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Old 21st September 2007, 10:27 AM   #1
RyanW is offline RyanW  Canada
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Default Cabinet brace act as helmholtz resonator?

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
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Old 23rd September 2007, 03:31 PM   #2
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Can you provide a drawing of the basic design?
That would lessen guesses.
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Old 24th September 2007, 07:06 AM   #3
RyanW is offline RyanW  Canada
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See attached drawing for clarification.
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File Type: pdf box bracing.pdf (50.0 KB, 161 views)
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Old 24th September 2007, 07:20 AM   #4
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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.
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Old 24th September 2007, 08:20 AM   #5
RyanW is offline RyanW  Canada
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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.
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Old 24th September 2007, 05:57 PM   #6
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Yes, there would be some sort of helmholtz like effect.
I would expect it to be insignificant though.
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Old 25th September 2007, 05:37 PM   #7
Svante is offline Svante  Sweden
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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.
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Old 28th September 2007, 05:40 AM   #8
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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.
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Old 28th September 2007, 06:07 AM   #9
Svante is offline Svante  Sweden
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Quote:
Originally posted by noah katz
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.
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Old 28th September 2007, 02:03 PM   #10
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Agreed, there will be a notch somewhere in the response.
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