cylindrical type diffusor

[foreverain4]

wasnt there a thread a while back about a half round type diffusor. something like taking a piece of plywood and bending it between two cleats? or, if anyone has a drawing, that would be cool...

[knightfly]

Here's one link - these guys have a lot of useful info - just back up to the root of their URL and start browsing -

http://www.electroacoustics.co.uk/ar...on/fuzpoly.htm

A good investment for all the things you want to check out for your studio would be Everest's Master Handbook of Acoustics - he shows basic plans for the poly's, and a TON of other info. Amazon carries it, it's around $25... Steve

[foreverain4]

am i correct in assuming that there is no insulation or anything behind these?

[RICK FITZPATRICK]

I have a a few questions regarding the poly's Steve.

Masonite clear bottom and top, and allow generous air flow behind it,

It seems I have either misread another description of poly construction, or I misunderstand the principle by which it absorbs. Are these the same as panel absorbers, which I am under the impression must be airtight? I do not understand how the curved surface absorbs low frequency, as the curve is under bending forces. Perhaps that has nothing to do with the absorption principle? Could you explain this a little more Steve?

Also, do they have to be curved by way of bending, or can you use preformed cylinders, such as Sonotube or similar thin products, cut at the appropriate arcs? It would seem, if you could use this type of product, if used in the corners, and lined with fiberglass, they would approximate the claims of Tube traps. Or am I missing something? Thanks Steve.
Cheers
fitZ

[DDev]

Here's a link to a tube-type absorber that I have in my bookmarks. I don't know how effective they are, but the concept seems neat. I would think that you could cut them in half and seal the flat side to a wood membrane pretty easily, but I don't know what affect that would have.

http://ic.net/~jtgale/diy2.htm

As far as the half-round thing goes, I'm thinking that one of the more recent studios posted on John's website in the gallery had them. There is also Todzilla's concept where he used a half-round unit along his ceiling.

Darryl.....

[knightfly]

I've got more Poly info on my laptop than this 'puter, but kinda busy for awhile - maybe later today (that's an area I also need more study on, BTW) -

If you just wanted diffusion, sealing them wouldn't matter. If you wanted them to act as a panel resonator ALSO, then they should be sealed. The dramatic Poly wall you see on America's Funniest Videos is just there for diffusion and looks, the individual poly's are too small for anything else... Steve

Hey Rick, you got wind and rain, I got freezing rain and powder snow - Thank God for low-tech, like wood stoves (and wives to bring in firewood :=)

[RICK FITZPATRICK]

Hello guys, say, I'm interested in building some kind of poly diffuser/absorber. But again, seemingly conflicting information confuses me. Sooooo, here I go again.

Darryl, thank you for link. The article is interesting but as usual, my pesimistic nature directs my focus on data which doesn't agree with my limited understanding of acoustical science.

First, in regards to the authors instructions to glue gyp. board discs to the ends of the tube to make it "airtight", I don't understand what he is talking about. How in the world can a cylinder that is completely made of 703, of which air can move through in 360 degrees, be "airtight"? Same with ANY "bass trap" of that nature. PANEL traps yea. I understand airtight in that sense, but with open 703 panels or cylinders, WTF!!
Also, it would seem, if memory serves me, that for the 703 to absorb in the most effecient manner, an adjacent boundary must exist, which in this case, would have to be another cylinder within the 703 sleeve, but somewhat smaller than the inside diameter of the 703 sleeve. HOWEVER, it would seem, being a cylinder, a soundwave with a wavelength larger than the diameter than the 703 cylinder would simply wrap around it as the 703 has more resistance than air. Also, a sound wave must strike a boundary PERPENDICURLY to reflect perpendicurly. How would an internal cylinder reflect perpendicurly? That is if I understand this concept correctly, which is sound(air molecule?) velocity is at its maximum velocity at 1/4 wavelength from a PERPENDICULAR boundary. What boundary is in his 703 cylinder? Here is a quote from the author of the article(please accept my apology for quoting him, but how else can I do this?)
(quote)
"A rough rule of thumb is 10-in. tube will absorb down to about 70 Hz. 20 in. tube to around 40 Hz." (unquote) Hmmmm....

Well, I don't know where he gets this data, but again, FROM MY UNDERSTANDING, NO LABRATORY on earth can validate an absorption test below 100 hz. Please correct me on this if I am wrong, cause I sure don't want to be accused of spreading MORE unfounded "net fact", but if true, then this is just one more example of possibly unsubstantiated data that may lead the reader to beleiving it is fact, and spending time and money building something that may indeed not perform as claimed. Even Owens Corning 703 absorption coefficent spec's don't go below 125 hz.
So, at this point, I must also ask for Steves forgiveness, as I must quote HIM too. This is from a previous thread he replied to in regard to "corner bass traps". I do this because I haven't got the acoustical moxie to state it myself. Steve, your explanation to the workings of a "bass trap" is very informative and enlightening. However, in my mind, this explanation brings up the very issue I am refering to. Which is, the validity of the authors claims as to the absorption ability of his "cylindrical bass trap". Here is what Steve said.(I know I'm going to get maimed for this....)
qoute....
"First, the way bass traps work is that they present an "acoustic resistance" to sound waves.
This is the function of the rigid fiberglas inulation board. Moving air (sound wave) can't get thru the insulation board as easily as it can move in free air, so the energy it uses
to pass thru is partially converted to heat. Less sound energy, less sound.

Every sound has its own wavelength, which is expressed in (normally) either feet or meters; in feet, the formula is Wavelength=1130/f, with 1130 being the speed of sound in feet per second (approximate) at sea level, and f being the frequency in Hertz.

A wall, floor or ceiling is referred to as a "boundary" in acoustics - if we consider these boundaries to be impenetrable for sake of discussion, then at any boundary the air velocity from a sound wave striking it perpendicularly, will be ZERO, while its sound
PRESSURE will be maximum.

If you move away from the wall a distance of 1/4 wavelength of the sound (just one frequency for this example) at that point, the sound VELOCITY will be maximum, and the sound PRESSURE will be zero. This is important, becauseThe only place it does any good to place an acoustic resistance in order to trap sound, is where there is air movement, or velocity. The more velocity, the more sound energy is converted to heat.

This is why absorbent traps need air space behind them - the more distance included in the trap cavity, with all other things equal, the
lower frequency the trap will attenuate. The closer to 1/4 wavelength the absorbent is placed from the boundary, the better that frequency is absorbed.

Using the wavelength formula, if we plug in 100 hZ we find that the wavelength is 11.3 FEET ?!?!! sooo, 1/4 wavelength would be 2.825 feet, or nearly 34 inches. That would be the optimum depth of a trap that would work at 100 hZ - just one example, so you can see
some of the problems of making low bass traps.

One of the benefits of a corner trap is that the depth is varied, so the trap works at a broader range of frequencies.

Keeping in mind that in order for the trap to work, the sound wave must pass THROUGH it, not AROUND it, then it makes sense that one important criteria is that the absorbent material is SEALED around the edges so the sound must pass THROUGH it. The frame is mainly there to support the absorbent, and give you a place to tack a cloth cover, and for looks. The important part is the type and thickness of absorbent, the depth of the cavity behind the absorbent, the thoroughness of seal,and somewhat the angle of incidence that the sound strikes the absorbent. Perpendicular gives more absorbency than a glancing blow, for example. (unquote)

Cool. I just had to read this again and post it for all to read. Thanks Steve. Now, could you explain just one more thing Still don't understand the "sealing" or "airtight" criteria though. How can 703 be "sealed"? If the frame is only for holding the panels in place and for a fabric fastening point, what if you removed the frame? How and WHAT do you seal? Hmmmmm......I must be having an anurism or at the least brain constipation....

Like,

If the speed of sound is "constant" at a given humidity and temperature, what does "sound velocity" refer to? I always thought that "velocity" is the same as "speed". And the speed of sound is constant. So how can sound "velocity" be maximum which implies that the speed of sound is NOT constant. In that case it must start at zero at a boundary, and ACCELERATE to its maximum speed within 1/4 wavelength. Correct? Hmmmm....what causes ACCELERATION? Is the distance an air molecule moves equal to a wavelength, but at the velocity of sound? Man, this stuff is mindboggleing.
This is why I ask questions here. To either get the REAL answers, or get the verbal shit kicked outta me. Either way, if it serves to bring light to this dark corner of ignorance on my behalf, then so be it. Kick away. I can take it

Wait a minute....I take that back. I've already received my verbal abuse for the day from....well, nevermind. Ha!

Anyway, lets move on. This stuff brings up a few more things that I need cleared up so I don't lay in bed till 3 this morning trying to come to logical conclusions.

How bout this. What does the total absorption of a slot absorber depend on? Square footage of the slots? And what is the absorption coefficeint of ONE square foot of slot?
And how do you actually determine the absorption goal? Like, if an open window 1 foot square equals 1 Sabine of absorption, how do you know how many square feet of slots equal ONE Sabine?........not to mention the Hemholtz equasion.....nevermind, I have a headache. Time for bed and a couple of Sabine tablets. Thanks for reading my dribble.....later

Oh, Steve, hope your wife is more firestarting fluent than mine is. And may the gods of shoveling chores be merciful to you. The rain gods are getting ready for a party here. A "flood party" The last one was in 64'.......glurb...gurble....uh,anyone got a life raft? how bout an innertube?....
fitZ

[knightfly]

Rick, remember when I had you stroke your leg? Did you notice that your hand, as it went lovingly back and forth, didn't always go at the same speed, but rather it stopped, then started going the other direction, accelerated through mid point, then slowed down, stopped, reversed, etc... - this is what a sound wave does, and that's the "velocity" that is being talked about. It's the velocity of the air particles that are quivering back and forth. A room boundary causes the sound to stop, so it's always going to be a point of minimum velocity - since velocity and pressure are opposite in phase, that same boundary will be at maximum sound pressure.

This has very little, if anything, to do with how soon someone 1130 feet away will become aware that you've just farted (although wind direction may affect this, if you include olfactory stimulus) (btw, the answer is one second, assuming normal altitude (pressure) temperature and humidity)

It's getting too late for me to answer all your "booklet"; are you SURE you read the Master Handbook, BTW? :=) Oh, by "sealing", they mean making sure that the sound has to travel thru the 703 instead of thru the caps - BTW, I'm not convinced that tube traps are much either - got more study to do on that front meself...

Here's a look at the "foin, foin weather" we be havin' - glad when it warms up and melts all this crap -

[knightfly]

And me favorite cure (or at least, where's a good place ta be while slurpin' it...)

[RICK FITZPATRICK]

Howdy Steve. Hope yer enjoin the foin weather....great for a BBQ huh?

It's getting too late for me to answer all your "booklet"; are you SURE you read the Master Handbook, BTW?

Yea, I'm sure. But even you don't believe everything in there, otherwise why would you be unsure of tube traps. Like I said before, I have the same problem with acoustitions as muffler bearing salesman. If everything he shows in there regarding
studios were fact, why would John say LEDE doesn't work? Opinion. Thats why.. science
is one thing, and I don't have a problem with that. But I do have a view that differences of opinion regarding the actual performance of acoustical design, even by credentialed people, should heed "buyer beware". But in the whole scope of things, don't even bother answering all my brain farts. I am the type of person who questions
things I don't fully understand. Once I do, then I can offer my own difference of opinion. If every person who flew were told it was "a difference in air pressure" that kept that thing they are flying in up in the sky, most would shake their head in indifference, or trust. I on the other hand want to know how and why before I climb aboard. The mere fact a plane flys is not enough to convince me, otherwise why would planes crash. A simple stall at 10,000 feet in a plane with my dad, illustrated the concept vividly. Now I know and believe.

Hey, thanks for the pics. Have a great week. And thanks too for not maiming me.
The hot thumb was enough. Ha! Hey, don't hurt yourself on slippery sidewalks...me, I've got to swim to work this morning!

fitZ

[DDev]

Rick,

I hope my link didn't contribute to any loss of sleep on your part. I posted it mainly because it was round and this post was about round things for diffusion. Personally, I considered something like this for my studio but wound up doing a corner membrane-style trap because it was something that I could make sense of. These cylindrical traps look cool, but I didn't have warm fuzzies about the science, either (although I never put pencil to paper to run any calculations, like you seem to have tried to do). The only thing that makes any sense to me about it is that it is a pile of insulation that has some absorption characterstics that may or may not be effective for the lower frequency ranges.

Darryl.....

[knightfly]

Hey Rick - Just flickin' ya a teensy weensy bit, you know me... But I agree that acoustics seems more like black art and snake oil than science sometimes. I have the same problem with making things out of cloth - damn stuff won't stay the same size. Now, wood or steel - no prob, you cut it to 25 1/4", that's the size the sucker IS, til you find out you cut off too much and hafta cut it again, in which case it's STILL too short :=(

I also think that there are a few too many snake oil salesmen in the acoustics biz - the inexactness of the "science" seems to draw them, 'cause there's only a small handful of propeller heads that know enough to contest their claims, and most of those guys consider it beneath their dignity to even notice...

I do try to keep in mind, though, the times when my pre-concieved notions get blown to hell - that's part of what makes me dig deeper instead of just saying "MBE!!!" (which, of course, stands for Male Bovine Excrement...

One such epiphany - about 15 years ago I had a band that got sucked into one of those "battle of the bands" deals for charity - the gig was outdoors, and some idiot had decided locations in such a way that our 50 foot snake lacked about 45 feet of being long enough (up to then we played clubs and stuff, so more than 50' was just extra crap to carry) - anyway, I had recently scored almost 500' of Belden 27 pair DATA cable, individually shielded twisted pair, so I thought I'd just build a 120' snake with some extra XLR's I hand laying around - Did so, had to use a few transformer DI boxes, so assumed I'd be losing quite a bit of high end, so I preset the board with all the hi EQ's set to +4 instead of "0" - when I started sound check, the highs nearly took my head off. I reset all the highs to 0, ended up with most set even lower.

When I got time, I checked out the specs on that cable - the per pair capacitance was listed at 10 picofarads per foot. Normal audio cable comes in at around 30-50 picofarads per foot. Up to that point, I was saying "MBE" (see above) to the guys that claimed capacitance made a lot of difference in cable runs for audio.

One thing I hope to accomplish this year is to see if ASC (they're here in Eugene) will give me a tour, and let me hear for myself how much of their stuff is snake oil and how much is "HCIAW" - Which, of course, stands for "Holy Crap, It Actually Works"... Steve

[foreverain4]

i started putting the polycylindrical diffusors up in my studio last night. i got one whole wall done. (the one with the mic stands along it) and i plan on doing the one behind the couch. i do not need to do all 4 walls, do i? i see larger studios and it seems as if they typically only do one wall... i must say, these things look really cool. i used 1/8" plywood with an ash veneer. i can already tell there is alot more high end without all the foam in there. i dont have any flutter echo either. they are really easy to do too. i had 3 of them up in about an hr.

[foreverain4]

here is a floor plan of what i am doing.

[DDev]

When you get the chance could you post some pix of your new diffusors and how you constructed them? Sounds like you're on the right track.

Thanks,
Darryl.....

[foreverain4]

i will get some pics of the wall i have done over lunch...

[foreverain4]

here are some pics. it was as easy as fastening rails to the walls and bending plywood between them..

[foreverain4]

another...........

[foreverain4]

and lastly........

[Michael Jones]

Nice job!
Don't they need to be sealed on the top and bottom?

[knightfly]

Everybody, but especially Fitz - Just re-read your quote of one of my posts, and find it necessary to apologise for one of my "brain farts" - To quote Fitz,"Still don't understand the "sealing" or "airtight" criteria though. How can 703 be "sealed"? If the frame is only for holding the panels in place and for a fabric fastening point, what if you removed the frame? How and WHAT do you seal?" -

The simple answer to that is, you don't and you can't. The only explanation I can give for saying that would be either sleep deprivation causing me to "shift gears" and start talking about panel type traps, or just basic stupidity...

You're right, the compressed fiberglas is porous so it's not possible to "seal" it, per se. When it's used as a simple broadband corner trap, you can actually just lean it up in the corner and walk away, as far as acoustics is concerned. There is some slight likelihood that if you were to mount a piece of 703 on spacers off the wall, that a slight amount of lower frequencies could "flow around it", as lower frequencies are omnidirectional. However, I'm not sure how you would ever measure the difference. Plus, if the material is working to that low a frequency, it would be a few FEET off the wall.

The rest of that quote sounds like I may have actually been AWAKE while typing it... Stevezzzzzzz...

[darnold]

I think this brings something important about acoustics. That specs and the science of it all can sometimes be irrelavent. That what it all comes down to is whether or not it sounds good.

I say this because alot of professionals have a basic understanding of acoustics, but do not have near the knowledge you have in acoustic science, but they still know that if they put this here and this here, it sounds good. Just like with preamps, microphones, or any other gear, sometimes the specs just dont really make sense, but it sounds good.

Bleh, just a thought i had while analyzing all of this.

Danny

[knightfly]

Good point, Danny - I've talked to pro's who know you sometimes can get the sound you want by moving a mic as little as 1/4" - but they can't tell you why. I feel that knowing why can speed up your choices of things to try, and how much change to make at a time. Both approaches will get you there - the pro because he's done it a thousand times, and just knows what will probably work. The "bookworm" because he knows the logic and will play with it til he hears what he knows is possible.

Now, if you happen to have BOTH of those going for you... Steve

[foreverain4]

Originally posted by Michael Jones
Nice job!
Don't they need to be sealed on the top and bottom?

actually, i dont think they are supposed to be. from what i understood from my research..

[DDev]

Those panels look great. They definitely add a classy aesthetic touch, and they should provide good diffusion of the sound. Having several together like that should cover the broad dispersion that is needed for diffusion, where a single one by itself might not get that random mix that the theory says is necessary.

If the intent of these is just for diffusion, then my understanding is they don't need to be sealed. Additionally, with the stress applied to the panels by bending them, I would think that they actually would not make very good resonators like you would get if you constructed a membrane-style absorber, so they wouldn't need to be sealed from that perspective.

Now, back to the music.....

Darryl.....