Lilmike's Cinema F-20 - Page 48

This horn was not exactly designed to work that way. K-horns require a corner to work properly, the F-20 does not, it performs as "advertised" in a groundplane setting.

Certainly, loading it into a corner will make an SPL difference due to the presence of the boundaries, but it will not play significantly lower.

The other thing to remember here is that the effective low corner is not just a result of the horn. I get to pick a low corner based on horn length, but it is important to remember that the driver also dictates how low a horn can play by virtue of its displacement limits. I am pushing the drivers I recommend in this design about as far as they can go and still maintain decent power handling and a reasonable margin of safety.

Well, you can juggle the widths a bit. There are limits though. The driver loading varies with width, which messes with the response.

I have modeled narrower ones, but I am not sure I modeled wider ones. I could without too much trouble.

Biggest issue is that this plan is for 3/4" thick ply, and you have 1/2".

My experiences with large drivers and thin plywood (even fully braced) have taught me not to go there, so for me, drivers larger than 12" get 3/4" thick ply.

The only 15-loaded tapped horn I designed with 1/2" material literally tore itself apart when run at war volume. It was built like all the others, cut to the same tolerances, and braced properly. It was a PA sub, not a home theater sub, and did not have to cope with nearly 2" peak to peak excursion. Still failed rather spectacularly.

I'll try to run a model for you tonight if I have the time. Files are all on the home PC.

If you're proficient with Hornresp, simply take each of the S-values (1, 2, 3, 4, and 5), divide them by 18.5 and multiply by 24 or whatever your baffle width is. Then - do the same for Vrc. That will get you in the ballpark

This approach to the model won't reflect the increase in area due to the thinner internal parts. Full bracing may offset some of that increase though.

Not yet.

Been busy trying to get my Jeep back on the road, sorry. Will try to run things ASAP.

Edit:

Took a peek. Looks like it will work, sort of. I am not a fan of the acoustic loading at 23.5" (stock THT width). I guess - the easiest way for me to put it is that I personally would not waste the time and plywood. LTD's comments were spot on.



Dark trace is the 23.5" width.

Honestly - you have the wrong thickness of ply for the F-20, but it is the proper thickness for the THT. The wood you have is already cut to an appropriate width for the THT. If it were me, I'd just build a second THT. The differences between the F-20 and THT are not that significant, and what you'd be building would not really be an F-20 anyhow, so it would not be a valid comparison at all, and your efforts would largely be wasted.
Edited by lilmike - 12/5/12 at 1:30pm

Thanks. Looks like the hard work is done. I am driving it again for the first time in a month plus.

There is a LOT more to a horn design than the frequency response. I am not happy with the acoustic loading of the F-20 at 23.5" internally.

My direct experience says otherwise.

No.

Best of luck with whatever you choose to build. If it is not an F-20 as I presented in the plans I prepared, please don't call it one.

To be fair - the design I am referring to that failed was mine, no one else's. I'm not bashing the THT or any of Mr. Fitzmaurice's designs at all, and recommended that the poster build one rather than the F-20 because he had the proper material for the THT.

Mr. Fitzmaurice's designs are his to worry about, not mine. He specifies 1/2" material for his own reasons, and lots of people have great results with them.

I'm not second-guessing that. I just shared that I have seen fully braced cabinets constructed with 1/2" material fail when loaded with a modest 15. In my design's case, the failure resulted from the baffle separating from the side of the cabinet. Fully bracing the internal structure to itself was not effective in preventing this. Bracing the sides (as the mouth of the THT is, at least in the plans I bought a LONG time ago) may have been helpful, but might not have prevented the failure, and would have precluded installing the driver.

Keep in mind, this happened with a relatively modest driver, with an xmax of 5 mm, that has a relatively low mms (at least as far as a 15"), that was loaded in a cabinet tuned in the mid 30s. The forces at work with larger-throw drivers that have higher mms values increase a LOT. I will not design a cabinet for a 12 or 15 that is made of 1/2" material, which is consistent with the vast majority of commercial horn and tapped horn subwoofer cabinets out there. I have no problem constructing smaller cabinets out of lighter material for use with a smaller driver, I just don't like the idea of using lighter material with a heavier driver, even when fully braced.

All I am trying to do is share my experiences, and the rationale behind the choices I made when I came up with this design and recommendations I make. Though feelings should not be ignored, at least in my book, "experience" trumps "feeling" any day, which is why I felt obligated to share mine. I've also "disassembled" quite a few unbraced and fully braced cabinets, the braced ones were not that much more difficult to flatten for hauling to the recycler, though to be honest - neither style was easy to flatten.

Experiences have also taught me (subsequent to this design) that proper and complete bracing is simply not optional in a sub horn. These cabinets simply can not be built rigidly enough without it, even if I used steel plate.

We all live and learn I guess.

Not trying to ruffle feathers or anything, just sharing what I have learned through some "educational" experiences I've had along the way.

Would I design/build the F-20 or T-6 with bracing today? Yes.