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Shaped Ports


peniku8

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  • 11 months later...

I liked this port so much I'm basically copying it for my 12TBX100 sub.  The height wasn't listed so I guessed it as 44mm from the pictures (3*18mm boards with 10mm of rebate).  Tuning came out at 38Hz.  How close did I get?

image.png

I'm also guessing from how your Hornresp response compares to mine that you used complex inductance parameters?

 

 

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I did use complex Le and my vent area are more than yours, even if you double the numbers to combine the two ports. I will PM you my HR inputs, but I usually manipulate the inputs there in a way to get results closer to real-world expectations.

My thoughts on the design after a while:

-I've used them at a few weddings and the performance was great. I have a sub-octaver on the cajon, which adds a nice 40-45Hz low end to the kick drum, which would be hardly achievable with run off the mill 12" PA designs tuned to 55Hz and what not.

-They can take a lot of power and I've never noticed port noise, but theres some early compression creeping in, which is just natural for a design this size

-The expansion of the port towards the outside is too much I think. It might cause separation (and turbulence) there, which could be remedied by an extra element to guide the air, but I'd maybe just make the expansion less logarithmic... I'm learning a bit from a youtube channel called Kyle Engineers, which is an ex-mercedes F1 engineer

-A bigger port gives you more performance of course, but the goal here was space-efficiency obviously, and it needed to be a one man setup which won't break my back.

The design is lacking a bit of punch compared to my 21ds115 cabs, but I guess that's only natural since those are front loaded and.. well 21"

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Thanks for the info! I had already designed a sub with just pipe ports to try and get something built quickly, however I needed to redesign it as my model would break on changing the material thickness parameter and it was overweight for the application (ultra portable human carried ~5km) resulting in a change from 18mm to 12mm (yes I know 12tbx100 is heavy, but I got them cheaply and this is the easiest thing to change).  But then I realized how much output at tune I was potentially giving up (JBL HLA port design information indicates around 9dB at tune).  In the future I would be interested in a even more port optimization but its a bit outside my area of expertise as I'm an electronic engineer and my only physics is in electromagnetics.   From the off the shelf options ( https://www.l-acoustics.com/en/product/ks28/# ) it looks like your on the right track.  If you wanted to try lots of port designs the best approach for testing would be to have a box that allowed attaching ports to the outside so that each test only required building the port and not a whole box.

Size is very constrained for these subs as they have to go on someones back and also fit under my desk at home.

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  • 2 months later...
  • 2 years later...

Heya, I wanted to update this thread with my recent findings comparing straight ports with shaped ones from two 21" subs.

I build two cabs of identical size and tuning loaded with 21DS115-8 drivers, where one cab has a straight port and the other one has a port with continuously changing area. The shaped port has a moderate ratio of 2.5 (largest to smallest area).

I put both cabs up for compression sweeps at 2m half space and the results are quite interesting.

Port compression is visible at around 20V for both cabs, but I don't care much for fractions of a dB of compression. It starts getting interesting at 64V, where the shaped port went 0.5dB into compression and the straight port was at 1.5. The difference remains 1dB at 90V, but now we see power compression starting to kick in as well, centered at around 75Hz.

Here are both cabs at 128Vrms input:

QEyO75U.jpg

The difference is still around 1dB at tuning, but now we see a stark difference of around 2dB near 70Hz. The cab with the shaped port behaves much better overall producing a smooth curve, which would still sound excellent at those levels, while the straight ported one starts being kinda all over the place.

The difference at tuning is easily explained: the aerodynamic port reduces 'air resistance' so the cab is more efficient there. The expansion at 45Hz I can explain too, or at least speculate: This is where impedance and particle velocity is high, which means no power compresison in this region and vortices forming along the surface, reducing friction with the cabinet walls. The dramatic difference at 70Hz I have no clue. The port does nothing at that frequency...

Interestingly, the sweeps at 181V look similar, because both drivers are heavily into power compression at that point, but the shaped port wins out by 1dB on average.

Distortion looks pretty similar across all levels, except for the 128V sweep, where the cab with less compression also has quite a bit less distortion (around that 70Hz).

So, conculusions.. It looks like overall a shaped port can extract 1-2dB more out of a subwoofer of a given volume, depending on which frequencies you need for your application. The cab with the straight port performed almost identically from 40-60Hz, but for other frequencies, it might be the difference of bringing 6 to bringing 8 cabs to a show.

On another note: hornresp predicted this subwoofer's max output at 44Hz to be 124dB (1m full space), limited by Xmax. But I measured 129dB (2m half space) with just 7% THD... 5dB is a pretty big difference, especially since hornresp doesn't factor in any sort of compression...

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