[dynamowhum]

I have a rolling TV cart I built for my 65" DLP. It was made from 3/4" oak plywood and the inside dimensions are 21X27X57. I figure I can line this with some mdf and put some bracing in it. It ought to be pretty stout that way. I want to use the maelstrom 18. I will start playing with WinIsd to see what configuration I want to go with. This will have to be front firing. Gotta come up with a grill to keep grand baby away or maybe just hide it behine some grill clothe. Figuring on using the Behringer for the amp. Your opinions and insights would be welcome.
This would help me achieve my lust for a big sub and not change the foot print in my theater.

 

[Mike P.]

Lining all sides with MDF would give internal dimensions of 18"x24"x54", about 13.5 cu.ft. Going sealed (yellow) would be a net volume of 13 cu.ft., ported (green) would be about 12.5 cu.ft. With the ported you'd be over 120 db at 18 hz with room gain.

 

[dynamowhum]

Thanks Mike last night I finally did the WinIsd tutorial and I pretty much got what you did there. I should have been more clear I guess and explained that the enterior dimensions of the current box is what I posted so it is a little bigger. I plan on an 8" port tuned to 14Hz. I figure maybe a 2nd order filter at 10 hertz should keep me well below the 3" excursion limit of this driver. If I have made a mistake pleae cue me in.

 

[Mike P.]

What net volume did you model with?

 

[dynamowhum]

Mike I used the dimensions of 19.5X25.5X55.5". In the light of day I should have done 18.75X25.5X55.5 because I will double up on the front baffle. So the gross box is something like 14.97ft^3 according to EVC. I didn't know the driver volumne so I guessed .5 ft^3. Now minus bracing and port I came to 12.97 ft^3.

 

[Mike P.]

Your numbers work, I came up with 13 cu.ft. net also. Here's the new Maelstrom-X Gen II modeling. 13 cu. ft. tuned to 14 hz, 1800 watts input power, HPF at 12 hz, 8 inch diameter port 47 inches long.

 

[dynamowhum]

Thanks Mike that's what I came up with as well. Real good to have confirmation on first time designing a sub.
Posted via Mobile Device

 

[dynamowhum]

I have a port question. I will need to put a 90 degree bend to put my port facing the front. I can't go out the sides as the TV cart is in a cubby hole with about 1 1/2" clearance. So anyway I need to put an elbow in an 8" tube. What do I use?
I went and modeled a slotted port but everthing I tried kept giving me to much cone excursion at 25Hz that I didn't have with the round port. I went online and priced an 8" pvc elbow and that was $68 plus shipping. That is a price I don't want to pay. So now I am wondering if I shouldn't just cut a 1 ft hole in the side of my cubby and align the port to it. What do you guys think?

 

[Mike P.]

I went and modeled a slotted port but everything I tried kept giving me to much cone excursion at 25Hz that I didn't have with the round port.

If the slot port has the same surface area as the round port then there should be no difference.

 

[dynamowhum]

That is the problem Mike the cabinet is not deep enough for that to happen. So I think I will just do the big hole in the side of the cubby deal. It will not be noticable from the front.

 

[Mike P.]

If the cabinet isn't deep enough then the slot port turns 90 degrees and continues up the back wall until the proper length is reached.

I'm not sure what you can use for a 8 inch elbow other then PVC, which is expensive.

 

[dynamowhum]

Ah thanks Mike I didn't realize you could do that with a slot. Should be easy enough. Once again thanks very much.

 

[dynamowhum]

I wonder if something is wrong with WinIsd Pro. Based on what was stated about diameter of slot versus circular I put in a 5X10 slot and still got a 1.408 cone excursion with the slot. On the circular which works out to a 50.27 cu^2 I get a 1.2 cone excursion. Maybe it's just sleep depravation on my part LOL.

 

[Mike P.]

Port air speed, cone excursion, and SPL are identical on my modeling. Are you using the same units of measurement on both your models?

 

[dynamowhum]

Hey Mike I had time to check my inputs today and everything was the same. I reloaded WinIsd and now both round and slotted ports show excursion over max. I just thought of something what should I be putting in for ohm load for series resistance? I had been inputting 4 is this right?

 

[Mike P.]

I reloaded WinIsd and now both round and slotted ports show excursion over max.

With what power input and what HPF? Series resistance is 3.8 ohms, you can download the Generation II file here:

http://www.hometheatershack.com/forums/diy-subwoofers/14410-downloadable-winisd-pro-files.html

 

[dynamowhum]

Power input 1800 and no HPF. Put in a 4th order Linkwitz-Riley and it tamed it down to a 32.63 excursion. This seems to be cutting it kind of close. Do you reccommend less than that?
A couple of other things. I am showing no front air port velocity ( scale 0 to 100 ft per second) and no port gain.
And finally my little 8" quick crete tube measures 7.5". Are there any true 8" concrete form tubes that measure 8".
Sorry for all the questions I just want to get it right before I start building. Thanks again.

 

[vili]

33mm max excursion is fine. The Gen I models maxed out at 35mm and I believe Kevin let out the suspension a little bit and the GenII will go to like 38-40mm

 

[Mike P.]

Power input 1800 and no HPF. Put in a 4th order Linkwitz-Riley and it tamed it down to a 32.63 excursion. This seems to be cutting it kind of close. Do you reccommend less than that?
A couple of other things. I am showing no front air port velocity ( scale 0 to 100 ft per second) and no port gain.
And finally my little 8" quick crete tube measures 7.5". Are there any true 8" concrete form tubes that measure 8".
Sorry for all the questions I just want to get it right before I start building. Thanks again.

Sorry I missed this post. The excursion is fine.

Ported subs use the rear port velocity and gain when you are modeling. The front port velocity and gain is for modeling 6th order bandpass subs.

Some brands ship their tubes one inside the other to save on shipping cost. The outside one will be 8 inches diameter, then progressively smaller so they fit one inside the other, and they are all considered 8 inch. Take a tape measure with you and verify the one you want is 8 inches in diameter.

 

[dynamowhum]

Thanks again Mike.