HT Subwoofer port tuning mistake... Looking for redesign insight

Hello all,

Pretty new to DIY Audio world, but recently finished a build of my first ported subwoofer enclosure. Overall I am happy with its performance. However, I feel I made some design choice mistakes and am missing out on some performance and personally find it a little too large for my taste.

Equipment and current box parameters:
Dayton Audio RSS315HE-22 12
Crown XLS Drive Core 1002 bridged
Net internal volume of 3 cubic feet
3x slot ports 1.5x5.25inches at the opening with a length of 44inches each with a tune of 21hz
WINISD F3 was ~20hz

For context, I do about 50/50 music and movies and am a huge fan of deep tactile bass. Not a huge SPL guy since I usually don't listen much past 90-95db at the listening position but do value some head room for intense movie scenes on occasion. When designing the enclosure I tried to be greedy and get as deep as possible will maintaining a reasonable size. After having it completed, I did hit my target of low bass for the most part, but I also noticed the ports have very minimal air movement even at full volume. I am now wondering if my design was too greedy tuning that low and I should have tuned the box closer to 27-32hz and let room gain carry the rest. Or, I made an error in my design calculations and missed something important. For note, I do not notice any port noise or resonance, and overall very "quiet" sub.

I am thinking of rebuilding the enclosure for the sub with a few primary options that help reduce port length, but also reduce the sub size:
1. Using 6 inch round port tuned to 28hz that is 36inchs long, with net volume of 2.4 cubic feet which gives me an F3 of ~24hz
2. Using 6 inch round port tuned to 30hz that is ~36inchs long, with net volume of 2.1 cubic feet which gives me an F3 of ~26hz
3. Using 6 inch round port tuned to 32hz that is 31inchs long, with net volume of 2.1 cubic feet which gives me an F3 of ~27hz
4. Using 6 inch round port tuned to 32hz that is 26inchs long, with net volume of 2.4 cubic feet which gives me an F3 of ~27hz

My concern now is that a higher F3 would cause me to miss out on some subsonic tactile bass or that I will somehow be making the same mistake again and be wasting time and material. I am also concerned that maybe 36 or 31 inches is still too long and maybe need to tune even higher but using a smaller diameter shows too high of air velocity at the port in WINISD. I considered using a passive radiator, but modeling it in WINISD shows a significantly lower F3 of 29hz, costs more, and reduced volumes, albeit a much smaller enclosure which would be nice.

Any insight to my issue of minimal port air velocity in my current build, or comments on my concern of higher F3 for HT use would be helpful. Thanks in advance!
 
I suppose you are not getting much port noise because:
  1. The port area is quite large
  2. You don't listen at high volumes
  3. There isn't much content at 20Hz so the port isn't doing much.
Did you actually measure the port tuning frequency? It may be lower than you think as slot ports have a large end correction factor. A quick look indicates that the box is too big, and that you can tighten up the bass just by reducing the volume by ~0.7ft3 (add a lump of something...) leaving the port exactly as it is.
 
Here is response achieved while varying volume from 45L / 1.6 ft3 (dark blue) to 85L / 3ft3 (red) in 10L increments while maintaining the same port dimensions as you currently have. You could reduce volume by adding bricks or whatever and find out which you prefer, then rebuild if you want with the smaller volume.

RSS315HE-22 45L to 85L.gif
 
Ever verified the tuning by measurement? WinISD is programmed to simulate tuning frequency to length of a mid baffle round port, and might be able to do mid baffle slot ports as well.. but no guarantees. What WinISD definitely can't accurately calculate for example is ports bordering on walls.

Port surface area is around 1/4 sd. For a driver this potent, not even 1/2 sd will be entirely compression and noise free at large signal excitation. Add to that that they are narrow slot ports, which isn't the ideal in regards to flow and turbulence.. the ideal would be quadratic, or actually round.

When you talk about hardly any response from the ports, the problem might be wrong alignment, meaning the ports don't really get excitated by the chassis. Or, as a previous poster noted, they might not get excitated because of a lack of signal. Apply sine signals around the simulated tuning frequency of your port to find out.


Complete side note, but since they were mentioned: correction factors for various port types are forum myths / lore. They float around, but there has never been a single empirical verification that show they work. It's really not wise to spread unproven claims as facts.
 
Complete side note, but since they were mentioned: correction factors for various port types are forum myths / lore. They float around, but there has never been a single empirical verification that show they work. It's really not wise to spread unproven claims as facts.
I'll add to the myth / lore and re-post this useful diagram ;)

effective Port length.JPG
 

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This pretty graphic has been circling around for years or maybe decades. People repost it probably / obviously because it's pretty and has formulas in it that look convincing to the layman, like worryingly often happens.

Claims / theories have to be proven with empirical data, meaning via experiments / study. As long as no proof is provided, it's just that - a claim / theory. Once that unproven claim / theory has become repeated often enough, it should be called myth / lore.

In case that's being mistaken as well: the burden of proof always lies on the person who made the claim.
 
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Your room/box does things more to sound than port end correction. I always ignore.
Has anyone reported drastically different freq response due to port end correction? I would like to know. I have bigger things to worry about. Probably your port measuring and cutting skills has more drastic effect than port end correction.
 
Agree to the first sentence above, especially for long ports. But....

There is some published research on this effect. Trawling through the internet it's hard to find a succinct reference, but below is a graph showing the effect on tuning frequency of locating a square orifice at different locations on the face of a cubical cavity (10cm side length). As the orifice moves from the center towards the corner, the tuning frequency drops by more than 30%. This is an extreme example as the end correction has a much bigger impact on a short port (i.e. an orifice), but the effect is obviously real. I'd love to know where the german worded diagram in the graphic I posted came from, but no one seems to know.

The graph below is from a thesis referencing "Effects of geometry on the resonance frequency of Helmholtz resonators" by R C Chanaud. I can't find a free copy of the original paper.

asymmetric orifices.jpg
 
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Agree. But since we were discussing about effect of port end correction, to actually help it defend needed nearfield measurement.
As I said before I ignore port end correction, as it hardly had any effect in grand scheme things. Just needed to see if anybody could prove it otherwise with measurements.
 
??? The end correction is about either shortening the vent and/or making the vent bigger, which is what I usually had to do and the extra port response was both felt and heard, so no need for nearfield measurement even if I'd had the option. ;)
 
Well, it was obvious standing next to it on my shop workbench the first time I learned of pipe end corrections during my 'adventures' in TL/horn design. In modding my now long gone full range horn stereo system's pair of dual 15" 20 Hz subs (only speakers left now), the difference was enough to make me lower tuning to its current ~14 Hz (pretty drastic
to me) that with 100 W/(4) 3 mm Xmax woofers caused the ceiling, stuff stored to 'rain' down on us during an action movie (previously documented), so pretty powerful and considering some other damage, my ancient floating floor 'stick' built/frame house apparently has a ~14 Hz fundamental resonance. :sigh:

The original room was 26 x 46 ft x 16 ft half cathedral ceiling, but lifestyle changes over time has them in a ~16 x 24 x 8 ft room that's open enough on the back/one side to the rest of this relatively small house to still have a ~12 Hz fundamental with a full height bookcase set up to make the parallel back wall diffuse. Corner loaded in both rooms.