from ethan winer's acoustic education page that the farther from the wall a bass trap was, the more effective it was for lower frequencies ...

when does diminishing returns enter the picture, say I had a bass trap 12" x 12" x 36" to be placed in the corner, can I place it about 20" from the corner and expect better lower freq. absorption?

 

[Ethan Winer]

Mike,

> when does diminishing returns enter the picture <

Maybe a foot away. Once you're farther out than that, the trap is "in the room" rather than near a reflecting boundary. My Acoustics FAQ has a section that explains this in much more detail. Look for the section Optimizing the air gap in the Table of Contents.

> can I place it about 20" from the corner and expect better lower freq. absorption? <

That's probably too far away. But you could always measure at different distances if you have a way to do that.

--Ethan

 

[mike c]

thanks ethan. so how effective are in-room bass traps? e.g. blocks of bass traps the size of 20x20x20 in the middle of the room? or will these mess with my highs and mids?

edit: air gap optimization ... 1130/60hz =18.8 /4 = 4.7 feet (from the wall)

 

[Savant]

edit: air gap optimization ... 1130/60hz =18.8 /4 = 4.7 feet (from the wall)

Probably repeating, but 4.7 feet is probably stretching the definition of "air gap" a little. That said, if you can place a porous absorber at 4.7 feet from the wall, you will be zeroing in on 60 Hz. Provided the absorber is substantial enough (20" cubed would qualify, IMO), this would help towards minimizing any problem - assuming you have one to begin with - at that frequency. I have used 2' by 4' by 4-9" thick absorbers to address certain low frequency problems by placing them "in the room" as opposed to "away from the wall." In general, it's the same principles at work. It's just weird (I guess?) to call the space behind something an "air gap" when it's more like a piece of furniture than a wall panel!

 

[mike c]

haha. thanks savant. I will add more "furniture" with big "air gaps" now.
is there a risk that I kill my mids and highs when I add too many of these DIY rockwool bass traps?

 

[Ethan Winer]

Mike,

> so how effective are in-room bass traps? <

Jeff gave you good advice, as always. I have to ask - why are you determined to put bass traps out in the room? They generally work best close to corners, and that also keeps them from being underfoot. Is there a specific reason you want them away from the corners and walls?

--Ethan

 

[mike c]

initially it was the 16" recommended air gap around the room idea ...
and I thought to myself how far off the wall would it be to actually stop being effective. til I re-read your article and found that the air gap was actually a bandaid for NOT having enough bass trap in the first place. (suggestion to fill the entire air gap with the bass trap was better for more freqs. than having the air gap which is better with a limited range of freqs.)

I could fill the left wall and left corners with bass traps, but can do nothing with the other side - but the other wall is farther away than the left wall.

my side table and coffee tables are empty in the middle ... I was thinking if I could fill them with the bass traps, that it would be better than not having bass traps.

so a better question now would be, would 2-3 big bass traps in front of the LCR speakers mess up my mids and highs. what is the quantity of bass traps that is equivalent to too many bass traps in a room?

 

[Nick250]

Hey Ethan, on the bass traps, do the ones that go halfway up the wall do most of the work, or do you need to go all the up to really get the job done?

Nick

 

[Ethan Winer]

Mike,

> would 2-3 big bass traps in front of the LCR speakers mess up my mids and highs. <

If you put any absorber directly between a loudspeaker and your ears, "mess up" is surely the correct word.

> what is the quantity of bass traps that is equivalent to too many bass traps in a room? <

As far as I'm concerned it's not possible to have too many bass traps. You can definitely have too much absorption at mid and high frequencies, but not below about 300 Hz. I have 40 traps in my living room home theater, and every time I added more the low end got progressively better. Of course, the first 8 traps definitely helped more than the last 8, but they all contribute in a positive way.

Look at it this way: A room with no bass traps has a response that's riddled with numerous peaks and deep nulls. The resonances at the peak frequencies also ring (sustain) even after the sound source stops. So it really comes down to how skewed a response, and how much ringing, are acceptable to you.

--Ethan

 

[Ethan Winer]

Nick,

> do the ones that go halfway up the wall do most of the work, or do you need to go all the up to really get the job done? <

It depends on a few factors, including how many traps you're willing to look at and how much absorption at mid and high frequencies is also needed. The advice on my company's site can be taken as broadly applying to all "porous absorber" traps, but there are specific properties of our traps that make the advice there slightly different from advice appropriate for more conventional absorbers. This applies to all porous traps:

If a single panel is placed halfway between the floor and ceiling, a fair amount of sound can get to the rear of the panel and be absorbed. However, that misses treating the tri-corners at the floor and ceiling. So usually it's best to treat from floor to ceiling, unless cost / effort or appearance preclude that.

--Ethan

 

[mike c]

I forgot to qualify my question ethan: what is the quantity of bass traps that is equivalent to too many bass traps in a room - that will start to mess up the mids and highs?

much appreciated ethan

 

[Ethan Winer]

Mike,

> what is the quantity of bass traps that is equivalent to too many bass traps in a room - that will start to mess up the mids and highs? <

It depends entirely on the type of bass trap and its "curve" of absorption versus frequency. As I said, I have 40 of them in my living room, and it's not too much at all. But that's not necessarily the case for other types of panels.

If you've never seen my living room with 40 traps, have a look at the photos below.

--Ethan

 

[mike c]

wow.

since you have access to the stuff (e.g. real traps) how come you don't use bigger - thicker - blockier bass traps? or does the diminishing returns kick in after 4-6" thick?

 

[Ethan Winer]

Mike,

> wow. <

Yeah, and it sounds great too. Most of my friends are professional musicians and/or audio engineers, and they're always amazed at the sound quality. Especially considering that most of my electronic gear is mid-level consumer stuff.

> since you have access to the stuff (e.g. real traps) how come you don't use bigger - thicker - blockier bass traps? or does the diminishing returns kick in after 4-6" thick? <

Not sure if by "use" you mean use or manufacture. We do have two models that are 4 inches thick, and I have a few of those in this room. But most of the traps are 3 inches, and they do a great job. One trade-off is the physical size FedEx and UPS will handle. We're right up against the "Oversize 2" limit now, so if the shipping boxes were even one inch larger the cost would jump considerably. But six inches thick isn't needed anyway, at least the way we make them.

--Ethan

 

[Savant]

does the diminishing returns kick in after 4-6" thick?

Whether or not thicker traps would be of benefit depends on the specific situation. It all depends on what the goal is. In general, the lowest useful frequency of an absorber continues to go lower as thickness is increased. Diminishing returns won't typically kick in until an absorber is quite thick. (It should be noted that there are diminishing returns at high frequencies. E.g., 4" and 6" thick porous absorbers have about the same performance above 250-500 Hz.) Practically speaking, if a room is going to benefit from an 9" absorber, then a 9" absorber should be considered. Of course, like any problem, it is likely that there are other solutions worth considering - some of which might be thinner or thicker than 9".

But I digress. The answer to your question - in a general sense - is that there are no diminishing returns to increasing the thickness of a porous absorber. There will be some point where increasing the thickness further does not produce usable results in the audible frequency range...but that's up around the 10-15 feet thick range...

 

[mike c]

to confirm ... as the traps get thicker, the traps absorb lower, but still have the same high freq. absorption.

what about absorption coefficients ... 1.00 is 100% why are there ratings above 1.00? e.g. 3.00 = 300% ... 100% is 100% absorption, what does the excess 200% do?

 

[Ethan Winer]

Mike,

> what about absorption coefficients ... 1.00 is 100% why are there ratings above 1.00? e.g. 3.00 = 300% ... 100% is 100% absorption, what does the excess 200% do? <

Most of the excess is due to the edges of the material absorbing sound, but not being included in the conversion from sabins to absorption coefficients. The quote below is from the article Measuring Absorption on my company's web site.

--Ethan

You may notice that absorption coefficients sometimes have a value greater than 1.0. Although it is impossible for any material to absorb more than 100 percent of the sound, measurements can yield a value greater than 1.0. The main reason this occurs is that all material has a finite thickness, and the edges - which are not included in the stated surface area - absorb some of the sound. So for an acoustic panel two by four feet and four inches thick, the real surface area includes the four-inch thick edge around the material. If included in the measurements, this would add four square feet to the stated surface area of eight square feet - a 50 percent increase! (See The Numbers Game below for a more detailed explanation.) Even when the edges are included in the total surface area, values slightly greater than 1.0 are still possible due to diffraction effects at the material's edges and corners. When the edges are rounded, this effect is reduced.

 

[Savant]

to confirm ... as the traps get thicker, the traps absorb lower, but still have the same high freq. absorption.

what about absorption coefficients ... 1.00 is 100% why are there ratings above 1.00? e.g. 3.00 = 300% ... 100% is 100% absorption, what does the excess 200% do?

The numbers to which you are referring are not percentages. They are simply a representation of the relative absorption. All other comparative factors being equal, a material with an absorption coefficient of 1.40 at, say, 2000 Hz, is simply better than one with an absorption coefficient of 1.17, or 0.90, etc.

Having said that, due to the "finite" nature of the parameters involved in the testing of acoustically absorptive materials, one rarely sees absorption coefficients above 2.00 or 3.00. Not to say it's not possible. Just that it's rare.

 

[mike c]

Most of the excess is due to the edges of the material absorbing sound, but not being included in the conversion from sabins to absorption coefficients. The quote below is from the article Measuring Absorption on my company's web site.

I read about that, but failed to put 2 and 2 together. thanks

The numbers to which you are referring are not percentages. They are simply a representation of the relative absorption. All other comparative factors being equal, a material with an absorption coefficient of 1.40 at, say, 2000 Hz, is simply better than one with an absorption coefficient of 1.17, or 0.90, etc.

Having said that, due to the "finite" nature of the parameters involved in the testing of acoustically absorptive materials, one rarely sees absorption coefficients above 2.00 or 3.00. Not to say it's not possible. Just that it's rare.

thanks Savant.