Hey Guys,
haven't been on this forum for a long time, but i got a question. it seems like helmholz resonators aren't that popular, which surprises me. it seems not that many people build these. but as far as i know, these are pretty unbeatable for very low frequencies!

what im also wondering about is, if you search for helmholz on google or gearslutz you almost only find helmholz slot resonators.
are the classic helmholz "boxes" outdated or not as effective?

If you talk to 100 designers/acoustic consultants you will get 100 different answers when it comes to helmholz. I personally have not problem with them, but have seen way to many diy guys build them and it did nothing to help there problem. Honestly there are many different ways to tame low end and all can work if done right.
Understanding Different Bass Trapping - GIK Acoustics

Elsewhere I have shown that the resonance of a device (drum) changes with location in a room. Proximity to boundaries, room volume etc. Let's call these frequency changing factors 'room load'.
The HH formulae knocking around do not factor this in.
So we need a device that can be tuned in situ.
Then there is the size factor. The MHOA shows just one tub doing a remarkable job on a 47 Hz mode. I could not replicate that at all. I reckon it is utter fake.
We see mild success with the very large Varitunes.
There are many threads about showing failure with various different types of tuned traps. Apart from the tuning issue there is another of critical damping. Too much and they do nothing, too little and they add to the problems. On the other hand deep fibre and even VPR are difficult to get wrong.

DD

There is another factor involved here, as in general internet acoustics forums. The average reader is terrified of simple algebraic equations like axial mode calculations and even straightforward calculators for things like abeorption of porus panels. Helmholtz equations have radicals!

Andre

Our very first product was a resonator type trap, but it was discontinued right after we released our MiniTrap (a broadband absorber with a sculpted absorption curve). In small rooms, broadband absorption pretty much always works, but as said above helmholtz resonators pretty much need to be designed exactly to fit the room, and then built very precisely to make sure it performs the way you think it will. For this reason they generally don't lend themselves well to mass production. At least that was our experience. Our products since then are much more consistent and perform much better, which is why they have been discontinued for more than a decade now.

I am sure it was. We DO BUILD and sell something tuned and broadband and it works GREAT in not only small rooms but larger rooms if you know what you are doing.. The key is to source material that is consistent, know how to place them and have tools that are above home grade when building.
https://www.gearslutz.com/board/7597560-post146.html

People have this misconception that a helmholtz should work flawlessly placed INSIDE of the room, the only criteria being a location of high SPL for the resonant frequency. In fact they work much better placed inside of the wall of the room. Pressure is biggest on the boundary, not away from it. So in a way, that kind of construction would be like having two rooms, and the boundary that separates them needs to be heavy. It's a big deal when it goes wrong, money down the tubes and wasted time. Finding the appropriate damping is near impossible.

but building a helmholz is not very expensive. you jus need 4 pieces of wood, any material should work. then make holes... done. or am i wrong?

Just to clarify, the difficulty building consistently is not what I was referring to regarding mass production, it was the fact that narrow-band absorbers like helmholtz resonators generally need to be built at precise frequencies for the specific room they are going in. DIYers tend to have a harder time with consistency. Bottom line is, these kind of absorbers, when used correctly, are not a one-size-fits-all kind of thing.

That, and our more recent broadband products greatly outperform resonators in general.

Some folks wholeheartedly embrace Helmholtz treatments as part of their approach to studio design.....

Boom.. Just because we don't actually sell a Helmholtz does not mean they don't work well. I am one to embrace all the concepts, not just what we sell.

+1

A bit expensive so not very popular in small studios because of money

very beautiful rooms!!

well my question was more in the realm of diy. why do not more people build these for their home studios? yea, they must be built precisely, but then they work quite well i would imagine.

and as i said/asked. it basically 6 pieces of wood glued together (with a few holes), right?

The problem is most I have seen are not built right, not placed correctly and or not enough of them.

That has also been my experience. Some sources suggest a very small amount of damping near the 'mouth' of the HH. Others suggest filling the whole space, others again suggest fibre at the back of the void.
I would suggest that the amount of damping may also change the tuning.
So I have to wonder about those very beautiful looking rooms.
Are they actually tuned, and if so how are frequencies picked, as they will change after treatment? Or is it a broad stroke scenario, with a varied depth void caused by angling boundaries? And how about the damping?
Unlike the IMO fallacious example in MHOA I completely failed to get any useful performance out of a single HH device. Furthermore I found tuning and damping to be highly critical and very dependent on location in the room and all other 'room load' factors. I am not suggesting the approach doesn't work. I am just curious as to how HH it is.
DD

OK - a number of folks answered this question (& pretty specifically at that) covering a variety of reasons.

For example Andre said:

jwl said:

and then added:

Now (among other comments that were made) those are some very valid points......

First off - a room (even a simple rectangular space) is quite complex when you examine everything that takes place inside of it from the perspective of sound.

Add to that the fact that as you begin to deal with certain aspects of the space - things can (and more often than not do) change.

You solve an issue with a particular specific frequency and find that it was masking an issue related to another particular frequency.....

Then you add to this the fact that when these types of devices are used they not only need to be properly designed - they also need to be very accurately constructed....... and you are asking (specifically) about resonators with holes.

Although they can surely be built - the size and spacing of the holes is critical to the performance of the device.... and there end up being a lot of holes even for a relatively small treatment....

The task of laying this all out and drilling the holes is a crap load of work...... and (when all is said and done) if the analysis of the room was not spot on - what you have when all is said and done is useless........

Add then to this the fact that when this approach is used for room treatments - a small amount of devices (especially in small rooms where things tend to be worse from the perspective of modal activity) aren't going to deal with the task at hand...... you need a lot of treatment with this approach...... so you have a lot of very exacting work that is going to take place....... and (if someone screwed up in their analysis along the way) if you don't get it done correctly right out of the gate - you get to begin all over again.....

Rod

I love them in small room set ups.

+1, 2 and 3!

Singularly,
Andre

thanks for the long reply!
all this makes sense to me and i understand that.

im all for doing bass trapping in the corners etc. first. you can get your room pretty good sound with just bass trapping and RFZ absorption.
what im thinking is, that lowest mode every room has (for mine it would be 45hz) will still be sticking out more than the others.
using a helmholz for that lowest mode just seems logical to me. but again, i havent constructed a helmholz yet... so i guess that might be the reason im thinking this way :D

Indeed your intuition seems logical nix. Modes tend to be high Q and a high Q resonating absorber should address this specifically. These lowest modes have massively long wavelength and high energy levels. Let's call it roughly 10 Metres wavelength. How much surface area of HH do you think would make a noticeable dent in that? Look at the pictures above, it's the whole wall.
Hi Q will require massive structure, no thin layers here. How to calculate the frequency and how to deal with the fact that it is very room and damping dependent?
I would suggest non tuned is safer. BBC tests show 12" of cheap light insulation working well at 50Hz. Let's go 2' thick to be sure. Such thick traps, if you have the space, will surely work. Take a look at boggy's MyRoom Acoustics FB page.
Fibre batts on edge, 2' thick traps. Now look at his LF frequency responses.

DD

The fear of using HH devises is a bit unfortunate and often uncalled for in my opinion. Let me demonstrate:


6”, 9” and 14” deep slotted panel (Helmholtz array)


If you only have 6” of available build depth, and want to tackle a low frequency issue (naturally also assuming you know that the surface area you plan to treat is directly related to the problem (modal and/or SBIR), then you might find that the panel will end up being a relatively high Q device and thus care must be taken to make sure that the tuning is correct, since if not; there’s a risk of missing the target frequency due to the narrow bandwidth.

However; if you have 14” of available build depth, you can design your panel so that it is effective over a wide range (bass) and it´s then not that critical to get it perfect. As can be seen above; the 14” panel would cover a range of about 30 to 300-400 Hz with very high efficiency, so this design could be used on pretty much any surface (assuming not an early reflection point naturally) without any “risk”.

Thanks Jens that is interesting. If you were tasked to make a 14" box absorber as in the green plot-- Would it contain fiber damping? About how much damping? Or would it be preferable to hand-tweak the amount of damping after installation?

Would a box of fairly thin plywood sides compromise performance, or would it be mandatory to make a big heavy box of something like thick mdf?

Would performance be compromised unless rigidly sealed against a wall?

I try to avoid “boxes”. If possible, it´s better to use the existing wall as the back of the panel and just use wood studs (MDF if deep panels) and make sure that any gaps between studs and wall are sealed (only important at the end of the actual HH array assuming all other gaps in-between are sealed).

Yes, the panels I design are usually filled with wool (type depending on the specific design) or at least about 2/3 filled. There’s naturally also a fabric (and sometimes also a fiber cloth beneath) in-between the slats and the wool to prevent wool particles from escaping.

Thanks, Jens

Was trying to avoid any permanent room changes if possible, so was interested to know if the assembly would need to be caulked up against a wall to work properly. Regardless whether it actually has a "back" on the box itself.

Do you think the slats need to be rather thick/rigid to perform well? Or is something like thin perfboard with the appropriate percent of open space "close enough" to work about the same?

Thanks

If you do free standing boxes; make sure that the back cannot vibrate easily (since that would change the behavior of the devise), either by securing it to the wall or by squeezing something soft in-between the wall and the backing of the box preventing it to vibrate.

The same goes for the perforated front; if to flimsy, it will easily vibrate and thus the devise will partly behave as a membrane absorber and the combined behavior (of membrane and HH) is not easily modelled (although I have seen papers discussing this).

Thanks, Jens

One more curiosity question-- I was playing with the models at acousticmodelling.com and get puzzling results--

In multi-layer absorber calculator, if I specify a slotted panel, air gap, absorber depth. And then specify a perf panel, with identical percentage of open area, identical panel depths, identical air gap and absorber depth. The only difference is that one model is a slotted panel and the other is a perf panel. I get different curves, different center frequency and different Q.

To further confuse the issue, as best I can tell, I get another slightly different curve if entering the exact same parms into their Helmholtz calculator. Three different answers for the same problem.

For Instance--
The helmholtz calculator (calculating a perf panel on certain parms) gives a center frequency about 115 Hz.
The multi-layer absorber calculator perf panel model, with the same parms, gives a center frequency of about 90 Hz.
The multi-layer absorber calculator slotted panel model with the same parms, gives a center frequency of about 71 Hz.

I'm not blaming you or anybody because the answers differ. Just reporting the results for opinion if you have any.

One thing of interest-- If two otherwise identical helmholtz absorbers were constructed, one for instance with a 12 mm thick slotted panel with 3% open space, and another with a 12 mm thick perf panel with 3% open space. Same air gap, same depth, same amount of damping in each trap-- Is there a real-world reason that slots might have a different resonant frequency and Q, compared to an identical box with holes instead of slots?

Can the bass really tell a difference between the slots and the holes? Or would such disagreement be most likely artifacts in the different models, but yielding about the same performance in the real world?

Different styles of perforation will give different results (even if the perforation percentage is the same) due to different "end/neck corrections", more here:

New Porous Absorber Calculator

There are a few other things that will affect the predicted result, such as different models for the porous layers, different models for the HH function, angle of incidence (normal incidence, a specific angle or diffuse field (random incidence)).

This is extremely unlikely. Both web pages use the same calculator. Are you sure you used the default porous model in the multi-layer calculator?

If you can't find the reason for the difference could you please post your results here so I can look into it.

Thanks,
Demetris

I've modeled (twice) perforated panels in both models - and the results have been identical given the same parameters.......

My best guess is that there must have been something different in your models when you input the data... so please re-check your data. (and post the results here as mentioned above if you can't figure out what's going on)

I would expect different results trying to compare slotted panels with perforated panels (even given the same ratio of total opening to hard surface) for the reasons mentioned by Jens above....

Rod