# Helmholtz Resonator

Discussion in 'Archived Threads 2001-2004' started by Dennis B, Jul 4, 2002.

1. ### Dennis B Stunt Coordinator

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Hi,

I was thinking about building my own Helmholtz resonator based on the cylindrical model as shown in Everest's MH of Acoustics, but unfortunately he won't give the appropriate formulas for this model. I have ETF and it does the calculations for me, but when I go buy the material, I'd like to have the formulas on my PDA so that I can choose from whatever's available more easily.

BTW, I see that many companies offer bass traps as a byproduct of polycylindrical diffusors, and I was wondering if that's a good solution. I already have a primitive-root diffusor on my ceiling that I'm experimenting with, and I wanted to have different toys for different applications for better control, but having stuff in the corners is not a bad idea, so I'm considering it.

Thanks for any opinions.

2. ### Dennis B Stunt Coordinator

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I found this one on a paper by Rienstra and Hirschberg, just published (6/10/02):

w = c * (Sn/(Leff*V))^1/2, where

c = speed of sound
Sn = neck x-section (area)
V = main volume
Leff = L + 2d, L = neck length, d ~ (Sn/pi)^1/2
w = 2*pi*f

I'll check tomorrow if it matches ETF's calculations.
Thanks.

3. ### Joe Hays Agent

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4. ### kevin_u Agent

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Dennis,

tell us about the ceiling diffusor you built. Its dimensions, materials, your room dimensions, placement, how it changed the sound.....

Kevin

5. ### Dennis B Stunt Coordinator

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Joe,
Thanks for the links. I have investigated the Roomlens before, but I thought they weren't the right solution for me. I already have Sonex on my first-reflection points, and I thought they did a great job of improving imaging, soundstage and detail. While you never know and the Roomlens could work better for me, I was a little more worried about taming the bass peaks right now.
I felt, from John's comments, that the Roomlens is primarily a diffusor/diffractor, that can also be tuned to absorb ceiling/floor modes, but I found that it could be a little difficult to tune them to the right frequency.
I have a large (in amplitude) but rather narrow peak at 30 Hz, along with its second harmonic at 60 Hz, and this is what I'm most concerned with right now. This is why I thought of building a tuned Helmholtz resonator, with a reasonable Q, to see if I got anywhere...
I haven't been able to find an appropriate cylinder-shaped volume to build the body of the HH resonator yet, though... any hints?
Thanks!
PS: Kevin, another post follows on the diffusor.

6. ### Dennis B Stunt Coordinator

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Joe,
After reading more on John's site, I came to realize that I may have to build a bunch of HH resonators to get where want, and that his "tube-trap-like" approach is more likely to work, so this is what I'm going to try next... big mess at home!

7. ### Dennis B Stunt Coordinator

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Kevin,
I didn't build the diffusor myself. There's a company here in Brazil who manufactures them in polyestyrene according to the primitive root theory, in partnership with a university. They build blocks of 24.4" x 11.8" which you can join according to a particular pattern to cover larger areas.
Before buying them, I had this company fax me all kinds of info and test results for the diffusor, and I thought they performed well enough in theory for me to give them a try.
Let me give you just a little backgound. When I first intalled Sonex on the walls at the first reflection positions, I noticed a big improvement in soundstage, imaging and detail. Then Sonex on the ceiling contributed less. I measured the energy-time curves using ETF and could see a great improvement in my room, where I was able to create a gap between the direct sound and the first reflection, but I thought the reflections that came afterwords weren't "scattered" enough. So that's why I decided to get some diffusors.
My back wall is where I'd ideally like to put the diffusors, but it is part of the dining room, way behind, so I decided on the ceiling, my second choice anyway.
Since the diffusors are so light from being of polyestyrene, I had no trouble putting them up. I used a doubleface tape and glue, so that only the tape was in direct contact with the ceiling, and not the glue. Some of the blocks fell on the first night, a very humid, rainy night, which might have acted on the glue, which was supposed to take 24 hours for a complete adhesion anyway...
So next morning I put them back up with some extra glue, and they've been there ever since.
First I installed 4 blocks, right behind the Sonex that's positioned to absorb the first reflections, and to be honest I felt very little or no improvement at all, really hard to tell. Then I went thoughtful for a couple of weeks, couldn't decide if I would take them down or put more.
I finally decided to give them a try and bought 8 more blocks. I arranged them in a 4 x 3 config, starting from right behind the Sonex and ending past my seating position below. Now I could hear something different, I got a little more ambience, especially for movies. It also made the stereo sound a little bit more "live". If it weren't all glued up on the ceiling, I'd maybe move them a little bit farther back to see if I got a better effect, but now I'm settling down a bit on highs and going after the lows...
Here's a pic of the final result.
Regards.