A couple port questions

[Adam_M]

I'm getting back to a sub project I started a long time ago - 15" Tumult in a 5.9CF(net) box.

I'm toying with either a 2"x21.5" slot port or 3 4" precision ports tuned to 19Hz. Velocity in either case peaks at about 31m/s at about 18Hz, and stays below 26m/s from about 21Hz up, and 17m/s from about 25Hz and up. I don't think those velocities are particularly concerning, given they take 1kW to hit, occur at such low frequencies, and the fact that it's a sub. Yes, I know there will be port compression and at large signals as the small signal models will break down a bit...


I'd probably point the ports down, and put the sub on feet that are ~3" high, but I suppose I could fire them back. They won't be firing at the listening position in any case.

1. Is there any benefit to the slot port other than cost and perhaps a little simplicity - not needing to build brackets for the 12 feet of 4" (cumulative) of port that'll need to be in the box?

2. Has anyone ever used a dust collector elbow as an elbow for the precision port? They are (at the mating surface) 3 61/64" - I'm sure some sort of epoxy can bridge a .025" gap. They are also 3 3/4" ID. Would there be major turbulence issues? I'm sure the port would have to be extended (slightly - I can do the math) to account for the slightly lower captured air volume of the elbows vs. the straight pipe sections.

3. Any chance I could get a away with a single 6" port? (1kW, 117dB predicted) Max velocity is 42m/s (UGLY!) at 18Hz, hits 26 m/s at 23Hz, and 17 m/s around 27Hz. This is of course the easiest build and pushes the port resonance up almost another octave and well out of the passband. No issues at more sane volumes and power levels. The more I think about it, the more I think this could be feasible...

This sub is basically for dinosaur footprints, Apollo moon launches, big explosions, etc. Music LF is handled elsewhere.

[fpitas]

No advantage to slot ports, except when they fit the box better than tubes. A round port will always flow better for a given area. Not sure what a dust collector elbow is, but any smoothly curving elbow will work. I'd go with the 6" port, with VERY generous flaring. Or perhaps two 4" ports. But that's me ;)

[billfitzmaurice]

I don't worry about port velocity below 25Hz as there's not enough content down there to matter.

[fpitas]

Plus everything in the room is probably rattling at 18Hz lol

[fpitas]

Downfiring the ports will help with port noise, in any event. Especially if the sub sits on a rug.

[Kornbread]

Throwing this out there. If you use a round port, why not use pvc commonly available at the local hardware store? It's cheap, comes in many flavors, and offers a selection of elbows that fit. Generally speaking, the larger the port dia the more likely you might get some pesky port resonances, although this may not be a concern with a sub. Just me, but if a round port was going to be super long, or take multiples, I'd consider a slot port. Anyway, at the volume 1kw is likely to hit, you ain't gonna' hear a little port chuffing.

[billfitzmaurice]

PVC is all I've ever used for round ports.

[augerpro]

I think the first question is are you working from a known design? If not, you're calculated port length may not be exactly right. If so the Precision ports are easily trimmed while the slot port is probably impossible to change. Attached is study on ports I recently did. Surprisingly the slot port was almost identical to the round port, possibly a bit better with harmonic distortion at the highest levels. One thing I would recommend either way is having an internal flare. Moving to the Precision port clone stepped up performance in power compression and harmonic distortion due to that internal flare. The best was the Harman inspired port, which would difficult to print for subwoofers, but I'm going to investigate whether it can be made as a slot port and perform as well. If it can, it would be straightforward to construct it out of bender board. Anyway here is the study:


ROUND 3!
I'll be rolling out results over the next week, but the first batch is the port performance. As mentioned before, the inspiration for this experiment was the Kef LS50 white paper and a Harman study by Salvatti, Devantier, Button (attached). The Kef port's innovation is using a foam center section to dampen the higher port resonances. The Harman paper looked at different port terminations and their impact on sensitivity, harmonic distortion, and power compression. Their own designs in the test were ports that were not straight walled, but a constant radius. They related this radius to the overall length via an equation, but the one labeled N=0.5 was the best balanced and the one that I selected to copy. Simply, the radius of the wall curvature is equal to the port length:



There is also a small 1/2" roundover on either end that isn't shown in that pic. What was interesting about Harman's results were slight sensitivity gain at low power levels vs the competition, and very good harmonic distortion and chuffing onset performance. I did not see the sensitivity gain, probably due to length, which I will address below.

For the Kef port I used common foam pipe insulation and also decided to print a version using TPU filament. I used Hatchbox TPU with a Shore 95A rating. I used 2 perimeter layers and 18% fill, which is about as low as I think a person could go. I copied the roundovers of the Precision port so it would just be the flexible center sections that were the variable.

The Precision port is just a 3D-printed copy of a 2" Precision port.

The "typical DIY" port is a simple 2" ID pipe with a 3/4" roundover at the baffle, as a DIYer would typically do. This would be the reference port.

The square port is similar to the typical port, except rectangle (it just now dawned me I labeled all the pics square, oops) with a 1:4 aspect ratio. The cross-section is equal to a 2" round port at 3.14 sq.inches. This also has 3/4" roundover at the baffle.

Speaking of diameter all of these are 2" nominal ID ports. For the Harman this would mean the very center of the port where it is skinniest.

The box is a 4th order bandpass tuned to 52hz. The driver is the SB15NAC-8. Front volume is 15L, rear volume is 10L (net, but see below). Front was lined with wool batting, rear was lined and somewhat stuffed with the same wool batting. I adjusted all the port lengths so the Fb was the same, as verified by impedance:



Length can give you an idea of how restrictive a port is. The less restrictive it is, the longer the port must be to maintain a given Fb. The lengths were: Harman 5.4", PP and Kef 4.25", and typical and rectangle port 3.75". This is where I deviated from the Harman paper. There they equalized the ports by length, which doesn't make much sense to me, in the real world you would want to know relative performance for a given tuning. For this reason, you might notice unexpected results vs the Harman paper. For example sensitivity, which was worst for the Harman in my tests, instead of the best in their paper.

This brings me to caveats. Since these are all different lengths, I had to add a little closed cell foam so the port volume was *roughly* equal. It wasn't exact, but I'm satisfied with my mitigation. Next caveat is that all of these are more or less close to the driver. A problem? I don't know. See pic for an idea:



Another caveat is surface finish. The Harman paper showed surface finish makes a difference and smooth is best. These are 3D printed and so not as smooth as PVC pipe you might typically use. A problem? I'm not sure, but I would say a necessary evil to keep the playing field level. Otherwise I would have just used the actual 2" Precision port I had. Another benefit was that I could make the mounting flange the same for all of them for quick attachment/removal.

Test setup> MLS measurements for the SPL: 500ms gate, 1/24 smoothing, mic at 1/4" from the baffle. I tested a number of distances and settled on 1/4". I had to change things up for the harmonic distortion measurements to not overload the mic and soundcard, here it is 20.5" from the baffle. On a related note, I could not use the MLS function for these high SPL measurements, so to judge power compression look at the level of the fundamental F1 in the HD measurements. Also when trying to get an idea of the onset of chuffing, you can see the harmonics starting to cluster. Why this is so is what you can't see in the plot: the entire spectrum is lit up with something almost like white noise, with other concerning sounds mixed in. You'll also notice the fundamental F1 goes from smooth to jaggy. I know the plots are limited, but I'll try to add subjective commentary to give clearer perspective. I added the drive voltage to the HD plots, if you are bored you can look at the impedance and calculate power and SPL.

Wow, lots of setup, let's just show the damn data!

Typical DIY port








rectangle port








Kef port w/foam








Kef port w/TPU








Precision port








Harman port








Some comparisons:









I need to delve into the results much deeper but here are some initial thoughts. The typical DIY and rectangle DIY ports were surprisingly similar in frequency response and sensitivity. I'll need to dig into the HD results to pic a winner.

The Kef ports didn't show their stuff as much as hoped. Foam looks better than what I printed, so chasing down high performance flexible materials will be the key to making the concept work. One thing I should mention, since the Harman port was the largest and not length adjustable, everything had to match that for Fb. This resulted in the Kef center section only being about 1 5/8" of exposed pipe for flexing. Perhaps if it was longer you would see a bigger impact on damping those port resonances.

The Harman was the most interesting to me. I was let down by the sensitivity being lower than all the others. But as soon as I did the HD measurements it clearly pulled ahead. I've only just glanced at the HD plots to confirm what I heard, but I can tell you sitting here running the test the difference was huge. Most of the others were having difficulty by 8.75V, and are downright scary sounding by 10-11V. The Harman was just so much cleaner. I remember thinking at 13.52V, yeah that's chuffing. But lower than that and you kinda had to listen closer to pick out audible distress, it just wasn't that noticeable, certainly not if I were actually playing music or movie to mask it. I need to compare it against the actual Precision port with smooth walls. If the 3D printed surface is not an issue, I would encourage anyone to try the Harman for any fullrange or subwoofer duty where ultimate SPL performance is important.

[Adam_M]

Augerpro: Wow, that’s quite a port study! I can't wait to see what insights you have as you analyze the results more. That Harman port looks promising! We also may have to chat later, my next build is going to be a purifi 6.5 and a tweeter in a waveguide. I've been watching your other thread for candidates.

Thanks for all the advice. I think I'm going to go the easy way and use a 6" precision port so I can get the big flares. I've built dozens of speakers with square edge and rounded PVC ports and this will be something just a little different.

[Dukk]

6" Precision Port flares are huge! I used some a long time ago and IIRC they were around 10" in diameter.

In this case I would go for a slot vent and just use the largest roundover I have on all corners/edges.

[tvrgeek]

Spurious thoughts:

Not finding reference to the Harman port or the Kef study. Can you point to them?

The above plots are interesting. I am more concerned with performance at way lower mach numbers.
In manifold design. it was determined that a 260 degree round over was optimum but need not be larger than 1/2 inch radius to prevent the artificial narrowing of the port though a 90 degree elliptical onto a flat plate was almost as good. Too bad I have not found an elliptical radius router bit.

It would be logical, that to reduce resonances, a variable diameter would be adventurous, but much harder to tune.

A material for an absorbent center might be semi-rigid pipe insulation. It would need to be lined of course. I can think of gluing many materials inside a cardboard tube.

[augerpro]

I don't have the Kef study on hand (search for Kef LS50 white paper) but attached the Harman paper. One version of the Kef port did use pipe insulation foam, but I'm going to investigate tapping the center of the pipe and damping that energy instead of trying to find the perfect flexible material of the right diameter. Might be easier to read about the project here (round 3 had the ports): https://www.somasonus.net/box-construction-methods

Harman Port Study.pdf

[augerpro]

Watch this thread over the next couple days: https://www.diyaudio.com/forums/mult...thread-26.html. I'm measuring some new ports and the results are promising. I've abandoned the Kef method of flexible center sections and went with tapped center sections feeding a cavity damped with wool. The initial results are very encouraging. Tomorrow I do harmonic distortion tests then I'll post results.

[Chris Roemer]

Have you ever seen 4thtry's project where he successfully trapped the port resonance? This was from 4 yrs ago at Iowa DIY.

[augerpro]

Interesting. Is there documentation on this project?