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Triticum Agricolam

6th Order Port Area Experiment

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This is great info.  I wish I understood it.  I have a 6th order now and I love it the way it is but I’d love to do some experiments on the ports and fine tune it more.  I just need to understand more. Don’t wanna blow anything.  I don’t have the money to replace it.   

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After looking over the data, here is what I think is going on.  With the small rear port configuration as input power increase the rear port begins to compress and restrict air flow causing low frequency output to suffer.  This explains why the output down low is so much less with the small rear port config vs. the large rear port config.  In the pic here I circled the area around 30 hz where you can see a big difference in output.  The purple line is the large rear port config, the red is the small rear port config, and the green is the 4th order config.  At 30 hz the small rear port config has about 3 dB less output than the large rear port config.  The 4th order config has even less output, as is to be expected. 

image.png.ce072f1997c5f2023b631e1a9db779ad.png

At some point the small rear port is restricting enough the box begins to function like a leaky 4th order bandpass instead of a 6th.  When the small rear chamber box becomes more like a leaky 4th order we see a very slight boost in output in the 40-50 Hz region.  The actual 4th order box shows an even larger (though still small) boost. I circled the area here.

image.png.c3f3385014cec3e04559247f312fcd07.png

For more evidence that the small rear port box is beginning to function like a 4th order we can look at the impedance graph.  

image.png.da748ca380dfa290b353d23f3b11ad8b.png

Here is the impedance graph from the small rear port config shown with the impedance from the 4th order config (yellow line).  At low power levels there is a pronounced dip around the 25 hz tuning with a peak below that.  At high power levels though the dip is very slight and the peak below it is almost gone.  You can see the line begins to look a lot like the 4th order impedance curve. 

 

I performed this experiment in the first place because all over the 'net you see folks saying to make the rear port area small on series-tuned 6th orders because it increase SPL, and it does, at certain frequencies.  However it does so at a significant cost to low frequency output.  Whether that slight increase at certainly frequencies is worth it or not is up to the end-user, in my opinion it's not something I'm going to want.  If that additional output in the 40-50 Hz region is important, I would lean towards a 4th order bandpass box which does even better in that regard.

6th order bandpass boxes are complex creatures.  Hopefully what I've done here can help us all understand them better.  They involve a lot of tradeoffs and I think it's important to understand exactly what's going so people can make informed decisions.  

 

Conclusions:

Port compression sucks.  Making the rear port area very small is shooting yourself in the foot.

1. If you want maximum low frequency output (below 40 hz or so, depending on tuning) make the rear port large enough to prevent port compression as much as possible!!!

2. If you want maximum output in the 40-50 Hz range (depending on tuning) so you can put up big SPL numbers and don't care as much about lows, make a 4th order and tune accordingly!!!

3. If you want maximum lows AND max output in the 40-50 Hz region, one possibility is to make the rear port large and then block/plug the rear port when you are going for max SPL.

4. 6th order bandpass boxes are cool, but they aren't easy!

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"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

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I love this thread. Thank you for putting in the work to come up with all of this data. Currently designing a series tuned 6th for 4 18" dc lvl6s :) with the goals we have for the build this was very informative..... I will sacrifice front chamber volume to make sure I have adequate rear chamber port area if I have to.... crushing 25hz is a must

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Great info TA!

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