I ended up doing a little different design, partially by choice and partially because I did something(s) stoopid
. This project will now be known as the "Corner Loaded Base Snorkel".
I am usually very careful and precise with the engineering side of a project. First I drew up a spread sheet parameterized by the important geometries:
1) the width of the baffle.
2) the depth of the sides perpendicular to the baffel.
3) the height of the enclosure
4) the number of braces
5) the length of the port
6) the radius of the port (both inner and outer)
I plugged in numbers till I found the tuning I wanted, ~25hz with a 4" port (flared at both ends). The front baffle is about 14 inches and the height is 49". Then I drew it in SketchUp and made a cut list.
Then I got busy at work and life. When I finally got back to it I pulled up the spreadsheet and headed to the store. I bought 4" PVC. This was my first mistake. I had modeled ABS which is ~3.8 diameter on the inside. PVC is 4"
inside diameter. To make matters worse I had hard-coded the outer diameter in a formula in the spreadsheet... I don't know why. But of course I did not notice this and thus miscalculated the length of port needed and started cutting.
This design was pretty complex to manufacture. I ended up doing 3/8" rabbit's on all edges that join at 90 degrees to increase glue area. The 135 degree joints I used biscuits. I did two dadoed cross braces, one of which serves as the flare for the port inside. The front baffle is 1.5" thick where the driver is mounted.
Here are some pictures gluing on the sides:
Next up is the port. I went back to double check my spreadsheet... and found the hard-coded cell
. This is the point where my project got it's name
. I needed an extra 4.75" of port length. I wasn't about to start over at this point so I decided instead of terminating the port on the top of the sub I now needed to extend the port above the enclosure!
Here are some pictures gluing up the port. The cross-braces are not glued in, I just inserted them in the dadoes so the port attached to the two cross-braces would fit perfectly. The bottom of the PVC was not exactly square so I cut a very slight grove to accept the end of the port and cover the slop. I used gorilla glue for the port-to-cross-brace joint.
Can anyone see my next mistake? See the clamping pressure on the top of the top brace? That pulled the cross brace slightly out of vertical. And therefore it would no-longer fit perfectly in the dadoed back pieces. Oh well, a chisel and more gorilla glue
.
Next up I sealed the inside of the cabinet with Zinsser BIN. This is to keep the inside from absorbing moisture and telegraphing joints to the paint job. I had never used this stuff before but it is
really nice. I won't use sanding sealer again.
Some pictures before the final glue-up:
Here you can see the 3/4" round over on the inside port end:
And here you see some plywood I glued to accept the screws for the driver. I don't trust screws in MDF:
And one more for good measure:
I glued it up about 2 weeks ago and have been listening to it since. I plan on going back and building a small extension to hide the snorkel and provide a round over for port on the outside.
I tested it with an SPL meeter and it gets plenty loud down low. I can however hear port noise, but it is not as loud as the front door behind the wall rattling
.
Honestly my initial impression is that I prefer my 4ft^3 ported Mistral. On the CSS when playing 20Hz I hear 40Hz +. I don't remember the second harmonic even being audible on the Mistral. Note that this is not a final conclusion and it is not a fair comparison. The Mistral is mated with
much better mains and is a larger driver.
One day when it warms up I will get this beast painted and post more pictures.
Thanks again Shack for the driver!
