Quote:
Originally Posted by LTD02 /t/1518239/questions-on-an-unusual-variant-of-the-martysub#post_24413436
just a heads up...check your first port resonance in winisd. that can create a hiccup in the frequency response if too close to the passband.
Thank you very much for this question, because due to the lack of response to my answer I posted to LTD02's question, I was forced to go digging for an answer. From my research, I realized that a first port resonance of 119.53 Hz is too close to the passband, since I plan to cross over to my subs at 80 Hz. And it definitely would be a problem if I decided that I needed to cross over at 120 Hz for some reason. So, this sent me doing more research and re-reading of posts that I initially just kind of skimmed over.
Upon doing this, I realized that my numbers were off quite a bit.
Starting with the original MartySub design
here , I noticed that LTD02 was figuring his port with the braces as just one port, not 3 separate like I was. It didn't change my numbers much, but it did change them some. Then I noticed how short his port design was compared to what I was thinking. I realized that I was shooting for too low of a tuning point, and that I don't necessarily have to keep the port velocity below WinISD's recommended 17 m/s. Not to say that this speaker won't handle the lower tune, but as several more experienced have posted, due to end correction, WinISD is figuring the tuning too high compared to real world results. I'm not looking for high SPL single digit output, but I'd like for some good output in the teens, and 16-18 will be fine. For goodness sake, like I stated earlier, my last sub was a single 10" Velodyne that I last used over 2 years ago, and now I'm starting over with these beasts. TWO 18" SUBS!!!! I must be nuts.
I'm sure I'll enjoy them at that tune just fine. I also found this
thread where jfreeman420 is building subs that are very similar dimensions to mine. The dimensions are only off just a little, and driver and port placement will be different, but I was able to glean some good info from there as well. So, anyway, on to the numbers.
Sub design 2.0
. I decided I will adjust the exterior dimensions just a smidge to 21" tall x 28" deep x 40" long. For anyone checking my numbers, I am figuring everything with .75" thick wood. With a double baffle, and a 1.5" recess of the baffle for a grill that puts me at 10.8 cu. ft. before the port, driver or any bracing. The port will have 2 supports in it, so I will end up with a 2.5" x 18" x 33.75" port (these are the internal dimensions of the port, actual port length will be 36.75"). I'm figuring the port will eat up 1.2 cu. ft., so with the port, driver, and bracing, I'm figuring that puts me at about 9 cu. ft. WinISD figures that to be tuned to 17.8 Hz with a first port resonance of 184 Hz, and that puts the port velocity at 18.7 m/s with a 600W signal. Hopefully these numbers will look much better. If someone sees that I'm off, please tell me. I'd rather know now, than after I've got wood cut.
