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Discussion Starter · #1 ·
Hi all,

I’m thinking to a room treatment in a new chamber where I will soon move my two channel audio system.
I would like to build an absorber for low frequency in a corner, just to hear (and measure) the difference before and after, and evaluate if continue on this way.
I’m looking at Helmholtz resonator, about 20x20x28 cm.
Here comes my doubts.
Better use a panel (28x200 cm about) with few holes tuned with a tube on the volume behind, or use a panel with many little holes, as suggested in the Alton Everest's master handbook of acoustics, that’s I’m reading?
In this second event, all formulas I found are applied on a parallelepiped volume. Are this resonators working well also in a prism triangular based volume?
I’m also thinking to fix sound dampening material inside, just to obtain a wider frequency absorption.
Has anyone try to build a corner Helmholtz resonator?
What looks me strange is that resonators works fine especially in the corners, and if I can’t find anyone that use them (Helmholtz), may be I’m thinking wrong.
Thanks an excuse me for my poor language.
 

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Venice is a lovely place. You are lucky to live there.

I think the best approach for a Helmholts resonator is first setting everything up and measuring the room. This is a very particular tool and you need corresponding conditions to make it worthwhile and set up correctly. They are tuned to a particular frequency band and determined highly by the dimensions and setup of the room\system. I think a Helmholtz is considered an "icing on the cake" application.

A helmholtz would be the very last stop on my drive around the Adriatic.
 

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Discussion Starter · #3 ·
Thanks for your kind reply and for loving Venice.
Yes, I done several measure with REW in several positions of the room.
I always find three main resonance frequency 42 HZ (room width 3,9 m), 60,9 Hz (room length 5,45 m), and 70,4 Hz that I don’t understand (the ceiling is sloping from 2,47 to 1,93 m … may be this is the reason?).
This is the waterfall in the listening position:



For this reason I would like to reduce this resonance tuning some Helmoltz resonators.
Before do it fixed on the walls corner, I thought to build a single resonator for calibrate volume, tube length and diameter just to obtain the right frequency tuning.
I also wish to compare the effect of the two resonators kind, with tube and with Pegboard.
I have began with made one cell corner resonator:



And I have forced the microphone inside:



On the next days I will try to measure the frequency tuning …
But I don’t know if this method will give me reliable result.
Surely only after make the definitive resonator I will understand the difference between with and without absorber.
Any advice will be precious.
 

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Looking at you waterfall I noticed somewhat peaked frequency response, along with the obvious long decay times at the frequencies you mentioned. Is the room treated for any other absorbtion or diffusion. I guess what I should have made clear is a helmholtz would be the last treatment I would try- after other treatments were deployed.

I do think its really cool you are building one though! Nice work.
 

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Discussion Starter · #5 ·
This room will be used also from my son for meet friends, so unlikely I can’t think to a dedicated listening room.
However I can place some absorber on those parts of the walls that are still free.
At this moment the room isn’t treated, even if a wall completely covered with stones, probably diffuse the high frequency range, because I felt an unexpected improvement after applying this stones, that’s have a very irregular surface. It would be interesting to understand if the cavity between the stones are (a bit) working as a Schroeder diffuser …



I begun with Helmholts absorber trying to reduce the few resonance mode of the room, but I also thought to make a membrane absorber for a wider low frequency absorbtion, and a panel with sound dampening material inside (compatible with my wife tolerance !).
I have several places where locate little boxes, so flatten the most unpleasant resonation with some Helmholtz seems me a good start.
I thought that once found the correct volume/hole ratio, it will be possible to place Helmholtz resonator as stands under the speakers (to reduce rear wall reflection), in the free walls corner, between the beam of the ceiling and so on, until get a passable acoustical response. Then pass to the other absorber.
Do you mean I’m engaging the problem at the contrary?

I took a look to your Oyster Bay Maritime Theater … that’s a dream!
 

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I think what you are trying may work in your situation, but in any space I would look at dealing with the overall room response first. Usually extreme room modes such as yours are taken down considerably from corner bass trapping. I cant add anything to Alton's work. (I have that book too. A great resource.)

Here is a good vid that shows a room from bare to completely (and I mean completely) treated. It is put together well and shows the room response via graphs and music samples of various genres. They break the vid down from just corners, to full absorbtion, to absorbtion and diffusion. They use graphs you will interpret well after using REW. Wear headphones to get the full impression. :). The room began with nasty modes like yours. I know this treatment is unrealistic, but you can gauge your expectations based on it.


My room should be great once completed. If I didn't enjoy the research and theory behind acoustics, it would have been much easier (and cheaper timewise) to hire a pro to handle it all.
 

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Eureka!!

I found a tool you can use to build your helmholtz resonators. If you have a Iphone or Ipad, download the app called "acoustical".

It has many useful tools for calculating many common audio related problems.

One of the tools under the rooms section is for a helmholtz resonator. It allows you to put in the dimensions of your unit to calculate volume, then you can specify the port diameter and length to get the tuning frequency. Very cool!
 

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Discussion Starter · #8 ·
As first many thanks Serenity for your useful advice.
These are some measurement done with the Helmholtz resonator. The calculated frequency with the used dimension would be 67 Hz.
I went out of the room, in the garden, just to avoid to superimpose the response of the room with the response of the resonator.

Microphone outside the resonator:

OK. No visible effect on the tuning frequency

Microphone outside the hole of the resonator:

A feeble effect on the tuning frequency

Microphone inside the hole of the resonator:

Marked effect on the tuning frequency

Microphone inside the resonator (without dampening material inside):

Here the tuning frequency is clearly 70 Hz, not so different from what calculated

Microphone inside the resonator (with dampening material on two back surfaces):

A more wide frequency effect. Seems that the dampening material reduce the total volume, shifting the tuning frequency higher.

Microphone inside the resonator (with dampening material only behind the front surface):

Something in the middle between with and without dampening material.


Well, with this results I would like to anticipate if a corner Helmholtz resonator with 10 tubes, so ten time larger than this, may reduce significantly the room resonance.
This is the superimposed curves, with and without this resonator in a corner of the room:

Absolutely identical !!!
I didn’t expect a revolution with only one hole, but not a so insignificant effect.
I mean that also with a larger resonator the acoustical improvement will be unimportant …
I’m a bit demoralized.
 

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My uncle has two helmholtz resonators. He built them. One is a giant sono tube thats about 10" in diameter thats cut at a certain length I think it's about 16 feet long and it's filled with fiberglass insulation and fabric wrapped and hung along the rear wall at the top near the ceiling.

Another he built looks like a giant box and it's on the center wall standing on it's own legs... it has holes drilled in a certain distances with a certain size hole.

both of these was done because he had very bad peaks at certain frequencies that his treated room couldn't eliminate completely. He is a 2 channel listener and does not EQ.

He uses treatment to dial in his room. I don't have any graphs but his room has a very flat response now. He built an addition on the home for a dedicated listening room and it looks like a recording studio with all the acoustical treatment and the room sounds great.

I've never seen a box with a single hole in it but I guess that could work. He built his room based on the recommendations from the everest acoustical book and this is where he got his designs for the resonators.
 

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btw... it needs to be big... not that little box thing.... big and then completely filled with fiberglass insulation that is dense to absorb the energy.
 

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I'm impressed you were able to properly tune and test at your predicted frequency. Good work.

I wouldn't be too disheartened. Maybe just think of this was a working proof of concept. :) I would have to agree that the results you are hoping for will probably come from many of these units stacked upon one another in room corners, or a large tuned chamber like BMXer mentioned.

You did succeed. Just not at the level you were hoping for. You have all the tools you need to proceed! :) Just have to decide if its worth the work for the outcome.
 

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Discussion Starter · #12 ·
Yes, this is the doubt, fifteen or so resonator like this seems have a consequence less significant than a trap make with dampening material, or than a vibrator panel absorber, that also works on a larger frequency band.
Reading Alton Everest's book, I understood that Helmholtz resonator was able to a stronger (selective) absorbtion that I haven’t observed.
So probably I have to change strategy (or made a larger Helmholtz resonator. May be opening a big hall on the wall, communicating with the adjacent room? This time my wife will ask to divorce!!!!!).
 

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Discussion Starter · #13 ·
Yesterday I have measured again the room response moving the resonator in several places of the room, just to understand if the first test was made with the resonator in a location where the resonance isn’t at it maximum level. Nothing is changed.

Before thinking at Helmholtz, I have reduced the three main axial mode of the room with a parametric filter. The result was really worthy, and for this reason I thought to work on this three frequency with a selective absorber.
Now, due to this experience that haven’t produced an appreciable results, I have decided to change strategy.
I would begin with two vibrator panel absorber, one corner dampening absorber and one QRD diffuser (that likes very much to my wife).
This will improve all the low frequency response and if the three main axial resonance mode will remain excessive (probable) I will adjust them with the parametric filter.
I hope to obtain a result that would let me avoid the electronic processing, but I haven’t preconception on using this technology.
 
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