Problem with Helmholtz Perforated Panel Absorber Calculator Logic

[LCRLive]

Hi I was wondering if anyone may be able to help solve an Acoustical issue for me for a new space I have to listen and monitor in. I'm wishing to get a better handle on perforated helmholtz panel absorbers. But alot of these online calculators including the Acoustical modeling one and the whealy one do not account for the side walls (frame) of the actual perforated panel. Not to mention alot of these results not even matching up a cross the calculators.

So they do have fields for the hole spacing, hole size, absorber thickness and total depth, a field for fiber flow resistivity and a field for the thickness of the front board with the holes in it. (whealy is doing strange things with that one in moving the 'no space green line when sliding the airgap spacing fader around, http://www.whealy.com/acoustics/PA_Calculator/index.html

and Acoustic modeling is doing masive jumps when entering any value after 0mm in the airgap 1 field where it really makes extreme jumps in the resonant frequency Q spike and this is not believable at all and going 0mm to 1mm lowers the resonance frequency but then going ffom 1mm to 2mm highers it so its really doing absurd things ... http://www.acousticmodelling.com/helmholtz.php

Also on these calculators (except for mh-archived website which I will get to below) there are no fields for where the panel's top, bottom and sides end (where is the frame?) to enclose the panel and how far the end comes from the last row of holes. Clearly the tuning of the panel will change if one extended the box whatever distance or if they reduced it to being too close to the hole.

I thought I found a solution at the mh-audio archived website hich seems t have an easier way to come up with a panel enclosure for this but there there is no entry even for the flow resistivity of the fiber, thickness of that fiber in there not the ability to set and airgap distance from the front board/

Does anyone know of any beter tools to accomplish all this in a more accurate way because I have scoured the internet for al sorts of calculators and cant seem to find anything useful out there.

Thanks

 

[Sonnie Parker]

This might be something @bowl_actually can help you with.

Last time I researched Helmholtz panels was many years ago. I can't remember the company, but I spoke with a guy on the phone and they would have to come out and do measurements in my room in order to know where to place them and what size panels to build. It was a complicated process. I have no idea if there is an easier method to accomplish it now.

 

[bowl_actually]

Yea, I have done some reading on helmholtz perforated panels but have actually never built one. From my research, they seem to be too narrow and also very fickle to get right. But I guess that's also true with membrane absorbers and pretty much anything other than velocity absorbers (rockwool, fiberglass, etc.).

There are some pretty good calculators and references for circular helmholtz absorbers and a relatively convenient way to tweak/tune them. Like most of these things, probably the best way to build them is to build a test one and measure what it does, learning how to tweak what you build to give you what you want. There are just so many variables in things like stiffness of material you use, how "YOU" seal them vs how someone else does it, etc. that it's hard to get a perfect correlation between the theoretical performance and the actual performance.

Another way that is common among professionals is to build a bunch of them, varying the parameters of each. The thought is to produce a broader range of operation and therefore not needing as specific frequency of operation. For example, if you're tying to treat something in the 60hz range, build three with a theoretical center of 60hz, 3 at 50hz, and 3 at 70hz, giving you 9 panels that cover from about 40hz to around 80hz. If you're design/calcs are off by a bit then it's not as big of a problem.

Another thing that is tricky with helmholtz absorbers in general is that they need to be placed in very specific places. Think of blowing across a beer bottle, if you don't blow exactly in the right angle it doesn't resonate. Helmholtz are the same way, if you don't place them exactly in the right spot, they do nothing.

Now, having said all that... Specifically your question on how close the last row of holes goes. The way this works is, think of each hole in panel and the associated area around that hole as a little "bottle." One half the distance from one hole to the center of the next holes in every direction, gives you the volume of this little bottle. That's the resonating cavity that you're solving the frequency for and that's what the calculators calculate. The reason you want a bigger panel is to create many of these little resonating bottles. So the distance from the last row to the edge has to create a little bottle of the same dimensions as the rest (otherwise the edge rows absorb a different frequency). So the distance from the center of the holes to the edge needs to be the same as halfway between the rest of the holes. This is why none of the calculators say anything about the size of the overall panel, they are just calculating one little bottle, how many little bottles you want in your array is up to you.

It might be worthwhile to study some of the physics around how these work to give you some confidence in building them. Here's a lecture about how they work along with formulas.

 

[LCRLive]

Yea, I have done some reading on helmholtz perforated panels but have actually never built one. From my research, they seem to be too narrow and also very fickle to get right. But I guess that's also true with membrane absorbers and pretty much anything other than velocity absorbers (rockwool, fiberglass, etc.).

There are some pretty good calculators and references for circular helmholtz absorbers and a relatively convenient way to tweak/tune them. Like most of these things, probably the best way to build them is to build a test one and measure what it does, learning how to tweak what you build to give you what you want. There are just so many variables in things like stiffness of material you use, how "YOU" seal them vs how someone else does it, etc. that it's hard to get a perfect correlation between the theoretical performance and the actual performance.

Another way that is common among professionals is to build a bunch of them, varying the parameters of each. The thought is to produce a broader range of operation and therefore not needing as specific frequency of operation. For example, if you're tying to treat something in the 60hz range, build three with a theoretical center of 60hz, 3 at 50hz, and 3 at 70hz, giving you 9 panels that cover from about 40hz to around 80hz. If you're design/calcs are off by a bit then it's not as big of a problem.

Another thing that is tricky with helmholtz absorbers in general is that they need to be placed in very specific places. Think of blowing across a beer bottle, if you don't blow exactly in the right angle it doesn't resonate. Helmholtz are the same way, if you don't place them exactly in the right spot, they do nothing.

Now, having said all that... Specifically your question on how close the last row of holes goes. The way this works is, think of each hole in panel and the associated area around that hole as a little "bottle." One half the distance from one hole to the center of the next holes in every direction, gives you the volume of this little bottle. That's the resonating cavity that you're solving the frequency for and that's what the calculators calculate. The reason you want a bigger panel is to create many of these little resonating bottles. So the distance from the last row to the edge has to create a little bottle of the same dimensions as the rest (otherwise the edge rows absorb a different frequency). So the distance from the center of the holes to the edge needs to be the same as halfway between the rest of the holes. This is why none of the calculators say anything about the size of the overall panel, they are just calculating one little bottle, how many little bottles you want in your array is up to you.

It might be worthwhile to study some of the physics around how these work to give you some confidence in building them. Here's a lecture about how they work along with formulas.

Sounds amazing to me thanks.

The other thing i always wondered was if the whole design of GIK alpha series where its altering shapes of the hole to create that modern circuitboard like design. But for some reason i do not think this will have as good performance as having all the holes be the same size for the same reason that if a person makes a subwoofer box with two different width or length ports, the so called air spring tuning may 'short circuit' themselves into some sort of unintended mess.

 

[bowl_actually]

Sounds amazing to me thanks.

The other thing i always wondered was if the whole design of GIK alpha series where its altering shapes of the hole to create that modern circuitboard like design. But for some reason i do not think this will have as good performance as having all the holes be the same size for the same reason that if a person makes a subwoofer box with two different width or length ports, the so called air spring tuning may 'short circuit' themselves into some sort of unintended mess.

Yea, since I have not purchased or used any of the GIK alpha series, I'm not exactly sure how it performs. However, the science of how it all works would suggest they are building a variety of different "little bottles" into their panel to broaden the range of operation. Much like I described about building multiple panels with slightly offset center frequencies, except just building it all into one panel.

As to whether it performs better then all the same holes, that depends on your definition of performs better. It would operate with a wider range making it more effective for most things. However, as you point out, it's not going to be quite as effective at the center frequency if they are all the same. These things are so narrow in their operation and tricky to get just right at an exact frequency that most people would argue that trading a bit of effectiveness at one specific frequency for a pretty good effectiveness at a broader range to be a good tradeoff. Especially if you're making a commercial product, if you were making them very narrow then each one becomes a "1-off" design that you have to specifically make for each customer.

 

[ddude003]

I don't think the GIK alpha series has much of a helmholtz effect... I believe that is a strike plate or scatter plate reflecting some energy and absorbing some energy... Acting as a absorber and diffuser...

Here is review of some of these products by @Todd Anderson

GIK Acoustics 4A Alpha Panel Diffusor/Absorber Review

We as enthusiasts spend a tremendous amount of time focused on physical equipment, analyzing and picking apart the vast variety of sources, processors, amps, and speakers available on the market today. I’ll be the first to admit, gear and cutting-edge technologies are tantalizing and exciting...

www.avnirvana.com

 

[bowl_actually]

I don't think the GIK alpha series has much of a helmholtz effect... I believe that is a strike plate or scatter plate reflecting some energy and absorbing some energy... Acting as a absorber and diffuser...

Here is review of some of these products by @Todd Anderson

GIK Acoustics 4A Alpha Panel Diffusor/Absorber Review

We as enthusiasts spend a tremendous amount of time focused on physical equipment, analyzing and picking apart the vast variety of sources, processors, amps, and speakers available on the market today. I’ll be the first to admit, gear and cutting-edge technologies are tantalizing and exciting...

www.avnirvana.com

Ah, good info. Great review of those. I wasn't familiar with them, but as the article points out, GIK seems to be a really good company to deal with and about as cheap as any commercial product. Thanks for adding that info.

 

[ddude003]

GIK has plenty of statical information and testing data of their various acoustic treatment devices... The Master Handbook of Acoustics by F. Alton Everest and Ken C. Pohlmann has nice coverage of various acoustic treatment devices including helmholtz systems... You may be able to find a copy of a previous edition as a pdf on the internet...

 

[bowl_actually]

GIK has plenty of statical information and testing data of their various acoustic treatment devices... The Master Handbook of Acoustics by F. Alton Everest and Ken C. Pohlmann has nice coverage of various acoustic treatment devices including helmholtz systems... You may be able to find a copy of a previous edition as a pdf on the internet...

For most people they would be one of the top choices. Their service where you upload information and drawings of your room and discuss your objectives with them, then come up with a plan is a really good option for most people.

I'm an odd case, I actually enjoy the physics and math and have a nice shop where I enjoy building and fiddling around with these things. And I also get bored, so will learn and build one thing then find something different to fiddle around with. For most, the typical DIY panels of rockwool, covered with some cool cloth or even a nice print is the best "bang for the buck" thing to do. I especially like to cover them with nostalgic album cover artwork, you can find large cloth album artwork on ebay.

For just home entertainment spaces, I don't think I would do either helmholtz or diaphragmatic absorbers. They are so specific in what they do you have to do a lot of work to know how to use them.

Also, below the schroeder frequency, you only have standing waves and "hot spots" which, for general listening, aren't as much of a problem as reverb issues. Schroeder in small rooms is generally above 150hz, so treating the reverb can be done with corner traps and 4" and 2" panels. Many people go for these exotic treatments, but don't get the RT60 times balanced across mid-high frequencies, so their room just doesn't sound good.

My advice is, balance out your reverb times down to around 150hz by using the correct amount of 2", 4", and diffusion and see how it sounds. It'll probably sound good and you might just leave it. If you want to do more, add in corner traps to hit some of the 80-150 a bit and it'll probably be pretty great.

If you're mixing music/atmos for a living... Build a specific room and hire professionals to design a good space or if you're like me and have a shop, an engineering degree, and enjoy it, spend a ton of time and effort fiddling until you get it right. Just my opinion of course!