# Adjustable Helmholtz Resonator



## BoomieMCT

I have a bunch of Sonotube left over from a sub project and I got an idea (but don't know if it would work). My thought is this - use it to build a helmholtz resonator to fix bass peaks in a room. It is my understanding that helmholtz resonators work at fairly specific frequencies and if I made one for a specific room, it might not be useful in another setup.

I was wondering if anyone has ever made one that had plugs (or something similar) to change the area and depth of the ports. I was thinking of using PVC pipe wrapped in a weather seal strips or something similiar. 

What do y'all think? Have I been playing with this calculator too much?


----------



## BoomieMCT

I thought about this more as I was sleeping. Instead of nested tubes for ports I think I would cut a large hole in one endcap and mount smaller caps with the ports much like I was mounting drivers. I would step the hole and use a driver mounting kit to keep it in place. 

As rough calculations, a 120 liter cylinder (about 3' by 16") could be tuned to deal with my nasty 63 dip in a broad way (52Hz - 71Hz: 20 cm port, 4 cm long) or more specific (59Hz - 65Hz: 30 cm port, 20 cm long). A smaller volume might be able to be tuned for more results (higher dips).

------------------------------------

The question I have it how effective are helmholtz resonators (in their bandwidth)? How do they compare to OC703 panels? 

Also, do helmholtz resonators need to be stuffed to work optimally?


----------



## Anthony

Boomie, 
I'm sure you will get some good information here, but another place to check out is Ethan Winer's acoustics forum over at the Music Player Forums: http://www.musicplayer.com/cgi-bin/ultimatebb.cgi?ubb=forum;f=26;hardset=0;start_point=0;DaysPrune=0

He's part of the Real Traps company (competitor to GIK) and supports RplusD (a similar but expensive alternative to REW); however, Ethan visits a lot of web forums, including this one, and is always happy to give advice in the area of DIY room acoustics (even if it means losing a sale). He also has a lot of information on his regular website:

http://www.ethanwiner.com/acoustics.html

Best of luck.


----------



## Sonnie

Ethan actually helps us moderate here at the Shack! :T He and GIK get along very well.


----------



## BoomieMCT

So I figure I have about 4.5' of leftover sonotube so I'm going to experiment. The HR I'm going to set up will look something like the attached picture - possibly with 3-4 tubes. Is there any particular way that a HR needs to be positioned to work best?

I may experiment with having them all have the same tune, and having each tube have a different tune.


----------



## Doug Plumb

I've experimented with this quite extensively and built many helmholtz resonators & tested them in rooms. Overall I prefer Ethan Winers approach - adding helmholtz resonators to small rooms works well but not bigger HT rooms because you need so many of them.

The biiger the better because bigger ones give higher Q. Vary the length of the port to change the resonant frequency. Using multiple resonators at one frequency doesn't work because each time you add a resonator to the room you change the characteristic frequency of the room that you are after.

Get really big sono tubes and use plywood for the caps. I used two layers of plywood affectively over an inch thick. You need lots of thickness to get the plywood to stick to the often wax coated inside of the sono tubes. I used silicone.

Put the resonator in the room with a tube length close to your resonant frequency in a corner then change the tube length until you nail it.

The alternative is to build wider band devices.

Another alternative would be RealTraps.com (Ethan Winer). These are as good as wide band helmholtz resonators because they reflect high frequencies while absorbing LF's which is what you want to do.


----------

