Slat Absorbers

Hi everyone! i was just building a slat absorber/resonator and realized that i didn't quite know how to make one. could someone plz give me some tips? i've got all the measurements and stuff but am lost and puzzled as to how its supposed to be built and how it works. like, when you attach the slats onto the wood, wat material r u supposed to put as insulation or absorption? and r u supposed to put material inside the box as well? i'm soooo confused!:( i would really appreciate any help!:)

kayb7 wrote:

Hi everyone! i was just building a slat absorber/resonator and realized that i didn't quite know how to make one. could someone plz give me some tips? i've got all the measurements and stuff but am lost and puzzled as to how its supposed to be built and how it works. like, when you attach the slats onto the wood, wat material r u supposed to put as insulation or absorption? and r u supposed to put material inside the box as well? i'm soooo confused!:( i would really appreciate any help!:)

Have a look at this awesome animation from Kendale n my thread. I was a bit lost too! By the way. Always put your location in your profile so others know where you are. Insulation needs to be rigid/semi-rigid. Guys in USA use a product called Owens Corning 703 Fiberglass insulation, commonly referred to here as 703, OC 703 or a 703 absorber (an absorber using 703 insulation or equivalent). You can also use a rock wool based semi-rigid or rigid insulation. My thread has a lot of info for newbies in it cause I was very new to it as well. There is a standard of how much mass they need to be, I think there is a link to it in my thread, otherwise hopefully some of the more experienced guys put it in here, I don't know off the top of my head, sorry:(. Cheers Willo http://www.johnlsayers.com/phpBB2/viewt ... 8&start=30

thanks, that was really helpful! by the way, does it really matter if the thing is airtight? surely it wouldn't affect it too much right?

kayb7 wrote:

thanks, that was really helpful! by the way, does it really matter if the thing is airtight? surely it wouldn't affect it too much right?

The basic idea of a slat resonator is that it is based on Helmholtz principles. Hence a slat resonator IS an Helmholtz resonator. An Helmholtz resonator is a mass-spring system or mass-spring resonator. The mass here is the air enclosed by the slots between the slats. Hence you alter this mass by varying the width and/or depth of these slots (depth mostly equal to slat thickness in standard applications) The spring here is the air enclosed by the volume behind the slats. Hence you alter the stiffness of that spring by altering the cavity depth (in fact more correct is by altering the cavity volume). This air really works as a spring. Air is even a rather stiff spring. Take a medical syringe (but remove needle .... can hurt). Press your finger at the end to close it off and try to close that syringe. You feel the extreme resistance of the air acting as a spring. If you now let the resonator open on one or more sides, this air can escape (as if you should drill some mini to larger holes in that syringe, see sound as pushing and pulling on that piston in that syringe). Hence you alter the properties of that air spring, which will retune/alter that resonator (difficult to calculate). But you nowhere told what exactly you want to build (some small resonator, cladding your complete walls, other .......? Narrow band, broad band, other ....?)

i just want to make a small slat absorber built to absorb a particular frequency (800hz) - its for a school experiment. i'm just a bit lost as to how to put it all together but i have all the dimensions - eg. slat depth, slot width ect. btw, can you suggest something, like glue or whatever, that is relatively cheap to make the box airtight? i am also a bit confused as to whether the slat absorber is actually a closed box with slats stuck on to the front of it - is this right? and should i put some material (insulation eg, wool) inside the box as well as where i attach the slats to the front of it? i'm sorry if i do not sound very clear - this is a new topic for me and there is not a lot of information (that i can find) on the internet.

Kay, Can you make some sketch how you see this now? And what exactly you're stuck with? Have you searched this forum? The post and thread referred by Willo? Have you found the calculator? Do you now already which slat width and thickness and slot width you need? What the cavity depth or volume should be? There isn't any other forum on the web where so much pictures and drawings of slat resonators can be found than here. Have you learned at school what a slat resonator is in fact? What a Helmholtz resonator is? Did you understood my explanation?

firstly, thankyou for replying- yes, i did understand your explanation, i have read the thread referred to by Willo, found the calculator, cavity depth etc. and after searching this forum a bit more have a better understanding of what a slat absorber is. I am not too good at building things you see, and was just worried if i was doing the right thing or not. one question - how do you make the cavity air tight?

Just make that box on which you nail, screw, glue the slats (your choice) rather accurate (it doesn't need to be watertight). If it isn't accurate enough use caulk or whatever mastic. And if you need this narrow tuned at 800 Hz, you don't need any insulation in that box.

thanks for you help - much appreciated.

Just one last thing. I don't know how accurate this school project should be and if someone will test/measure if you got this 800 Hz correctly tuned. If that's the case then make that box as described, firm and stable, and make the slat width as such that the 2 edge slats have a 50% width covering the empty space of the more centered slats, but still leave them a bit wider. Then mount these slats temporary (as per your theoretical calculation) with something you can still adjust it (small nails, not completely knocked in or something). You'll notice that these formulas are stylized things ignoring some possible side-effects. There can be correction factors not exactly covered by these formulas. As such you can still easily adjust your tuning by shifting the slats closer together or shifting them apart a bit. When sure your resonator is correctly tuned then fix the slats better (and adjust width size of edge slats if you want it nice). If you find someone to help you you can make long-holes in these slats and use screws and washers. As such you can easily retune them. Could be a nice thing to demonstrate the principle in class. But I don't know the exact terms of your school project of course. :wink: I hope you got a clearer description of what's expected from you than what you gave us here.

i am trying to make the absorber as accurate as possible. can you expand a bit more on these correctional factors which are not taken into account in the formulas? and i do not understand what you mean when you say: "make the slat width as such that the 2 edge slats have a 50% width covering the empty space of the more centered slats"

Kayb7, Now it's first up to you. Can you please make a draft/sketch what you're going to build based on what you know until now? Who is and how are you or whoever testing the properties of that resonator (if someone is going to)? I'm not used to make drawings as some people are able to here. What I'm saying from these formulas that reality is complexer than a simple formula, hence you must take count that you can adjust them somewhat. And no I'm not going into that math, which is exponentially more complex than that base formula, and which certainly goes far beyond the scoop of your project. It's much more difficult to calculate then just fine-tuning your resonator in practice.

here's a very quick sketch. you need to adjust the width of the slats, gaps (slots) between them, and depth based on target design goals. it would be sealed to the wall or you could seal it with a plywood backing. adding insulation causes the Q of the resonator to lower a bit (+/- about 1/2 octave?) from the center frequency.

Glenn, :wink: I sinned .... Was easier to adjust your nice picture a bit.

Eric - feel free to sin more often :-) all excellent points. could you clarify the edges should have 50% volume? i'm assuming you mean (in this example) the end slots should be (50%) less open than the other slots. if this is correct, could you explain why this helps? as always, thanks for sharing your knowledge!

Hi Glenn, :twisted: Damned, I urgently need to learn using a drawing program (played this one together in MS Word). If your mass-spring systems aren't symmetrical than you get a phase shift lowering the resonance peak but broadening the frequency range. Even the slightest damping (the cloth) will influence this Q. It acts as a damper on the mass-spring system (certainly in this possible high tuned resonator). Thanks for your gentle reply. :) I still wonder what exactly the class project was/is the OP is working on.

Eric - Google Sketchup is very easy to use and it's free. :wink: cheers john

ah... so in a high-Q resonator, the air spring function needs to be balanced/symmetrical on the slats in order to prevent a phase shift which results in a drop in the resonant frequency and a lowered-Q. since i tend to think in broadband terms most times, i wasn't aware of this. this is a good bit of knowledge to have if one is trying to fine tune a high-Q resonator and finding it not behaving as designed... i agree with John - sketchup is free and pretty intuitive to use...

Thanks John, Glenn, Yes I'm convinced about sketchup. And your forum is living proof of the flexibility. It's more Eric, which is the problem. But I agree I must do something about it. It's fun if one can easily visualize one's thoughts with nice pictures and drawings. Glenn, Just a small thing. The way I sketched it, is in fact the manner the traditional referred formulas are intended and buildup. It's only because these formulas just give input values to be entered, that people don't feel what they stand for. I know that for wall covering slat absorbers, with diverting slat and slot widths, additional mineral wool and cloth, and angled cavities and so on, it all doesn't matter that much since one wants a broadband effect after all. But the base formula calculates the principle as I drafted it. The symmetry in the mass-spring systems is defining, not the symmetry in the slats/slots.

thanks Eric! yes, i understood the mass-spring symmetry to be necessary for this application and its well appreciated! when is that book of yours coming out? :wink:

goood question Glenn :) Eric??

Aloha, Sign me up for a copy. What a great idea! :wink: Aloha 8)

Thanks guys, I made a picture to explain my last comment, before I saw Glenn's reply. I just enter it anyhow since it visualizes the idea a bit. Maybe useful for others.

About that book: :) I'm looking for less, not more work. Rod has experience what an incredible amount of time and energy is involved. At the other hand, my sister often told me: "Eric why don't you?", you leave something behind you feel good about. Energy ........... ? Rod knows more about me, I wished we could have joined forces. Rod wrote an extremely gently thing in his acknowledgments to and about me in his book (vague to the outside world, but with his hart and I'm grateful for that). It's more to than about me.

Eric_Desart wrote:

About that book: :) Rod has experience what an incredible amount of time and energy is involved.

It almost killed me for sure........... :wink:

Rod knows more about me, I wished we could have joined forces.

As do I Eric, that would have been immensely fun - but also an extraordinary effort putting it all together while sitting 1/2 the world apart............ :(

Rod wrote an extremely gently thing in his acknowledgments to and about me in his book (vague to the outside world, but with his hart and I'm grateful for that). It's more to than about me.

My friend - I did not say a single thing that wasn't the truth....... you have gently guided me along my quest for information - and for that I will be always in your debt...... Sincerely, Rod