Greets!
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>Does anyone here have any experience using a Helmholtz resonator to null out port resonances?
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These are called band-stop filters and will only produce a deep notch at one frequency. To damp a series of modes requires a low pass filter, which is an expansion in the line like an exhaust resonator.
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>The design uses the 6.5" Tangband sub driver in a .75 cf vented box with a 100 watt plate amp in the back. Box size is a guess as I don't have it in my notes. The down side to building a very small sub tuned low is the port size needed to get a small box to resonate at 30 hz tends to be on the silly side.
>To get the box tuned with a port we ended up with a 3" port that is close to 5' long. When they were modling the sub the question came up of what to do with the port. After many silly ideas and good laughs I suggested that they use the port to make a base for the box. And so the mini sub was born...
http://www.pacifier.com/~gpimm/minisub.jpg
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Correct, you basically wind up with a zero expansion BLH. Cute design BTW.

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>The first port resonance is at ~120hz and quite high Q. We tried damping the port with both dacron and long fiber wool but by the time their was any results at the 120 hz resonance the port had stopped working as a port. With all the bends in the port there is enough resistance that the port resonance at box tuning is already low Q and adding any more damping is not a valid way to deal with the problem.
>My thoughts are that with the port size of 3" the port on the Helmholtz resonator should be 1 to 1.5" in diameter so that it can work independantly of the 3" port it is attached to. Does this sound like a good starting point?
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OK, maybe a BSF will suffice:
http://www.kettering.edu/~drussell/GMI-Acoustics/Filters-Frame.html
That said, rather than 'band-aid' this design, I recommend you build an end loaded reverse tapered TL, which if you think about it is where you're intuitively headed with wanting to use a somewhat smaller pipe in series with the larger one. It also has the benefit of being shorter for a given Fp than a straight pipe, the length decreasing with increasing compression ratio (CR) and provides more loading around Fp so you may be able to tune it a bit lower.
GM