DIY helmholtz resonator made easy

[Brainchild]

Based on several different online and print sources, I put together a spreadsheet that gives a fairly tight range of expected resonant frequencies for a given box size and port configuration. Changing the values in the green fields changes other field values and gives frequency results.

I put this file up on a crappy hosting site so if someone wants to post it elsewhere that would probably be better.

Resonant_Absorber_Take_4.xlsx

[Dange]

Originally Posted by karumba

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.

Yes the resonator requires energy input to resonate, therefore there a loss (absorption) is created. However the addition of porous absorbent inside will increase the absorption (and also reduce the HR's own decay time).

The material the box is made out of is really inconsequential, as long as it is relatively stiff and sealed it will work the same.

[by-tor]

Originally Posted by Brainchild

Based on several different online and print sources, I put together a spreadsheet that gives a fairly tight range of expected resonant frequencies for a given box size and port configuration. Changing the values in the green fields changes other field values and gives frequency results.

I put this file up on a crappy hosting site so if someone wants to post it elsewhere that would probably be better.

Resonant_Absorber_Take_4.xlsx

That doesn't work for me...

[scoring4films]

Originally Posted by AwwDeOhh

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.

Thanks for the reply AwwDeOhh.

The waterfall I posted has no smoothing, though the room is pretty bad.

It is somewhat treated already. Some of the corners are stuffed with Roxul (~3-4 bales worth in total) in the form of so-called "StudioTips Superchunks." I think the SSCs account for the ~200ms decay time on the waterfall. Also have a cloud installed.

I'm planning to empty the room and start from scratch. Will create new broadband/resistance-to-flow/roxul traps, as well as more diffusors, and install them all more carefully (with the help of REW).

It's my understanding that resistance-to-flow traps won't do much below ~100Hz, so I was hoping an HR would help with that huge hole at ~51Hz (and maybe reduce the severity of the next L/W modes at ~100, 150, 200, etc.?).

According to the RealTraps ModeCalc, some kind of an issue is expected at ~51Hz -- right where the first L and W modes coincide (see below). There does seem to be something going on at ~51Hz on the waterfall (also re-posted below).

Does anyone know if an HR will help with that big ~51Hz null? Karumba showed us how well an HR can work on a peak, do they also work on nulls? Any suggestions much appreciated. Thanks!

[shawn19]

hi Craig,
hopefully not hijacking this thread. i am sure there are others with more experience, but let me start this off.

Originally Posted by Craigster

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?

yes, for more absorption, just build more boxes. the limit on the size is wavelength/16, so for 80Hz the largest length will be half that at 40Hz. the references always mentioned that the resonator must be much smaller than the wavelength, but i could not find a reference. if memory serves, i think i found it here,
PolyU Institutional Repository: Acoustic resonators for noise control in enclosures : modelling, design and optimization
it is a great reference with a lot of useful information.
as far as the number of holes, in one of the original papers in the area by Uno Ingard 'On the theory and design of Acoustic Resonators' 1953, he writes
'It is therefore clear that in order to obtain maximum acoustic reactance of a given open area in a partition one should concentrate the open area into a single aperture in the center of the tube. This is important to realize in the design of resonators and acoustic filters.'
so the best would be to have only one opening of the correct size. the problem is that it is very hard to get the resonant frequency exactly right the first time. as Dandan has mentioned there must be some way of tuning it. karumba uses the method of adding holes and measuring until getting to the desired frequency. i would recommend the approach in the thread 'Panel and helmholtz absorbers' where one builds the box, then adjusts the tube length and measures until the frequency is correct. both methods will work, i dont know which is better. hope this is helpful.

[AwwDeOhh]

Originally Posted by shawn19

---
so the best would be to have only one opening of the correct size. the problem is that it is very hard to get the resonant frequency exactly right the first time. as Dandan has mentioned there must be some way of tuning it. karumba uses the method of adding holes and measuring until getting to the desired frequency. i would recommend the approach in the thread 'Panel and helmholtz absorbers' where one builds the box, then adjusts the tube length and measures until the frequency is correct. both methods will work, i dont know which is better. hope this is helpful.

Does the hole need to be circular and/or symmetrical?
Would it be possible to start with a larger hole (somewhere near but under the recommendation), and tune by grinding/shaving the side(s) of the hole?
Do you believe that's a viable method?

[Dange]

Originally Posted by AwwDeOhh

Does the hole need to be circular and/or symmetrical?
Would it be possible to start with a larger hole (somewhere near but under the recommendation), and tune by grinding/shaving the side(s) of the hole?
Do you believe that's a viable method?

Holes can theoretically be any shape, it's the open surface area that's important.

Circular holes are just the easiest I suppose, i.e. you can drill them and it's easy to know their area.

Making the hole bigger would allow you to tune up in frequency, so you need to have the initial resonance is below your target frequency

[AwwDeOhh]

Originally Posted by Dange

Holes can theoretically be any shape, it's the open surface area that's important.

Circular holes are just the easiest I suppose, i.e. you can drill them and it's easy to know their area.

Making the hole bigger would allow you to tune up in frequency, so you need to have the initial resonance is below your target frequency

Yes, that's what i meant (or at least was thinking in my head... lol..)

So it really doesn't matter what the 'hole' looks like, as long as it's the right amount of open area. Good to know! thanks

For instance; i could start with a circular hole (one that's smaller than needed) and tune by squaring off the circle. or making different patterns of cuts.. in theory i could make a smiley-face then? or a logo?

[BriHar]

Somewhere I saw someone using Sonotube cylinders. Most of the cylinders were of equal length, capped and sealed top and bottom (they were mounted on the walls like organ pipes). The bottom cap had a hole in it to accomodate a smaller tube of a certain length. Tuning was accomplished by trimming the length of the smaller tube. I assume different lengths of the main tube would give differnt ranges, while the smaller tube would do the fine tuning.

[Brainchild]

Originally Posted by by-tor

That doesn't work for me...

Here we go...

[karumba]

from my experience those calculators are just estimations. so my philosophy for HRs is to tune them with measurements. but maybe i just chose the wrong calculators.

[Brainchild]

Well of course, but they converge unexpectedly well with one another and with measurement in my experience.

[Craigster]

Brainchild --

What is the smallest room and the lowest frequency that you have achieved significant measurable improvement from?

(i.e. in a 1500 Cubic foot room, lowered a 79Hz room mode 6 dB at the MIX position)

[Brainchild]

I looked for the old test files last night but couldn't find them; the design for my boxes is very similar to the "single-cell" resonator boxes (small cubes with single ports) that shawn19 showed in his "Panel and Helmholtz resonators for studio" thread, except with 4 smaller ports in each box.

I posted the static frequency response results for about 20 of these things on here before...couldn't find the file, like I said, but the way I remember, the static frequency response was about 3dB down at the most problematic resonant frequency in the room (~37 Hz if I recall) at the mix position in a 1200cu.ft. room.

I've built a few more since then, but the current studio is in a state of perpetual disrepair as I prepare for a refit. This time, I'm planning for all four walls to be lined solid with resonator boxes.

[noize_organizer]

very interesting thread !
I would like to tame down 105.9 Hz with a narrow Q (92 Hz - 128 Hz) // any advice on how to obtain that narrow Q ?

Thanks

[AwwDeOhh]

Originally Posted by noize_organizer

very interesting thread !
I would like to tame down 105.9 Hz with a narrow Q (92 Hz - 128 Hz) // any advice on how to obtain that narrow Q ?

Thanks

Try building some panel resonators, and place them in a hot spot(s) for your target frequency. Go to the library or a bookstore and pick up the "Master Handbook of Acoustics" by F. Alton Everest
There's a formula for building the panel:

           170
      f= --------
          ?(m)(d)

f= frequency of resonance in Hz
m=surface density of the panel, lb/sq ft of the panel surface
d= depth of airspace in inches

(basically secure a 1/4" plywood sheet to a frame of 2x4 lumber [the 3.5"--4" being your airgap] with the plywood as the face, and the wall as the back. Use a flexible caulk around the perimeter to secure the plywood to the frame. this should target roughly 100Hz. Secure a sheet of rigid insulation 1/8 to 1/4 inches behind the plywood without it touching the plywood)

[noize_organizer]

Thanks for your input.
I am actually building panel resonators in the corners - not too sure what frequences they actually target as the distance to wall is less easily predictable than when built right against a flat wall.
I can actually already witness an improvement in the bass spectrum ...

[karumba]

i also think that helmholtz resonators are better user for frequencies <= ~80hz. from 100hz & up panel resonators or just rockwool seems to be the easier approach.