Lilmike's Cinema F-20

Here is the first of my Cinema Horn series. This model is pretty good sized, at just over 20 cubic feet, but has a manageable footprint of 4 square feet. Dimensions are 29.375" wide, 20" deep, and 60" tall. It cuts out of 2-1/4 4'X8' sheets of 3/4" ply, no bracing required. I designed the enclosure to stand as a tower or lay on its side. The driver orientation keeps the cone in a generally vertical plane in these two orientations.



I designed this enclosure for the MFW 15, which is an American-made (Eminence) driver that was used in a ported sub a while back. This is a healthy driver, and in fact, has a larger voice coil than the venerable Dayton DVC.

This is an offset-driver front-loaded horn. This is not a complicated cabinet. Though it is big, it is a simpler build than the Insubnia, there are less pieces. There are 5 internal panels and four external ones, all the same width. Two sides complete the package. That is only 11 boards total. Did I mention no bracing? Working alone, I was able to cut all the wood and make the baffle in one evening, then assemble the cabinet the following evening. Final assembly and some sanding and cleanup took me a few more hours before testing today.



Here are the results of today's testing. Thanks to maxmercy and his buddy for dropping by and helping out. It was a lot of fun, we had beautiful weather, enjoyed some excellent beer, and measured a great sub.



Measured with REW 5.0, with a UCA-202 and a Lilmike mic. First sweep was set at a drive level of 2.00 volts into a 4.00 ohm load resistor, each sweep higher is +5 dB from the previous, so the final sweep is 25 dB higher than where we started, or essentially at the limit of my 300 watt amp, and past the thermal limit of the driver. I'm not sure if the compression is the limit of the amp, driver, or both. We limited the start to 20 Hz for the highest level sweep. I was well within the limits of my soundcard and mic at these levels. No mic calibration was used, the soundcard was calibrated via a loopback.

Yeah, it is flat. I measured +/- 3 dB from ~20 Hz to ~100 Hz. This is a front-loaded offset-driver horn, in a groundplane setting.

Though the noise floor was high due to the highways and industrial activities near me, we also did a little THD experiment. I just raised the drive level to compensate.....

Here is 15 Hz. As expected, distortion is pretty high, and SPL is low.



Looks a lot better at 20 Hz. Well below 10 %, and the SPL is 105.9 dB.



Look what happens at 21 Hz, same drive level...3% THD, and 107.6 dB



25 Hz = 4% at 106.7 dB



Back down to 21 Hz, but more power...113 dB, 21 Hz, 5% THD.



Erich can help with drivers, I have a complete set of dimensioned plans and cutlists done already (currently only in 2D - not spiffy like radman12 did with the Anarchy). I also have impedance measurements as well as the model to post. More to come.

Edit:

Here is the impedance and phase plot - nevermind it says SPL - that is a REW thing... As usual, I measured this with my Woofer Tester II, I'm still working on proper test leads for the REW approach.



Here is a comparison. Modeled is red, measured is blue.



Hmmm, I might have a leak yet - that low peak could be a bit higher. I have a couple ideas about where it might be. I'll report back if/when I find anything.

Here is the requested group delay plot:



And the SPL and unwrapped phase:



Edit:

I've attached the model, as well as the sketchup diagrams I used and an optimized cutlist. I am currently working on some assembly instructions

Edit 4-25 - I updated the cutlist to match the instructions I am working on

Edit 4-28 Based on review comments like this:

"The instructions on the F20 are rather concise and easy to understand. If you can't build one from these instructions you might want to get on down to Best Buy and get your HTIB on."

and

"Okay, you suck! Not really, I actually suck. Your directions even have 3d renderings."

I figured it was OK to attach some step by step assembly directions to the post..... Somehow or another - the instructions are a 12-step program....

5-12-2011 Updated Zip with an additional Sketchup showing the flange details, fixed a few typos in the instructions

Edit 11-1-2012:

Since the direct links broke, here is the 5X5 Cutsheet post:

 

http://www.avsforum.com/t/1329971/lilmikes-cinema-f-20/420#post_20501440

 

Edit 11-1-2012:

Bracing discussion starts here if you wish to use it. Again, it appears that direct linking to attachments in different posts seems to be broken, so the post is linked.:

 

http://www.avsforum.com/t/1329971/lilmikes-cinema-f-20/960#post_21160972
 

If you wish to retro-fit bracing into one that is already built, these suggestions should work too, as you can access both ends of each brace by opening the panel, removing the driver, and working through the chamber. All braces are a cut-to-fit item.

 

 

 

cinef20.txt 0.443359375k . file

 

 

 

 

 

Cinema F-20 Cutlist by Mike Piechowski.pdf 6.3662109375k . file

 

 

 

 

 

Cinema F-20.zip 48.2529296875k . file

 

 

 

 

 

5-12 Cinema F-20 Instructions.zip 397.798828125k . file

 

 

 

 

 

MFW Specs.pdf 126.7783203125k . file
Edited by lilmike - 11/1/12 at 10:11pm

At over two sheets of 3/4" ply, I doubt it. Shipping costs and logistics will be prohibitive. The finished cabinet is a 2-man lift.

This one is not that hard to cut out and build. The layout really only keys off of four points, and it assembles easily if you do things in the right order.

Full plans, cutsheets and all details to make your own are coming. I want to make sure I have things clear enough for people to follow along, and I want to get a basic set of assembly instructions done, because if you do things out of order, you get to undo things.....go ahead...ask.....

I will try to get the plans posted tonight.

I ran out of amp before I hit the driver's limits, so a 300 watt amp is a great match. Highpass at 20.

Tacoma, WA

I have both, but they're on the laptop. I'll try to get that plotted up tonight, as well as the impedance data.

Think so, but that's not even all of them.

My nice TIG is not in the pic...

No Pre-pro in the system when I measure.

Source path:
Laptop running REW ----UCA202 Out----BASH 300----Sub

Signal Path:
Lilmike Mic-----UCA202 In-----Laptop running REW

Setup:
Connect source as described, except connect 4-ohm load resistor at end of speaker wires from amp.
Connect DVM (I use a TrueRMS DVM) across load resistor
Set amp gain to Max
Start REW, open generator
Set level to -40 dB
Select sine wave, set frequency to 60 Hz
Measure voltage, lower amp gain until you're at 2.00 volts across the load resistor
Stop generator
Connect speaker in place of resistor
Calibrate SPL at -40 dB signal level
Run measurement (may get low signal warning, but not always, data typically looks OK)
Increase REW signal level by 5 dB
Run next measurement
Repeat until you hit the amp limit or the driver limit

The Lilmike Mic makes this set of sweeps super easy, as it does not require any range settings, just set it up once and go.

My idea was to use a pair as a 10-foot wide "stage". The mouth opening can be moved if needed (sides or bottom, just change the cutsheet).

It can also get stretched a bit without running out of driver capability, but that's for another thread.

Not all horns have to be tapped....

This particular driver performs much better in a front-loaded horn, so I conjured up one for it.

Think I modded it to be flat to 20. Can't remember, might still be stock.