Port / driver placement

[Zippy2000]

Unibox looks like a bit more up to date software btw. I might give that a go as well. Does it also do active HP filters?

 

[Zippy2000]

There is no problem by itself with long port, except that you will want to lower the xover frequency to the point where standing waves in the port are not limiting performance. Since an open pipe will sustain wave lengths of 1/2 times an integer number, for a 1 meter vent, a practical cut off frequency would be 85 Hz. The reason is that you want to stay away about one octave from the lowest frequency that can be sustained by the vent, which is about 170Hz for 1 meter.

So it's indeed a resonance thing then when it comes to port length. In my case I want to cross over at about 300Hz, so according to your math I could use a max port length of about 57cm to stay below 150Hz, correct?

This is good stuff to know! Very little has been written down on this it seems.

 

[vacuphile]

300 Hz in standard air (V=343m/s) has a wave length of .87m, so I would come to a length of .435m.

This is rule of thumb of course and I found it to work well for 4th order filters (24dB/octave). If you go to a 2nd order filter (12dB/octave), there obviously is more energy below the xover frequency and you may wish to be more conservative (that is, xover at a lower point or decrease the length of the vent).

Some sim programs allow you to simulate vent resonance (Smith and Larsson Woofertester resp. SoundEasy come to mind).

 

[Zippy2000]

Thanks for that, very useful!