[BrunoB]

Has anyone experience building a Helmholzt resonator?



It looks easy: a box + a tube. The frequency of resonance is easy to calculate (http://www.phys.unsw.edu.au/jw/Helmholtz.html). I will start building my own pretty soon.
I want adsorption in the range 30 to 40 Hz.

Bruno

[bpape]

Be very careful with Helmholz tuned resonators that low.  They can cause their own issues and you need a pretty significant amount of 'tubes' to have much effect.  I'd recommend a sealed membrane type absorber instead.  They're a bit broader like you want and not quite as big to do the job.

Bryan

[youngho]

May I suggest taking a look at chapter nine of Alton Everest's Master Handbook of Acoustics?

http://books.google.com/books?id=sgwg1Vwm9VUC&pg=PA229&lpg=PA229&dq=alton+everest+helmholtz+resonator&source=web&ots=JGsFOCdzUE&sig=_dyroPDP4jJqOF2Pup7szYll50Q#PPA219,M1

[BrunoB]

May I suggest taking a look at chapter nine of Alton Everest's Master Handbook of Acoustics?

http://books.google.com/books?id=sgwg1Vwm9VUC&pg=PA229&lpg=PA229&dq=alton+everest+helmholtz+resonator&source=web&ots=JGsFOCdzUE&sig=_dyroPDP4jJqOF2Pup7szYll50Q#PPA219,M1

Thanks for the link. I have the printed version of the book.


I just found some nice on line calculators: http://www.mhsoft.nl/Helmholtzabsorber.asp

Bruno

[youngho]

Cool, I just thought you might be more interested in the perforated type, rather than the traditional style, of resonator.

[BrunoB]

Cool, I just thought you might be more interested in the perforated type, rather than the traditional style, of resonator.

It seems that you have a preference for perforated resonators. If yes, is there a particular reason?


I found a nice looking perforated resonator here:
http://highend.nu/index.php?option=com_content&task=view&id=11&Itemid=36


There are other interesting DIY room acoustics projects on the same site:
http://highend.nu/index.php?option=com_content&task=blogsection&id=3&Itemid=36


Bruno

[BrunoB]

Be very careful with Helmholz tuned resonators that low.  They can cause their own issues and you need a pretty significant amount of 'tubes' to have much effect.  I'd recommend a sealed membrane type absorber instead.  They're a bit broader like you want and not quite as big to do the job.

Bryan

Thanks four your input. I am interested for membrane resonators as well.

How do  you calculate them? The formulas I have found do not take the stiffness and the size of the membrane into account.
I think that  stiffness would increase the frequency of resonance whereas a  larger size would decrease it. Another problem is that I don't have an accelerometer to measure the frequency of resonance. Theses are the reasons why I am more interested to build a Helmholzt resonator.
An another reason is that, according to pages 227 and 228 of the acoustic book http://books.google.com/books?id=sgwg1Vwm9VUC&pg=PA229&lpg=PA229&dq=alton+everest+helmholtz+resonator&source=web&ots=JGsFOCdzUE&sig=_dyroPDP4jJqOF2Pup7szYll50Q#PPA227,M1
a  single resonator  seems to be quite effective.

Bruno

[BrunoB]

I built a very simple resonator just for testing purpose.




The bottle volume is 3 l, tube length is 24 cm and diameter is 7 cm. The calculated frequency of resonance is 114 Hz (using the calculator of http://www.mhsoft.nl/Helmholtzabsorber.asp.

I measured  the frequency response of the resonator near the one of my speaker using Room Eq Wizard. The results are shown below.



green line: no resonator
red line : resonator in place
blue line : resonator with a handful of fibrous material inside the tube or with the bottle filled with fibrous material


The measured frequency of resonance is 110 Hz (very close to the predicted value of 114 Hz)

The waterfoll plot shows a very strong resonance for the unfilled resonator and does not show any for the filled one.


Bruno
 

[bpape]

Membrane info:

http://www.customaudiodesigns.co.uk/articles/membranes.html

A good explaination and discussion

http://archive2.avsforum.com/avs-vb/showthread.php?t=568891

It's the mass more than the stiffness but stiffness will change things slightly. 

The size of the membrane will inherently change stiffness to that is something to consider also - but it doesn't shift it very much.

As you saw with your test, when you fill the port, you kill the resonance and also the efficiency.  When you leave it open, it tunes pretty well but at lower frequencies, can cause the type of issues I was speaking of when I suggested you try a membrane instead.

Bryan

[BrunoB]

I added one more tube to the resonator shown above. I wanted to know if there would be one or two resonances.

The bottle volume is 3 l, tube length is 24 cm and diameter is 7 cm. The measured frequency of resonance is 110 Hz.
The bottle volume is 3 l, tube length is 14.5cm and diameter is 2.5 cm. The measured frequency of resonance is 55Hz.

With the the two tubes attached to the same bottle, the measured frequency of resonance is 130 Hz.



Red line: no resonator.
Blue line: resonator in place
green line: fibrous material placed in the small tube
dark green: more fibrous material placed in the small tube


Adding fibrous material in the small tube shifts slightly the resonance frequency down, the Q goes down as well.


The microphone was placed closed to the big tube opening. Same results were obtained when placing the mic close to the small tube opening.

Conclusion: 2 tubes of different dimensions give a resonator with a single higher frequency of resonance.

[BrunoB]

Membrane info:

http://www.customaudiodesigns.co.uk/articles/membranes.html

A good explaination and discussion

http://archive2.avsforum.com/avs-vb/showthread.php?t=568891

It's the mass more than the stiffness but stiffness will change things slightly. 

The size of the membrane will inherently change stiffness to that is something to consider also - but it doesn't shift it very much.

As you saw with your test, when you fill the port, you kill the resonance and also the efficiency.  When you leave it open, it tunes pretty well but at lower frequencies, can cause the type of issues I was speaking of when I suggested you try a membrane instead.

Bryan

Bryan, theses are very useful links. Thanks.



Bruno