Question regarding slot port configuration

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I've designed and built a subwoofer that would be expected to be tuned to 33 Hz based on WinISD configuration.

However, the port is long profile and has at least one bend in it. From the research I was doing it seemed I could expect that the actual tuning would end up lower than this due to the extra resistance in the long profile.

Oddly enough, the tuning seems to have come out right where WinISD said, judging by cone excursion during test tones. I am now looking to modify the slot port height to modify the tuning lower.

My question is: In such a case with a slot port that has one 90 degree angle in it, is it acceptable to modify only one portion of the port path? Or must the entire port path be modified? Ie, must port profile be entirely uniform throughout? My understanding is that the Hemholtz resonance is affected by the length and the volume contained within, therefore if I reduced the cross section in one portion only the tuning should be reduced (though not as much as if both sections were reduced). Is this correct?
 
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From the research I was doing it seemed I could expect that the actual tuning would end up lower than this due to the extra resistance in the long profile.

The extra resistance should lower the output from the vent, not change the resonance frequency.

My question is: In such a case with a slot port that has one 90 degree angle in it, is it acceptable to modify only one portion of the port path?

Yes. You can place a small "pinch" in it, e.g. by sliding a piece of wood partway up the vent, in order to reduce the resonance frequency.
 

GM

Member
Joined 2003
FWIW, I view as a two step pipe reduction, i.e. crude inverse taper, which will lower tuning at the expense of increased port compression, so to maintain the same port efficiency will require the initial 'expansion' be larger, ergo reducing cab net Vb if built in, which in turn [somewhat] negates the lower tuning in a 'dog chasing its tail' scenario. Better overall in this case is to add an external vent extension if it doesn't raise vent mach too much, otherwise just damp the vent to lower tuning.

GM
 
Thanks guys, I did some testing of the tuning using cone travel as a guide as I swept through tones in the 40Hz to 20 Hz range. The box stock was putting out about 32 Hz. I found that by adding additional MDF in one portion of the slot port I was able to reduce to 26Hz tuning. The cross section of that portion of the box is now 0.75" x 12.75", still more cross section than a 3" pipe. The large (exposed on front baffle) portion is double, being 1.5" x 12.75" so I don't think port noise will be an issue.

I also stuffed the box with 16 Oz of poly fill but that only reduced the tuning approximately 1Hz.
 
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I just looked at the response curves at different tuning frequencies and I think I will leave the tuning right at 26 Hz. The 32Hz had a lot of peaking that would likely be excessive in room. The 26Hz seems to have similar -3dB and -6dB levels as the 22Hz with more output across the entire 30Hz to 100 Hz range.

CSS recommends 45 liters and 25 Hz tuning but this is the largest I could get the box and maintain the aesthetic I needed to.
 

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FWIW, I view as a two step pipe reduction, i.e. crude inverse taper, which will lower tuning at the expense of increased port compression, so to maintain the same port efficiency will require the initial 'expansion' be larger, ergo reducing cab net Vb if built in, which in turn [somewhat] negates the lower tuning in a 'dog chasing its tail' scenario. Better overall in this case is to add an external vent extension if it doesn't raise vent mach too much, otherwise just damp the vent to lower tuning.

GM

Unfortunately extended port is not possible for this installation but thank you for the explanation. The cross section of the port even when reduced is larger than the 3" port that was suggested by the manufacturer.
 
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