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DIY helmholtz resonator made easy
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karumba
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7th November 2011
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DIY helmholtz resonator made easy

i've seen a couple of questions here regarding how to tune helmholtz resonators. i think those are very easy to build & tune & very effective for room resonances <60hz. this is a tutorial i've posted ~1year ago in the german speaking board recording.de & translated now to english. here is the link to the original thread (surprise, its in german ;-)):
Forum - Raumakustik & Dämmung - helmholtzresonator DIY - RECORDING.de

what is a helmholtz resonator good for? it shortens the decay of a room on a narrow band frequency area. the benefit is, it is tuneable to a frequency and therfore a good addition to porous absobers
some more infos:
Helmholtz resonance - Wikipedia, the free encyclopedia

now a short description how to construct the helmholtz resonator (HR).
my test HR has the following sizes: 100x46x46cm
material: "particle board" 19cm thick
construction: bolted and sealed inside with acrylic
working time: case 20 minutes, tuning 10 minutes
cost: <20€

everything but the front panel screwed together. the inside is sealed with acryl. it needs to be 100% airtight!




before the front panel is mounted, put thick acryl on the top:




the fron panel is screwed:



this is the finished case (wait 1-2hrs unil the acryl is dried):



i put now 3 holes with 3cm each on the front panel & then made the first measurement with a microphone. the mic does not show or soak into the hole, but adjacent, so the membrane is not damaged by the air flow! :

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ok, now lets tune. As mentioned above, I drilled 3 holes a 3cm. volume of the box is ~170l.
with this calculator I roughly checked how much i would need. in my case (tune to 34.7hz) it was ~12cm:
http://www.lautsprechershop.de/tools..._helmholtz.htm

to the tune you just need a mic & an FFT analyzer. I've taken the Voxengo SPAN. you simply put the mode on the "high res" and then look at the interesting range 20-200hz.

Now knocked with my fist. the HR resonates at 28.9hz (I had my cursor on the resonance peak and then it is left below the frequency display):


hm...thats too low. i drilled another hole with 2.5cm:



this is a bit better, now we have 31.8hz. but we need 34.7hz.

i put additional holes until it resonated at 34.7hz. i needed all together:
3x 3cm, 1x 2.5cm, 5x 1cm, 1x 0.6cm:



btw, this is how it sounds when you tap on the box (if you would like to analyze this your self):
http://dl.dropbox.com/u/2216927/misc...ltz_34.7Hz.mp3
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now some measurments. first the room response without HR:


configuration 1:


result 1:


configuration 2:


result 2:


configuration 3:


result 3:
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configuration 4:


result 4:


configuration 5:


result 5:


configuration 6:


result 6:
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configuration 7:


result 7:


configuration 8:


result 8:
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I also tried other configuration, e.g. all holes with direction to the wall, but they have shown a significantly worse performance.

evaluation:

configurations 5 & 7 showed the best performance. If we compare results # 7 with the untreated version for 34.7hz we have the following result:
before: 65.6dB



after: 51.5dB



the target room resonance ringing within 1s decreased by ~14dB. given the small size of the HR it seems to work not that bad. the frequency response is also smoothed - that was expected.

I see now the following optimization options:

1. apparently it seems the room resonance has shifted a bit to a higher frequency - this is seen quite nice in comparison to the "untreated" graphics. I am therefore not sure whether I've caught the exact resonance and thus can be teased out a bit more performance. therefore it would be better to tune the helmholtz resonator directly via waterfall measurements in the corner.

2. the volume of HH is still not very large. the more volume, the stronger the damping. I could imagine with 4 of these HR the resonance at the 35Hz would be in a reasonable range.

3. it could be also configurably designed by using something like this: http://antstore.net/shop/index.php/l...n-Stopfen.html

4. this HR is currently still undamped. if some "card web" (?) or mineral wool is put inside the "effective voume" is increases and the resonance frequency is shifted. this also increases the bandwidth (and the effectiveness at the given resonance frequency may decrease).


i hope this is helpful. comments appreciated!
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Interesting.
Thanks for sharing.


Comment/question:

It looks as though the one HR is the only treatment in the entire room, correct? (at least applied)
I see a stack of (what looks like) diffusion panels next to your test HR box. Those appear to be moved when you put the HR on it's side.. since this is very near or at your right FR, could this be changing your waterfall results? (i know they're not installed, but as it's a large mass next to your First Reflection Point, I'm sure it would have an effect on the measurement..)
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thanks for your comment. this room is not fully unoptimized, i just removed some of my conventional basstraps & the 3D diffusors where not installed. from my experience i don't think it had an impact in that frequency range (i did similiar - but not that extensive - measurments before showing the same trend), but thats a good point!

the key part of this analysis is maybe just to give an easy tutorial how to build & tune a HR (it is really that easy) & also to show that the "configuration" might have an impact - so always make measurments. its also a good idea to not drill the holes in the middle of the panel (to have the option to put the HR "on its head").
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Quote:
Originally Posted by karumba View Post
... 3D diffusors where not installed.
3d QRD

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Quote:
Originally Posted by Jens Eklund View Post
oops...of course 2D. thanks for pointing out!
i'm always mixing that up...they are looking just too 3-dimensional to me ;-)

jens, do you have an idea why the resonance frequency is "moving" (shifted)?
before: 34.7Hz
after: 35.8Hz
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Quote:
Originally Posted by karumba View Post
oops...of course 2D. thanks for pointing out!
i'm always mixing that up...they are looking just too 3-dimensional to me ;-)
Most objects are 3D but the scattering is occurring in 2 planes (assuming 2D diffuser).
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Quote:
Originally Posted by Jens Eklund View Post
Most objects are 3D but the scattering is occurring in 2 planes (assuming 2D diffuser).
yes, of course.

btw, what do you think about my active version of this?
active helmholtz resonator
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Quote:
Originally Posted by karumba View Post
jens, do you have an idea why the resonance frequency is "moving" (shifted)?
before: 34.7Hz
after: 35.8Hz
Could be a number of things:

* Something is leaning against the walls changing the boundary condition.

* You have introduced a big box to the room ...

* Air temperature difference.

* The mode is totally absorbed by your “Helmholtz” box but since the box is completely undampened, the resonance of the box is reemitted too the room and the frequency is that of the box (slightly different from the room mode) ... Not very likely but theoretically possible.
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Quote:
Originally Posted by karumba View Post
yes, of course.

btw, what do you think about my active version of this?
active helmholtz resonator
To fight problems caused by energy clashing in different ways by sending in more energy is not a good solution.

I also don’t fully understand the concept of your "Helmholtz" box. Once you add the porous material (to get some proper losses in that system), the fc of resonance will change and since it will drop, drilling further holes won’t help.
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i fill a resonator allways about a third of 5kPa mineral wool. But i use bigger volumes from 500l to 2000l, which gives me perfect results all the time.

cheers
Mika
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Quote:
Originally Posted by Jens Eklund View Post
To fight problems caused by energy clashing in different ways by sending in more energy is not a good solution.
thanks for providing your view.

Quote:
I also don’t fully understand the concept of your "Helmholtz" box. Once you add the porous material (to get some proper losses in that system), the fc of resonance will change and since it will drop, drilling further holes won’t help.
my approach would be to e.g. put in 1/5 or more with mineral wool before drilling holes at all. then i would tune just as described above. should work, doesn't it?

mika, yes, 5-6kPa should be perfect for that.
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Questions

A few basic questions about this Helmholtz resonator.

1. Doesn't a "resonator" imply increased resonance? Is this a misnomer? It sounds like the goal here is to absorb a certain frequency and lessen the tendency to resonate -- the opposite of increasing the resonance. What am I missing?

2. How does one determine the size box needed or the placement in the room? i.e. To similarly pursue a room mode at 40Hz, can I build one of these HR boxes on top of a bookcase at the rear of my room against the ceiling, 10" deep, 18" high and anywhere from 4-10' long?

3. Am I to understand that the number of holes is what determines the frequency which is dampened?

4. Am I to understand from Mikahanau that 1/3 of Mineral wool will give superior results? Is this proportion supported by testing?

Thanks so much to whomever can clarify these things for me. I may pursue this tomorrow in my own room.
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1) yes, you understanding is correct. it resonates, but the resonance is 180° phase shifted canceling the standing waves from the room.

2) i think thats just experience. i would start e.g. with a 250l box & go from there. from my understanding it doesn't matter if you use 2x250l or 1x500l. 2x250l are maybe a bit comfortable to build & could be tuned independently

3) yes. the more/bigger the wholes, the higher the resonance frequency

4) i think anything from "nothing" to 1/3 should be fine. above its maybe getting too broad, since the intention is normally to treat narrow band resonances.
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fantastic! Vielen Dank!

I'd like to try this too.

I have a small, almost square room (3.42m X 3.35m X 2.80m)

According to the room mode calculators, the first two big room modes should be at ~51Hz

As you can see below, I have a giant null at, and all around, ~50Hz (as well as a lot of missing low end overall).

Will an HR like the one you built help in my room? Does an HR only work on peaks?

Any suggestions greatly appreciated

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For all intressted slutz, here a a link to a german Forum, where is a review of a rearshall room i designed for a customer, with helmholtzresonators as Basstrapping device, since mineral wool was to expensive in that country, and the budget was lower than low budget ;-)

enjoy:
[Review] Mikas Mietshaus - Beratung Raumakustik

cheers
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Nice work Mika. The REW graphs show impressive results.
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Quote:
Originally Posted by karumba View Post
1) yes, you understanding is correct. it resonates, but the resonance is 180° phase shifted canceling the standing waves from the room.
That's not how it works. The sound energy is absorbed by the resonator. There isn't a re-emission of the sound with inverted phase to cancel out the incident sound. (but that's how your active version works)

Damping is key as that is how the energy is removed from the room. Therefore the addition of mineral wool to your resonator should improve its performance
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nice! thanks for sharing karumba. Your thread is what I was looking for :D
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very nice thread. its nice to have the build and in room measurements. it would be great to see the effect of adding the internal damping.

i am no expert, but have been reading a little about helmholtz absorbers.

Quote:
Originally Posted by Craigster View Post
2. How does one determine the size box needed or the placement in the room? i.e. To similarly pursue a room mode at 40Hz, can I build one of these HR boxes on top of a bookcase at the rear of my room against the ceiling, 10" deep, 18" high and anywhere from 4-10' long?
the best size is probably as large as possible. the larger the resonator the more absorption. the limit on size depends on the frequency, because the resonator has to be small compared to the wavelength. so if you want to absorb 40Hz, the largest dimension should be about 21 inches (53cm). this is a rule of thumb, and larger sizes may work, but it becomes trial and error and depends on things like where the hole or holes are.


Quote:
Originally Posted by Craigster View Post
3. Am I to understand that the number of holes is what determines the frequency which is dampened?
the frequency is determined by the internal volume, the opening size and the wood thickness.

Quote:
Originally Posted by Craigster View Post
4. Am I to understand from Mikahanau that 1/3 of Mineral wool will give superior results? Is this proportion supported by testing?
internal damping is used to lower the Q of the device. if the Q is too high, then the absorption is over a small band, and the interaction of the resonator and the room can cause side bands to appear on either side of the absorbed frequency. this may have been what happened with karumbas resonator.
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Quote:
Originally Posted by shawn19 View Post

the best size is probably as large as possible. the larger the resonator the more absorption. the limit on size depends on the frequency, because the resonator has to be small compared to the wavelength. so if you want to absorb 40Hz, the largest dimension should be about 21 inches (53cm). this is a rule of thumb, and larger sizes may work, but it becomes trial and error and depends on things like where the hole or holes are.

the frequency is determined by the internal volume, the opening size and the wood thickness.
Shawn - This is really helpful. Thank you for interacting here.
So, to attack 40Hz, I can build a rectangular box with the length at 21" (in my case and intended location, that leaves me with 10" depth and 18" height to work with. I presume if I want more absorption, I could build a 2nd, 3rd, or 4th box of the same size (same thickness, #/size of holes, etc.). And if I want to also attack 80Hz, that then I'll use a larger box with more holes. Everything tracking so far?

Where did you (or where can I) find information regarding the maximum length of the HR (i.e. the recommendation of 21"/53cm) you gave which corresponds to frequency? And where are guidelines regarding width of wood, number of holes, and size of holes? Any links on the web?

Thanks so much. I'm hoping to try this tomorrow.

Craig
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@craig

I'd target the lowest problem freq.
In your example, 40Hz. by targeting 40Hz, you may well cause the 80Hz issue to resolve.
(same note, octave up)

Last edited by AwwDeOhh; 8th November 2011 at 05:05 PM.. Reason: for clarity
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Quote:
Originally Posted by Dange View Post
That's not how it works. The sound energy is absorbed by the resonator. There isn't a re-emission of the sound with inverted phase to cancel out the incident sound. (but that's how your active version works)
thanks for clarifying.
from my understanding also a passive HR is brought into resonance. thats the reason an undamped HR could "post-ring" or bring his own modes at a different frequency back into the room. due to that, for me "absorbing" is the same as "resonating with shifted phase". but my view is maybe just a more practical one.

Quote:
Originally Posted by Dange View Post
Damping is key as that is how the energy is removed from the room. Therefore the addition of mineral wool to your resonator should improve its performance
actually this undamped HR is "somehow" damped, because of the used material (particle board), which already damps. if the material would be e.g. ceramics, i'm sure additional damping would be really necessary.

@scoring: maybe you should attach the mdat file, since your settings & axes are not well adjusted.
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Quote:
Originally Posted by scoring4films View Post
fantastic! Vielen Dank!

I'd like to try this too.

I have a small, almost square room (3.42m X 3.35m X 2.80m)

According to the room mode calculators, the first two big room modes should be at ~51Hz

As you can see below, I have a giant null at, and all around, ~50Hz (as well as a lot of missing low end overall).

Will an HR like the one you built help in my room? Does an HR only work on peaks?

Any suggestions greatly appreciated

That looks like an untreated or Under-treated room. I'd first tackle the room with some bass traps. A HR is more of a finishing tool to tackle certain modes inherent in a room. I'd do the best i could with bass/broadband absorption and possibly diffusion first-- before thinking about a HR. Once the room is better under control, you'll know if you're a canditate (and what the biggest problem freq(s) are) for a HR.
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Quote:
Originally Posted by Craigster View Post

Where did you (or where can I) find information regarding the maximum length of the HR (i.e. the recommendation of 21"/53cm) you gave which corresponds to frequency? And where are guidelines regarding width of wood, number of holes, and size of holes? Any links on the web?

Craig
+1 hoping for box dimension guidance
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from my experience, the dimensions matter (or are important) for panel resonators. but for HRs, only the volume is of interest. it shouldn't be too low (maybe my version is a kind of lower bound). i have to say i'm experienced (i'm interested in acoustics since 10+ years), but i'm not an expert, so maybe someone else would like to clarify.
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