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Trying to understand the limitations of Helmholtz resonators in LF absorption

sarumbear

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* Ideal: full coverage of walls or whatever solution "that absorbs the room resonances so that the RT60 is uniform across the 20-400Hz range"
* Good enough or effective: what, for a certain user, gives a result worth the effort taking into account practical limitations. (Remaining imperfections can maybe still be improved with DRC).

Which of both applies to the tube resonator solution you implemented in your living room? I think that's what the OP wants to understand.
The first is the only one which is objective, measurable and repeatable. The second is subjective; it depends on the person.
 
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Delrin

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Thanks to all who have replied!

Perhaps I should formalize some ground rules for what I will call "the Helmholtz Open Science Challenge". I'd like to think this is in keeping with the ethos of a forum dedicated to the scientific understanding of audio concepts. As noted before, I am soliciting information about working Helmholtz absorber designs along with solid evidence that they actually contribute to the control of modal ringing and related phenomena.

To "win" the challenge, posts should meet the following criteria:
  1. The general characteristics of the room, such as the dimensions and modal frequencies targeted, should be given. If the room has a complex geometry that defies a simple length/width/height description that's fine - just give a reasonable idea of its shape and size (particularly the dimension(s) contributing to the targeted mode(s)). Any acoustic treatment present just prior to adding the Helmholtz absorption should be indicated.
  2. The approach used to model the Helmholtz absorber should be described in detail. What software or online calculator was used? What parameters were input? Give sufficient information for another person to replicate your simulation. Using expensive industrial modelling software is fine - just give the software name and version, and what the input parameters were. In addition to the general design parameters used to set resonant frequency, the approach used to determine the amount of Helmholtz absorption should be detailed (ie the area coverage of wall assemblies or number of large tube resonators along with their neck geometries). The rationale employed in the placement of the Helmholtz absorber(s) in the room should also be detailed (eg rear corners? entire wall? wall/ceiling corner?).
  3. The fabrication of the Helmholtz absorber(s) should be described in sufficient detail to allow another person to replicate the design. What materials were used? Was any special tooling required? Provide all necessary dimensions.
  4. The acoustic properties of the absorber should be measured in "unit testing" conducted by placing a measurement mic close to a neck port during a frequency sweep from 20-500Hz. Ideally this will be conducted both before and after the addition of absorbent material. Other parameters for the acquisition and display of these measurement sweeps should conform to the REW guidelines posted by amir as a sticky thread in this sub-forum. If you really want to use something other than REW it's fine, but the information presented should be equivalent. SPL/phase plots, as well as waterfall and spectrogram, should be presented. Bonus points for posting the mdat file.
  5. The effect of the Helmholtz absorber(s) on room acoustics should be demonstrated by room measurements conducted, at the main listening position, with and without the absorber(s). The REW measurement and display parameters should be set as described above (allowing for substitution of alternate measurement software yielding equivalent information). If moving the absorber(s) in and out of the room is impractical, it would be acceptable to neutralize immovable resonators by blocking their ports. The primary criterion is demonstration of reduced modal ringing at the main listening position, but if enlarging the sweet spot is the main goal (eg in a home theatre situation) then multiple measurement locations will be considered.
I realize this is asking a lot, and if you have proprietary designs you use to earn a living then this won't be appropriate. However, many people post here and on YouTube out of a genuine motivation to share useful knowledge and guidelines like those above might help maximize the benefit. The YouTube videos posted by Andy Mac Door come close to meeting this standard, while many other well-meaning efforts (on YouTube and elsewhere) fall far short.

The "winner" gets bragging rights and a coupon redeemable for a pint (or two) of their favourite beverage should we ever both be in the same town. I also hope to use the examples gained in graduate level university courses I teach, and will be happy to credit a winning contributor either by real name or pseudonym.

No doubt these rigid guidelines are not perfect, and I hope this will stimulate some discussion and constructive criticism (which I welcome). I think the spirit of the challenge is clear, and reserve the right to edit the "rules" where applicable.
 
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Delrin

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Is that really all Delrin? :rolleyes:
Good luck.

If someone is going to post about their great DIY Helmholtz absorber treatment, as I have seen done, then why not provide sufficient information for others to achieve a similar benefit? There are many YouTube videos and forum posts - on other topics - that already meet this standard (and, unfortunately, far too many that don't).

There's also the detrimental effect of making it look easy, which leads to the numerous online accounts of people who wasted time and money on a "Helmholtz Resonator" that does nothing. If we're going to present a design concept to people who might attempt to replicate it, I think we ought to demonstrate convincingly that it works.
 
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Bjorn

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This is research that takes a lot of time and best suited for a school or within a cooperation IMO. I don't see that strict type of study happening in the DIY community, but I might be wrong.
 

sarumbear

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Perhaps I should formalize some ground rules for what I will call "the Helmholtz Open Science Challenge". I'd like to think this is in keeping with the ethos of a forum dedicated to the scientific understanding of audio concepts.
Science without math? Good luck with that :D
 

Vladimir Filevski

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I have considered releasing Helmholtz resonators to the market for years and have design ready that I'm comfortable will work. But I'm not sure there's a market when the product is quite large and with a depth of either 20 cm or 30 cm.
Yes, for adequate absorption of low frequencies large absorbers are necessity and most of clients will not accept those in their rooms, hence lower demand of them, unfortunately. I wish you a success in your business.

Especially considering that the Modex plate (VPR) is only 10-11 cm in depth and also has the benefit of being combined with absorption or diffusion in front making it more broadband and usable at places with specular energy. Something that's not possible with a Helmholtz.
Effective commercial product, albeit with adequate (some will say - expensive) price.
 

Geert

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The first is the only one which is objective, measurable and repeatable. The second is subjective; it depends on the person.
Good observation, but it doesn't answer any questions.
 
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Delrin

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This is research that takes a lot of time and best suited for a school or within a cooperation IMO. I don't see that strict type of study happening in the DIY community, but I might be wrong.

I respectfully disagree. Many DIY enthusiasts are also students, sometimes in physical sciences and/or maths. Many university professors are also DIY enthusiasts (I am one of them). All the elements of my "challenge" can be met with a room, a sound playback system, a measurement mic, a copy of REW, some basic carpentry skills, and a willingness to learn some physics and the related math (at the risk of making it sound too easy). If Andy Mac Door's result is to be believed, he achieved a good result through careful research that most intelligent DIYers could carry out (given enough time and motivation). He is also a good presenter and has great taste in hats.

Not restricting the discussion to acoustics or Helmholtz resonators, it is not difficult to find (amongst all the other drivel) YouTube channels in which the resources and knowledge rival those of many educational institutions. In many cases, the explanatory skills of the presenters put some (not all) university professors to shame. I believe this is a good thing, in that universities are going to have to "up their game" in the post-covid world of online learning.

During my university career I have been involved in the planning and construction of new laboratories, and the requirements in my "challenge" are not unlike what we might ask in a call for tenders. I do not want to waste public funds on BS that does not work. Such information is commonly exchanged, under non-disclosure agreement, so that we can make an informed purchasing decision. It is not uncommon to include requirements for a "site visit" to another institution where the product is installed so that we can make our own tests (detailed in the call for tenders). If a corporation is evasive on such points, their bid will be rejected.

I realize there is a wide gap between an institutional public tender and shooting the breeze in an online forum (as we are doing here), but these basic "critical thinking" concepts are very helpful in everyday life. If someone wants to sell you something, but is evasive on the details or implies that you are too dumb to understand, I would shop elsewhere.
 
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Delrin

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Science without math? Good luck with that :D

Hi sarumbear,

Nowhere does it say that math is excluded. I thought it would be implicitly clear that Requirement 2 requires providing details on the math and physics used to design the resonator.

The math can be provided as a Mathematica notebook, equations scrawled on the back of a greasy napkin, or clear references to published articles or technical notes that are available for download.

I would consider anything computed using http://www.acousticmodelling.com to meet the standard, so long as all of the input parameters and model selections are indicated. I would accept this because that web site provides full references supporting all of the modelling performed, and all of these papers can be downloaded and reviewed. As I have noted elsewhere, Andy Mac Door's video series already meets this part of the standard. The only thing that is missing (for me) in Andy's video series is some "unit testing" of the resonators and more clarity on all of the acoustic treatment included in the "before" and "after" measurements.
 
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Delrin

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One or two 'PSI Audio AVAA' active bass traps the size of a subwoofer are usually insufficient to fix room modes...

Yes exactly - I've read through the marketing info on the AVAA, and been trying to track down the patents to read. The company talks a lot about their patented technology, but doesn't make it easy to find the patents (there should also be some academic papers on it).

The smattering of online accounts I've found (including one here on ASR) are equivocal at best. Typically, posters end up deciding they can get better results through resonant absorption (which is never really described as far as I can tell). This seems at odds with the company's marketing claims.

The AVAA is a fascinating concept, but really expensive for something that may or may not solve the intended issue. One thing I've noted is that they characterize the absorption efficacy of their unit in Sabins, which is to say in terms of the equivalent size of hole in the room boundary. The provide a graph of equivalent absorption area at this link, with a value of 5 square metres around the 50Hz range. While not solving all problems this sounds impressive for a device size of 0.2m^2.

If the 5m^2 figure is to be believed, it would be hard to rival this with other non-active absorption methods. I would love to get my hands on one of these for some detailed testing. We have a local pro audio dealer who offers rentals on a great many products but I don't think this is one of them.
 

sarumbear

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Nowhere does it say that math is excluded. I thought it would be implicitly clear that Requirement 2 requires providing details on the math and physics used to design the resonator.
My comment was based on the observation of the 70+ messages on the thread so far. Besides, I disagree that using a calculator is not applying science as it is based on many assumptions that are rigidly connected to the structure of the absorber.

The paper from where the calculations of www.acousticmodelling.com said to be taken from show various absorbers but they are mainly related to large spaces. In my opinion, unless you create various resonator shapes that are suitable for a domestic living room, describe them mathematically then run simulators for each, you will not go far.
 
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OldHvyMec

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If someone is going to post about their great DIY Helmholtz absorber treatment, as I have seen done, then why not provide sufficient information for others to achieve a similar benefit? There are many YouTube videos and forum posts - on other topics - that already meet this standard (and, unfortunately, far too many that don't).

There's also the detrimental effect of making it look easy, which leads to the numerous online accounts of people who wasted time and money on a "Helmholtz Resonator" that does nothing. If we're going to present a design concept to people who might attempt to replicate it, I think we ought to demonstrate convincingly that it works.
Exactly what are you asking for? The plans? The tricks? The unpublished "STUFF" that separates my STUFF from others STUFF or better the lack of their "STUFF"?
I love to share with people that I'm confident would appreciate it. There are also people I would NEVER share a meal with, because they think the world is
a place to cherry pick information and without that information no one could possibly figure anything out. There also gawkers that stand on the sideline
picking apart every thing someone says. I find it simpler to tell them, "do your own measurements you still have to have something to measure, FIRST. Words
without actions and hands on experience is all that is required for me. AGAIN some people work with what they have as far as materials while others object
and scrutinized their efforts because their lack of understanding, physical WORK vs pushing chalk across a chalk board.

I don't care for any skeptic requiring my knowledge as proof for their lack of doing anything but crunch numbers. They are worth far less to the actual DIY or anyone
for that matter other than pissing them off. Audio2design comes to mind along with his/her/it/them's 50 aliases.

Debate is great, there is also a way to achieve GREAT debates without demanding proof that was known about 6 thousand + years.
Beating a dead rabbit comes to mind.

I use the same style of tube resonators seen in post #58. It required 12 resonators in a 16x20x8 foot room. I removed 4, I started with 16 total.
ALL of the specs are posted and I don't need to verify their measurement for the simple reason the RESULTS are concrete. Not a theory or
something to be re-proven for the 10,000,000th time in nature alone.

Note to self: I would really like to know why people can't look at a graph play a tone and not be able to remember that tone for the rest of their life.
I would really like to know why so many people demand proof for what was proven by a simple test that holds up in a court of law.
Is that the person you heard in the NEXT ROOM? answer: YES. Can you listen to the 10 voices and pick out the person you heard?
YES. Now, have you heard that voice in this courtroom? answer; YES I HAVE. He did it, now hang the prick. It is NOT eye witness
testimony is it? It is a lot more reliable in a court of law than you think for a REASON. Try to be objective. OBJECT being the operative word about
the testimony.

A lefthanded thought, I suppose.

Time to feed the chickens
 
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Delrin

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Hi Sarumbear,

My comment was based on the observation of the 70+ messages on the thread so far.

Consider the plea for more math to have been sent...

Mathematica Comic.jpeg


Besides, I disagree that using a calculator is not applying science as it is based on many assumptions that are rigidly connected to the structure of the absorber.

We are in complete agreement, as use of the calculator alone will not meet the standards I laid out above (which anyone is free to criticize, ridicule, or ignore).

If, however, a calculator is used to to plan a design and that design is then built, tested, and found to behave as predicted, then I would accept this as a valid and convincing design/build process (feel free to criticize, ridicule, or ignore my opinion).

No matter what you do, you will be incorporating some assumptions. The trick is to figure out which ones matter in your application and how to test them.

The paper from where the calculations of www.acousticmodelling.com said to be taken from show various absorbers but they are mainly related to large spaces. In my opinion, unless you create various resonator shapes that are suitable for a domestic living room, describe them mathematically then run simulators for each, you will not go far.

The link you gave is not a paper, but Cox & D'Antonio's venerable (but somewhat antiquated) book "Acoustic Absorbers and Diffusers". I would not describe the "Layered Absorber Calculator" at www.acousticmodelling.com as being based on that book, as the calculator allows the user to select specific models for porous absorption and Helmholtz resonance that are based on different published research papers with the full citation given on the web site:

Absorber Models.jpeg


Helmholtz Models.jpeg

References.jpeg


Some of the older papers, like Delaney & Bazley and Allard & Champoux are referenced and used in Cox & D'Antonio, and Allard/Champoux happens to be the default on the web calculator. The Transfer Matrix approach is also heavily emphasized in their book. However, the web site designers offer numerous models and make no claims as to the relative merits of the different options. If someone like Andy Mac Door is able, through doing online or other research, to determine that the Komatsu model is most realistic and use it to build a resonator that behaves as predicted and gives a measurable benefit, then that's a step in the right direction.

I think it's also important to note that the level of simulation offered on a calculator like www.acousticmodelling.com is totally agnostic about the size of the room. The simulations simply predict the frequency response of an absorber array whose total extent must be determined according to other considerations (beyond the scope of that particular simulator). If the basic design does not resonate at the right frequency, or does not present the required surface impedance, then no amount of compensation for room size will make it work. Integrating the design into the actual room envelope is of course a critical step in acoustic treatment, but doing this with a fundamentally flawed resonator design would be useless.
 
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Delrin

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Exactly what are you asking for?

To quote @amirm:

"Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true)."
 
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Delrin

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I believe the main reasons why we see so many fail attempts with Helmholtz' (and other bass traps for that matter) is because of:
1. Not optimal placement
2. Too small units
2. Not covering enough surface area.

Below are some before and after meausurements in small rooms with the use of Helmholtz resonators.
View attachment 260253View attachment 260254

View attachment 260256View attachment 260258

View attachment 260261View attachment 260262

View attachment 260264View attachment 260265

Thank you for posting your room measurements and other comments Bjorn. I see you are a manufacturer, and would not expect you to post proprietary design details on a public forum.

Some of the "hints" you give are helpful, such as the general point that a large coverage seems necessary. This is consistent with Andy Mac Door's design (sorry to keep harping on it but there are not a lot of other options - please feel free to correct me).

The example photos of cylindrical units also provide a helpful reference. In another post, you mention a "gigantic Helmholtz covering the whole width and almost 2/3 of the height of the room." Is this comprised of the cylindrical units you showed? or is it a slot resonator design?

Thanks again!
 
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Bjorn

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Thank you for posting your room measurements and other comments Bjorn. I see you are a manufacturer, and would not expect you to post proprietary design details on a public forum.

Some of the "hints" you give are helpful, such as the general point that a large coverage seems necessary. This is consistent with Andy Mac Door's design (sorry to keep harping on it but there are not a lot of other options - please feel free to correct me).

The example photos of cylindrical units also provide a helpful reference. In another post, you mention a "gigantic Helmholtz covering the whole width and almost 2/3 of the height of the room." Is this comprised of the cylindrical units you showed? or is it a slot resonator design?

Thanks again!
Here's the large Helmholtz resonator placed on the rear wall. It's an audiophile in Norway and I had nothing to do with this FIY.
DSC_0220 (2).jpgDSC_0221.jpg

bassrespons før helmholtz.jpgbassrespons etter helmholtz.jpgsammen.jpg
 

sarumbear

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The link you gave is not a paper, but Cox & D'Antonio's venerable (but somewhat antiquated) book "Acoustic Absorbers and Diffusers". I would not describe the "Layered Absorber Calculator" at www.acousticmodelling.com as being based on that book, as the calculator allows the user to select specific models for porous absorption and Helmholtz resonance that are based on different published research papers with the full citation given on the web site.
All I can say is to copy what the calculations publishers said:

The formulae used by the calculator have been taken from the following sources:
Trevor J. Cox and Peter D'Antonio. 2009. Acoustic Absorbers and Diffusers: Theory, design and application, 2nd Edition. Taylor & Francis.

Whether I call the above a paper of a book is irrelevant. The only other sources cited are below, which are not directly related to resonators.

J. F. Allard and N. Atalla. 2009. Propagation of Sound in Porous Media: Modelling Sound Absorbing Materials, Second Edition. John Wiley & Sons.

J. F. Allard and Y. Champoux. 1992. New empirical equations for sound propagation in rigid frame fibrous materials. J. Acoust. Soc. Am., 91(6), 3346-53.Ed to resonators.
 
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