Cabinet brace act as helmholtz resonator?

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I am in the process of designing a sealed cabinet for some 12" woofers and of course I plan to brace the box. As is common, I am planning to cut several holes (4.5" dia.) in a piece of 1" MDF and divide the cabinet once or twice.

Upon drafting my plans, I noticed that what I had just finished drawing looked an awful lot like a helmholtz resonator. I calculated the natural frequency of the absorber to be 15Hz. Does anyone know whether this is in fact a concern? Does it react differently being that it is within a closed system (ie. both sides of the brace could be viewed as a helmholtz resonator????)

Any feedback is much appreciated. Am I getting ahead of myself? Is there a general rule of thumb for designing braces (ie. A brace should not block greater than 50% of the "flow area")

Thanks in advance.

Ryan
 
I'm not sure if a helmholtz resonator works in quite that fashion if it's not releasing into free air. If the bottom box was open (ie. 8 4.5" ports) and you were using 1" thick walls, and the rest of your box was about 110 litres, then you'd have a resonance frequency of about 145Hz.

But, I'm not sure you get the same resonance when you're doing that into a sealed space. I've got sealed bass boxes that do not seem to have any resonances of this nature, and I used 3" holes to allow airflow. I only use them below 120Hz - I'll go home and try them out full range and see if I notice anything 'weird'.

If you're really worried, line the walls of the bottom section with sorbothane :)! Or measure the impedance of the speaker in the box - if it's not smooth then you might be getting those effects.
 
Thanks for the input Cloth Ears. I guess it gets a little bit confusing because the term "helmholtz resonator" when paired with "speaker enclosure" usually means ported box... In this case, I am more referring to a "Helmholtz absorber" such as is used in auditoriums and lecture halls to absorb problem frequencies.

Maybe rather than worrying about this I should just change my brace design so that it is more open.
 
Yes there can be resonances from holes in the bracing. If the holes are narrow, the resonance frequency becomes low, and if the holes are widened, the frequency will increase. If the area becomes equal to the box size (ie the bracing is taken away) the resonance becomes one of the standing waves in the box, so to say.

...but given the amount of filling you have in the box, the resonances will not be problematic at all, is my guess. Also, it seems as if the holes are so large that the resonance would be rather high.

How did you calculate 15 Hz? It seems too low to me.
 
noah katz said:
I also think you must have made a calculation error to get 15 Hz with such a large "port" area and small volumes.

I think the cavities and dividers comprise a bandpass system.

Actually, having an internal resonator is more of a bandstop system. At the resonance, the cone will feel a very large acoustic impedance, which will more or less makes the cone stand still.

It is just as with the bass reflex, except that there is no contribution to the surroundings from the port to fill in the response.
 
If in fact the shelf brace creates a notch at 15Hz, I imagine it probably doesn't hurt anything. In fact, a notch at 15Hz could help to eliminate 15Hz subsonic flapping. It would be interesting to pursue this thought process further and see if one could develop a subsonic absorber (one that would absorb 1 to 18Hz rather than just a notch) for a vented box to eliminate the lack of control below box resonance... This is probably not entirely practical because electronic filters are so much simpler but it would be interesting.
 
Don't know, didn't 'run the numbers', but FWIW you can design this cab alignment using MJK's DBR WS and set the lower vent diameter and length to = 0.001 of your preferred units if you want to pursue it further.

Maybe not practical, but having used both I still prefer acoustic solutions to acoustic problems if size isn't a major concern.
 
RyanW said:
Thanks for all the feedback. I have redesigned my bracing, using dowels rather than shelves so as to avoid any possible problems.

I calculated the 15Hz frequency using a formula from an audioholics article. Follow link below.
http://www.audioholics.com/education/acoustics-principles/helmholtz-resonant-absorber

Ok, so that formula is correct, but I wonder what numbers you put into it?

You need the V, S and L.

Guessing from your picture V is maybe 30 litres=0.03 m³?

S is the area of 8 4.5" holes=8*(0.0254*4.5)²*pi=0.33 m²

L is the thickness of the MDF plus the end corrections. Now the end corrections are a bit hard to estimate in this case as it is inside a box, but to get a very rough estimate one could use an equivalent hole of sqrt(8)*4.5" diameter. Then the end correction would be 1.7*sqrt(8)*4.5/2*0.0254=0.27 m. The thickness of the board can be neglected, apparently.

This would result in a resonance frequency of roughly

fh=c/2pi*sqrt(S/LV)=345/(2*pi)*sqrt(0.33/(0.27*0.03))=350 Hz.

Could you fill in with the right numbers?
 
RyanW said:
If in fact the shelf brace creates a notch at 15Hz, I imagine it probably doesn't hurt anything. In fact, a notch at 15Hz could help to eliminate 15Hz subsonic flapping. It would be interesting to pursue this thought process further and see if one could develop a subsonic absorber (one that would absorb 1 to 18Hz rather than just a notch) for a vented box to eliminate the lack of control below box resonance... This is probably not entirely practical because electronic filters are so much simpler but it would be interesting.

Interesting thought...

However, I think such a resonator would become so terribly large and given the problems getting a large enough box in the normal case... Well I think that that is the explanation why this idea has not become a hit earlier. :D
 
" It would be interesting to pursue this thought process further and see if one could develop a subsonic absorber (one that would absorb 1 to 18Hz rather than just a notch) for a vented box to eliminate the lack of control below box resonance."

1 to 18 Hz? That's 4 octaves!

Resonance of any kind is intrinsically narrow BW; just try getting a regular ported enclosure, or anything that resonates, to do so over more than a fraction of an octave.
 
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