[william]

Okay, I have placed five fiberglass panels in my music room, which seems to have created one heck of a focused sound.  They are leaning, one at the side wall first reflection points (5x7), one in each corner behind the speakers (4x7), and one centered behind the speakers (5x7).  I still have an echo from where I am sitting, though, and I have done nothing for bass response.  Is there an exact science approach possible as to where one should place traps?  I am very nervous about buying traps and not needing them or not placing them in the proper locations.  Any assistance would be greatly appreciated.  Thank you.

William



Die Erleuchtung pre, brand new Odyssey Extreme Monos (not even used yet) amps, Snell C/II speakers, Sonic Frontiers transport, EAD - III D/A convertor, Sonographe turntable with unknow arm and Advent cartridge

[JLM]

William,

Most of the room acoustics "science" is good rules of thumb.  The real "deep" science would be based on acoustically measuring the room.

Imaging/soundstage is improved by absorption/diffusion of 1st reflection points from each speaker to the listening position.  Don't forget the floor and ceiling.  And if you're sitting close to the back wall, don't forget those 1st reflection points too.  In the case of imaging/soundstaging its higher frequencies that you need to be concerned with, so most any foam/fiberglass should work.

Taming bass reverb is a matter of thick/heavy bass traps, that ideally are located in as many corners as needed.

Here's a link you can play with:  http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

[Ethan Winer]

William,

> Is there an exact science approach possible as to where one should place traps? <

Yes, and it's not really that complicated. As JLM said, you put mid/high frequency absorption at the first reflection points, and bass traps in the corners. Then you can assess if more mid/high frequency absorption is still needed. For example, if the room has large areas of bare wall you'll want a few additional panels there to prevent flutter echo between those parallel surfaces. Likewise for the ceiling if it's made of sheet rock and the floor is also reflective.

Here's a drawing showing the basic arrangement of acoustic treatment:



The panels marked "1" and "2" are bass traps in the corners. The panels marked "3" are mid/high frequency absorbers.

--Ethan

[John Casler]

William,

> Is there an exact science approach possible as to where one should place traps? <

Yes, and it's not really that complicated. As JLM said, you put mid/high frequency absorption at the first reflection points, and bass traps in the corners. Then you can assess if more mid/high frequency absorption is still needed. For example, if the room has large areas of bare wall you'll want a few additional panels there to prevent flutter echo between those parallel surfaces. Likewise for the ceili ...

Excellent drawing.

But I might add that the reality of it is that treating "all" the intersecting room boundaries could benefit the bass.

That is, the lower frequency bass waves spread out "omnidirectionally" until they encounter a room boundary (wall/ceiling/floor) and then travel along that boundary untill they reach another.

This means the sound wave hits the wall/floor/ceiling and will travel to both the corners (where two boundaries meet) or the "tri-corners" (where three boundaries meet.

The energy traveling along each wall for example is somewhat "funneled" into the corners.  As this energy approaches the corners, it finds a reduced volume (air space) and actually increases in pressure (that's why the bass is so strong in the corners)

The waves then collide with each other at these intersections and and compress and have no where to go except to "rebound" back out into the room directly from these corners.

The bass traps and treatments function by several different principles:

1) the foam "in corner" LENERDS absorb some of the energy as it arrives in the corners.

2) The bass traps themselves do just what they say.  They "trap" the rebound wave as it is being pushed out of the corner

3) Another type of treatment, that is not discussed as often is wall obstructive devices.  These are placed along walls of "highly traveled" areas and reduce/interupt/absorb the power of the bass energy as it flows along the wall, ceiling or floor, and reduces the power/energy of the wave before it enters the corner.

Creative placement of some furniture, bookcases, etc, can also be mildly helpful to this end.

So I guess, the diagram shows a good starting coverage, IMHO you need more of Ethan's products than even he shows

Ethan do you have any type of "on wall" broadband absorber/trap?

Just a couple thoughts.  Hope they made sense

[ctviggen]

You can attach Ethan's traps to the wall (with or without something to space them off the wall -- apparently, they work a little better off the wall).  You can also get stands, which is what I've done.  I don't have any ceiling treatments as of yet.  I also have three doors along one wall, so it's hard to find a place to attach a trap, and I live in a "raised ranch," so my outer walls only go up half way, then go out about a foot.

[Ethan Winer]

John,

> the lower frequency bass waves spread out "omnidirectionally" until they encounter a room boundary ... <

Good explanation, thanks.

> Another type of treatment, that is not discussed as often is wall obstructive devices <

I'm not sure how that would work. You can't diffuse bass frequencies very well in a smallish room, and even if you could that won't solve the ringing which is fully half of the problem. Or do you mean something else?

> do you have any type of "on wall" broadband absorber/trap? <

All of our panels can be mounted on a wall, and if they're placed out from the wall they absorb to a lower frequency. But corners are still the best place to put them, unless the room is so large that treating all the corners still leaves a lot of bare wall surface. In that case additional traps can go on the walls, especially the front and rear walls.

--Ethan

[John Casler]

John,

> the lower frequency bass waves spread out "omnidirectionally" until they encounter a room boundary ... <

Good explanation, thanks.

> Another type of treatment, that is not discussed as often is wall obstructive devices <

I'm not sure how that would work. You can't diffuse bass frequencies very well in a smallish room, and even if you could that won't solve the ringing which is fully half of the problem. Or do you mean something else?

> do you have any type of "on wall"  ...

Hi Ethan,

What I am talking about is the absorber/obstruction type devices, generally placed vertically along side walls.

In some "specialized rooms" the ceiling is even "louvered" to the same effect.

Bass waves spread out in a room.  They hit the wall and presurize against it, as they travel along it.

The devices I'm refering to are of many types.

Some look like 1/2 columns (round half moon shaped columns) that are attached to the wall with the flat side in.  If they are heavy enough, they "disrupt" this "pressure flow" on its way to the corners.  It would seem that they would have high pressure on the frontal surface and lower pressure on the rear surface depending on the direction the wave was flowing.

I could see your minitraps performing this task even more effectively by simply attaching the leading edge to the wall, and raising the "trailing" edge slightly.  This would create an area of "low pressure" on the backside and also effectievly work as a "backwave trap" of the rebound wave.

In fact an alternateling leading edge out trap then followed by trailing edge out would seem to also offer some benefits, in theory (my theory that is)

Might be a new use for your traps.

Or you might consider mounting the trap "perpendicular to the wall, and then using a LENERD type fill on each side.

In essense, if you mounted 2-3 of these at 1/3 or 1/4 distance intervals on the wall it would seem that the bass frequency flow and subsequnt corner pressures would be reduced?

Sorry for brainstorming  but I can see your minis in that app.

I know I've mentioned this in other posts, but if you want to see how bass runs to the walls, compresses/pressurizes, moves along them, collects, and then "slams" back out at you in a highly pressured "rebound", simply do the following.

Go in your kitchen, and run a full sink of water.  Wait until the surface is very still, and dip your finger in quickly.  This will create a wave, that will travel out until it hits the edges of your sink and then travels to the corners and slams back out.

It is very interesting to see (from a simple science POV).

Now water waves and sound waves are not the same, but seeing this, will help you visualize some of what is happening to the larger waves (Low Frequencies) in your room.

Now, if there were little traps along the edges of your sink to "disturb" this pressure,  it could only help, reduce the sound damaging "rebounding slam" we are generally trying to "TRAP" from the corners.

Just my perception

[Ethan Winer]

John,

> Bass waves spread out in a room.  They hit the wall and presurize against it, as they travel along it. <

Yes, thanks, I can even follow all that.

But again, reducing ringing and lowering the Q of the modes is a very important part of bass trapping, and I'm pretty sure no amount of "wave redirection" will do anything to help those.

--Ethan

[John Casler]

Here are a couple shots of some of the devices I was talking about.

The first is the ceiling louvers, which of course are "broadband", but depending on construction would have the potential of disturbing/trapping bass wave travel.

The second shows the half moon columns that too disturb low frequency flow (at least the look like they would)





I'm of the opinion that your traps could also be used in a cantilevered (one side attached, and one side slightly out) attitude.

If the wave approaches the "open" end it is captured and directed into "closed" opposite end.  If it approaches the closed end, it is redirected back into the room, subsequently reducing its pressure and power, than if it had continued "unimpeded" to a room intersection.

This also makes the "corner traps" more effective since they have less wave pressure to deal with.

????

[JLM]

Diffusers are effective in improving imaging/soundstage above 1000 Hz, but here at AC I don't read of them much.  

[youngho]

Sorry, I'm a bit dubious of the explanations given in some of these posts. I only have a high school physics education, so I'm a bit uneducated, but some of this sounds a lot more like feng shui than acoustics.

My understanding was that approaching a room boundary meant approaching an antinode, where a standing wave has maximum amplitude, for all odd-order axial modes corresponding to the axis perpendicular to that boundary. A corner is the intersection of three boundaries and consequently the antinode of three standing waves. The reason that bass sounds boomy in corners is because it is the antinode for all axial modes in the room, not because bass is somehow attracted to the corner.

"Obstructive" devices, otherwise known as diffraction, have little to do with redirection of flow of "bass energy," and it's easy to imagine a room with concrete floors, walls, and ceilings treated with all sorts of foam and other fancy "obstructive" devices with little or no absorption at low frequencies. What will happen? The room will be very boomy.

The ripples that you describe travel outward from the point of disturbance until interrupted. They reach the corners because there is no interruption between the point of disturbance and the corners, not because of the sides of the sink. When they reach the corner, they rebound from two boundaries.

Incidentally, details of the construction of the room pictured in John's post:

http://www.positive-feedback.com/Issue16/lavigneroom.htm

[denverdoc]

Dubious? As you should be, if everyone here had a HS physics class behind them, we'd all be more enlightened
John

[youngho]

Sorry. I made a few mistakes. It's been too many years since physics.

All assumptions were made using a rectangular room.

I wrote that a boundary is the antinode for all odd-order standing waves. I was thinking of the center of the room. The boundary is the antitnode for all axial modes related to the axis perpendicular to the boundary.

I wrote that the corner is the antinode for three standing waves. I meant that it's the antinode for all standing waves related to the three boundaries that intersect in the corner.

Sorry! Cheers, and happy listening,

Young-Ho

[woodsyi]

I had highschool physics.  It was even an AP class.  But darn it!  I don't remember no stinking congruence of odd order axial antinodes of standing waves perpendicular to the three sides of a corner.  I only just found out about axis of evil waves while sitting on my potty doing some extra curricular reading!

[ctviggen]

Do they teach acoustics in physics?  They teach light propagation and electromagnetic propagation, some of which is transferable to sound and some of which is not.  I think that diffusers aren't discussed because they take a lot of space.  Look at the ceiling of the picture John put up, for instance.  That's a lot of space.  What John's saying though is an interesting idea -- combining diffusion with absorption, although I imagine that actual implementation is harder than it appears.

[denverdoc]

Not in my physics class or was that chemistry I took?   I guess my point was merely that modelling physical systems mathematically is for most of us a skill that is developed with practice, and having that background makes the self-education go a lot more smoothly. And it's not just an issue with forums, I have read all kind of non-sense from audio writers in most of all of the glossies--Audio mag seemed to be the best during its time,
Cheers,
J

[youngho]

AP physics at my high school included a section on waves, not only of the electromagnetic variety. I did browse through Everest's Master Handbook of Acoustics, the vast majority of which was way over my head, in the first few years of college. Also, there's a great tutorial on bass at the Harman website at http://www.harman.com/wp/index.jsp?articleId=122

I don't know much about anything, which is why I included the disclaimer at the beginning of my post. I did look at the ceiling of that room. You can read about the construction of that room in the link that I included. What seems to take up a lot of space in that ceiling is the absorption.

"Another type of treatment, that is not discussed as often is wall obstructive devices. These are placed along walls of "highly traveled" areas and reduce/interupt/absorb the power of the bass energy as it flows along the wall, ceiling or floor, and reduces the power/energy of the wave before it enters the corner."

Perhaps my reading comprehension skills have deteriorated, but the description of "bass energy" here sounds more like a stream of fluid than a wave phenomena.

"The energy traveling along each wall for example is somewhat "funneled" into the corners. As this energy approaches the corners, it finds a reduced volume (air space) and actually increases in pressure (that's why the bass is so strong in the corners)"

This paragraph sounds as though light objects in the corners would fly into the room, moving from an area of higher pressure to lower pressure or that Bernoulli's principle would dictate that the "bass energy" would flow more slowly due to higher pressure.

Anyway, like I said, I was just a bit dubious. Perhaps I am entirely wrong and bass really does flow around the room like a spray of water from a hose, dust in the corners of the room fly further into the room due to reduced volume and increased pressure of bass energy, and moving into the corners means that you'll hear the bass either decrease in frequency or even become inaudible because "the bass energy" is no longer travelling at the speed of sound. What do I know?

Young-Ho

[denverdoc]

Assuming then you have seen a set of partial diffy-q's for a wave eqn and have been able to follow the soln from the imposition of boundary eqns, you're way ahead. Seems to me the imposition of objects will help to break up some of the longer axial modes, period.

The paper on optimazion of sub placement on the Harmon site, is very good reading. Placing a pair of subs mid wall in opposition flattened my 1/12 octave response from something like +/- 6dB to less than 1/2 that. This is not a placement I would have tried. Also the simulation of 4000 subs in a room is interesting as well, the room gain vanishes and the response limits to that of the nearfield response of the driver/enclosure itself.
J

[woodsyi]

Youngho,

I was just poking fun at your detailed explanation of nodes and antinodes with what you claim was just high school physics education.  You obviously know more than what a standard US highschool physics class teaches!  I do think when wavelenghs are measured in yards rather than inches as in bass, little bumps on the wall or ceiling don't do much.  My extra curricular reading is a 4th edition of the same handbook you mention.  A lot of good information in that book.  Cheers.

[youngho]

Woodsiy: Ai, you got me. I did forget to mention that I took a year of physics in college, too, as a prerequisite for my major, though I was rather heavily involved in extracurriculars in the time and consequently absorbed little. I was "diffracted" by other things. What I vaguely recall are the basics, which I really did learn in high school.

Denverdoc: I have taken a look at the Harman paper. You and I had a discussion about several issues related to it a few months ago. The imposition of objects will not necessarily help to "break up" axial modes, period. Again, I suggest constructing a room with concrete floor, walls, and ceiling. Cover all the surfaces of the room with that egg carton stuff. The room will still be boomy. Those axial modes will still be there, even though these particular "obstructive devices" might be thought to disrupt the "flow" of "bass energy" along the walls. Take a look at the anechoic chamber at Bell Labs: http://www.bell-labs.com/org/1133/Research/Acoustics/AnechoicChamber.html. Notice that they don't mention absorption below 200 Hz. Take a look at the anechoic chamber at Meyer: http://www.meyersound.com/products/technology/chamber.htm. Note what they say about bass absorption. Now what do you think about objects and the "flow" of bass?

Young-Ho