Drilled-Wall Port Tuning for Subwoofers!

EDIT: This is entirely incorrect! This will NOT work. WinISD beta is flawed in how it calculates the use of multiple ports. Lesson: Use WinISD Pro alpha!!!


When modeling a vented sub with WinISD, I was toying with the vent settings to get the vent mach below 0.17 in a feasible configuration. My application with an RE XXX 12" driver requires a 2.6 ft3 enclosure tuned to 21 Hz, but in order to have a vent mach under 0.17, a single vent has to be 4.75" in diameter by 37.4" long, which is not feasible. But if I add more vents, I could decrease the vent size:

For 2.6 ft3 Tuned to 21 Hz:
1 Vents @ 4.75"D x 37.4"L = Mach 0.16
2 Vents @ 2.50"D x 20.8"L = Mach 0.14
4 Vents @ 1.25"D x 10.4"L = Mach 0.14

So with more vents, I was finding more feasible combinations. Then I realized something. If I could have enough vents so that the vent length could match the thickness of the enclosure's front panel, I would not need any vents because I could just drill holes into the front panel. Since my enclosure's front panel is 1.5" thick, if I target a vent length of 1.5" by adjusting the vent diameter and number of vents, I am able to accomplish this:

For 2.6 ft3 Tuned to 21 Hz:
26 Vents @ 0.1875"D x 1.52"L = Mach 0.15

So if I simply drill 26 holes of 3/16" diameter into the 1.5" thick front panel, according to WinISD, the 2.6 ft3 enclusure is tuned to 21 Hz!

Another benefit is that after the enclosure is built, I can easily change tuning by plugging holes:

For 2.6 ft3 @ 21 Hz:
26x 3/16" Holes = Mach 0.15

For 2.6 ft3 @ 25 Hz:
36x 3/16" Holes = Mach 0.06

For 2.6 ft3 @ 30 Hz:
52x 3/16" Holes = Mach 0.03

By drilling port holes into the enclosure, the overall volume of the enclosure is greatly reduced because I do not need any additional volume for the ports, and I can change the tuning by plugging holes. Plus it's a lot easier to build. What do you think?

Thanks for the interest guys. It would have been more logical had I said...

1 Vents @ 5.00"D x 41.6"L = Mach 0.14
2 Vents @ 2.50"D x 20.8"L = Mach 0.14
4 Vents @ 1.25"D x 10.4"L = Mach 0.14

Also, it's interesting to see how by increasing the number of vents,
the required 'total' vent area is reduced proportionately:

1 Vents @ 5.00"D x 41.6"L = Mach 0.14 (Vent Area: 19.625 in2)
2 Vents @ 2.50"D x 20.8"L = Mach 0.14 (Vent Area: 9.8125 in2)
4 Vents @ 1.25"D x 10.4"L = Mach 0.14 (Vent Area: 4.90625 in2)

...and when the vent area is reduced, the proportionately shorter ports are still able to filter the sound waves as desired.

Here's a thread about a sub with pluggable ports. They discuss the same counter-intuitivity that I encountered with the relation between the number of ports and frequency, but that's also how a flute works. Pitch decreases as as you constrict ventilation. Not sure why.

Hmm, there was nothing further on the 1st post and I didn't edit it. If there is a second half to that post, it would be where I discuss how easy it is to change the tuning by just plugging the little port thingies.

Edit: This seems so easy, one of the reasons I'm posting about it is in case there is someone out there that can tell me that there's a reason why this is not typically done, so I don't waste my time trying something that won't work.

Thanks everyone for your responses. I know that the biggest issue would be the narrow peak with higher tuning with the same enclosure size. It seems to me that this would be an issue even with standard ports. Actually it seems to be more of an issue when using standard ports, because as tuning is increased, port length is decreased, and therefore net internal volume is increased even more rather than decreased, making the narrow peak even more pronounced.

Reducing internal net volume by placing object inside is a great idea. So for higher tuning, one could use a combination of plugging holes and adding mass, with the amounts of each according to WinISD calculations. Since this is for HT, I plan to keep it tuned at 21 Hz, but it's nice to have the option to tune higher if need be.

Steve, I just read your post "LLT Explained" from your signature. It outlines the issues I've been trying to overcome. I ended up considering the RE Audio XXX 12" so that it's power and excursion can extend much lower than typically achievable for a single 12" driver in a somewhat smaller enclosure. It's modeled as well as more than 2 combined 15" drivers from most other brands in much larger enclosures. After reading your article, I think you may be particularly insteresred in the RE XXX series. They are known for SQL and to be sort of the execption to the rule of thumb that great excursion drivers fundamentally introduce THD. I think the only trade-off is the requirement for more power, but I've got some decent NAD amps lying around collecting dust. I may use a 2700 with great bass extension which is brigeable to 1.6 kW @ 4 ohm.

Anyhow, in your post, you address the need to reduce as much as possible the detrminental effect of phase canellation between the port and the driver, and the need for reducing port output so that the driver primarily produces output. For this issue, I think the use of multiple ports just may have a huge benefit, because with multiple ports, both total port area and mach are reduced in a seemingly unlimited manner. As such, it would seem to me that the smaller total port area would produce less output relative to the driver, and therefore introduce less phase cancellation issues.

Also, I had not considered tuning as low as 15 Hz to offset LF room gains that can make vented systems sound boomy and sloppy. I will be looking into the concept of LLT for my system.

Steve, after modeling for LLT, as expected I see that when tuning so low, we lose so much SPL due to excursion limits. If you look at the three configurations in WinISD below, you can see that when total output is reduced to excursion limits when tuning lower, more than half the SPL is lost when going from 21 Hz to 15 Hz. Do you feel this loss is worth it?



Edit: Also, how much gain can I expect from typical room gains to bring these up to THX levels (115db)?

On second thought, maybe I will just go with the RE XXX 18" in a sealed enclosure to maximize the utilization of the excursion throughout the frequency range, and just use an EQ to make adjustments:



Never thought I'd turn to the world of sealed, but I like how excursion can be easily maximized for any FR range you want with an EQ.

Now there's a duh moment for me. There's nothing wrong with self-taught though, I'm sure most of us here are; just takes a lot more reading than writing.

I'm still curious as to Steve's suggestion that the flat extended bass is not so good in-room compared to an extended tapered down bass due to in-room gains at lower octaves, because tuning at 15 Hz really does cut the usable SPL in half over just 21 Hz.

And, if a extended/tapered bass is really what we want, a larger driver in nearly the same size enclosure that is sealed and EQ'd can provide that anyway. Plus you then have the ability to easily utlilize the all the gains in the 30 - 60 Hz region when you want.

Among everything I come across, the single most overlooked and very important aspect when I see people planning their subs is how driver excursion limits what is actually achievable from WinISD. I think that until you've compared excursion limits and optimized for them, you're only half way there. I feel that the most optimized design for a driver is one in which the SPL curve matches as closely as possible the excursion slope of the driver. That is the only way to get the most out of the driver, and then you can EQ as needed. WinISD really needs to have excursion slopes added to the graphs so that so many people don't continue building subs that can't really achieve what is planned.

Thanks!

As far a beta vs alpha, I thought that alpha wasn't commonly used because it's so buggy. I installed it on 4 machines under 3 different OS and all I keep getting are Windows errors that won't go away until I reboot. Not fun. So I've been using an Excel sheet by John Kreskovsky to determine excursion limited SPL slopes.

Thanks. I checked it out and it's a nice tool, but it doesn't seem to show the slope for the excursion limit. Also, it's modeling the sealed box at 10% smaller than WinISD. Is either program considered more accurate?

That's the beauty of using multiple vents! The more vents, the more efficient the sound waves are filtered with less area. Model it and you will see.