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deep bass porting question

1731 Views 8 Replies 3 Participants Last post by  Mike P.
Hello all, I am new here but not new to speaker and subwoofer building. My reference books are the two Radio Shack manuals from the 1990s - Building Speaker Systems and Advanced Speaker Systems. Those, plus available on-line calculators, have allowed me to design optimum enclosures for a few subwoofer projects and a few speaker projects. I am happy with all of them.

So my question. If I have a nice 12" sub driver rated at 300 watts with an Fo of around 22 Hz, an appropriate Qts, and a healthy Xmax, is there a benefit to making the ported enclosure of sufficient volume and port size/length to get a tuning below the driver's Fo? Maybe down to say 18 Hz as a starting point? Can that approach actually give bass extension below the free-air resonance of the driver?

Where all this is going, I am inspired by the SVS cylindrical subs, I have a nice cast-frame driver from a Boston Acoustics BT1100, and am aware of the various sub projects using a Sonotube or similar cylinder with end caps as the enclosure. I have a local source for a 12-foot section of 16" diameter Sonotube, so I may end up having to make 3 or 4 subwoofers!

Anyway, that is my basic question for those of you experienced in ported subs, regarding deep bass extension, any wisdom you can share will be appreciated. I am more interested in experiences than with speculation. Mike
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Welcome to HTS!

Yes, you can absolutely tune below Fs. The most commonly sought alignment for home theater is the large low tuned (LLT) alignment, see LLT Explained, which requires big cabinet volumes and very low tuning. There are other alignments like 3dB and 6dB bass shelf that get a similar response that stays fairly uniform and goes low. You should easily be able to make a sonotube cabinet big enough for a 12" to get into those alignments.

Your 12" driver sounds okay, how much xmax is it exactly? Can you provide complete T/S parameters?
Thanks for the comments and the encouragement. No, unfortunately I don't have any T-S parameters nor do I have the equipment to measure them. My "rough" plan is to do some testing with test tones and SPL meter, using an adjustable enclosure, to try different volumes and different port lengths, to narrow down the field of play, then make finer adjustments with the semi-finished enclosure. Mike
If you are up for a DIY project you can rig up your own woofer tester with a few simple parts using REW. I have a woofer tester so I haven't used REW, but either essentially do the same thing. Download REW, hit F1 for help, then search for: impedance measurement.
Thanks fusseli, the link you gave me "LLT Explained" has a wealth of information and answered many of the questions I had. It all makes perfect sense. Embedded was a Sonotube calculator which I downloaded and made a few "what if" passes. It looks like tuning a 3.5 cu ft enclosure to the 15 to 18 Hz range won't be very difficult at all. One question not answered (in my mind at least) is the appropriate port diameter for such an enclosure using a 12" driver. A diameter of 8" was mentioned for an 18" driver, of 6" diameter for a 15" driver, so in that geometric progression I suppose a 4" diameter would work for a 12". I was hoping to use a 3" diameter, to keep the length moderate, but that may result in high air velocities and chuffing near the tuned frequency .

Anyone have a general suggestion for best compromise for a 12" driver tuned to the 15 to 18 Hz range?
Here are some general numbers by modeling a different 12" driver, which would generally resemble yours. With a 12" driver in 3.5 cuft tuned to 18Hz a 4" port would be safe for maybe 100-200W of power. For 300-400W you'd want a 5" port. With only 100W you might still get chuffing with a 3" port so I wouldn't recommend one that small.
I have a nice cast-frame driver from a Boston Acoustics BT1100
That driver was designed for and comes from a ported cabinet with a tuning frequency around 30 hz. I predict it won't have the excursion capabilities to produce any substantial SPL below 20 hz. I'm very curious as to what results you will get with this project.

Thanks Mike P. for the additional information on that driver. After breaking it in I put it temporarily into a 1.8 cu ft sealed box to see what it would do and it had decent output down to 20 Hz, which is the limit of my test CD. So I am hopeful! :)

If not, then buying a specific 12" driver that WILL work lower is the next option. Where can I find recommendations for such? I would like to stay under $200 for the driver if I go that route.
The Dayton UM-12, Dayton Titanic MK 4, TC sounds Epic 12, all from Parts Express, would have the Xmax capabilities for strong 20 hz output in a ported design. All are under $200.
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