Need help with a big stage sub

[Logain842]

Hi everyone,
I'm a beginner to the DIY audio realm but I have a background in electrical physics and computer science so I thought I'd take a stab.

My application is perhaps a little different than many I've read about on these forums. I play in a steel drum band and I'd like to add a little low end power to our stage show. We currently have a pair of nice EV 15" powered subs. They sound great but unfortunately they don't pack much of a punch in crowded venues.

I've looked at some of the larger 18" and dual 18" cabinets from Mackie and JBL but I'd like to try and save some money by building my own. I'm after that classic "W-Bin" sound from the 70's and after. I figured I'd pick up some cheap Eminence Sigma Pro 18A's or Omega Pro 18A's. I'd mount a single driver in a 6th order bandpass configured box. The bottom line is I'm not really after good sound quality (that comes from the 15" subs), but bass that will really shake the audience.

I used WinISD Pro to model some boxes but it has a flaw when it comes to 6th order BP boxes. I did the calculations by hand and wound up with a rear volume of 163.20L (5.763 cu. ft.) and a front volume of 92.70L (3.274 cu. ft.). My rear tuning freq. is 28Hz and the front is 65.69Hz. Using 3/4" baltic birch ply I have outer box dimensions of 36"x22.58"x24" (HxWxD); The front chamber height is 11.75" and the rear is 21.25".

So I'm ignorant and I have a few questions for the experianced DIYers out there.
1) Does this sound like a good plan? Am I off my rocker?
2) I know the enclosure is all based on volume but how much off is too much? If my dimenensions are off so that I wind up .1 cu. ft. short compared to a whole cu. ft. short what will I notice? Just how sensitive are the volumetric mesurments?
3) Is 6th order BP a good configuration for this setting?
4) 4" ports or 6" ports? I've read that larger reduces power loss but can add noise?
5) The manufacturer recommends a 35" HPF to prevent cone overexcursion. How important is this? Where can I find a simple, cheap, passive HPF?
6) How important is it to figure in braces and driver/port volume for total volume calculations?
7) Should I make these active? Are their any affordable plate amps with good wattages out there?

I appreciate the help a great deal. I'm looking to get this project started ASAP to use in the summer tour season.
Thanks!

[RobWells]

Hi,

There's lots of talk about w bins and pro subs over at speakerplans forum

Cheers,

Rob

[infinia]

Hi
Your calc's are probably OK. But forget BandPass for PA use, they go low at the expense of SPL's. Think of moving lots of air, not aiming for specifically for extra low frequencies. In a BP all the air has to move through tuning ports. Stick with cabinets with tried and true results for prosound esp. for a first time project.

[Logain842]

Awesome, I appreciate the advice.

[paulspencer]

1) Does this sound like a good plan? Am I off my rocker?

Not at all. DIY is a good idea. Done right you should get a better result with less money.

2) I know the enclosure is all based on volume but how much off is too much?

Do a simulation in WinISD and get it how you want, then change the numbers a little, and this will give you an idea.

3) Is 6th order BP a good configuration for this setting?

A bandpass has potential here, but they are sensitive to misalignment, far more so than a vented box. As you know, WinISD pro doesn't do them properly. I would stay away from this. You risk a stuff up here, and without willingness to measure and tweak til it's right, I'd leave it.

4) 4" ports or 6" ports? I've read that larger reduces power loss but can add noise?

Larger diameter means longer port to get the tuning required, but at the same time lower max vent velocity. You want to keep the velocity down or they will chuff. Also pay some attention to aerodynamics of the vents. A vent with flared ends can have a higher velocity before turbulence becomes a problem.

5) The manufacturer recommends a 35" HPF to prevent cone overexcursion. How important is this? Where can I find a simple, cheap, passive HPF?

35" HPF?! huh?
Perhaps you mean a high pass filter (low cut) with a corner frequency of 35 Hz?
Pro amps usually have low cut built in. With a vented sub, the cone excursion goes out of control below tuning, so you want to cut out any low bass that might be going through the system. If you did need to add this on, then you would be looking for some component that has an active HP filter, but you should have this in your power amp anyway.

6) How important is it to figure in braces and driver/port volume for total volume calculations?

Always work out NET volume as this is what is simulated. How important it is to calculate this properly in the final box design depends on how sensitive your box is to misalignment, and the design in particular. For a vented box, I'd make a quick guess for the driver and bracing, that should take you all of 1 minute to work out. If you have not that much bracing and a big vented box, say 300L then the 5L or so that bracing and driver take up is not worth worrying about.

7) Should I make these active? Are their any affordable plate amps with good wattages out there?

Yes active.
Don't bother with plate amps. A big pro amp with plenty of power is what you want. They have a low cut built in and are designed for abuse. They are economical and more flexible, allowing bridging and lower impedances. They have more authority as an AB comparison will clearly show. I use a pro amp even just for home hifi use - Behringer Europower EP2500. For about AU $600ish it puts out 2.4kw bridged into 4 ohms or 2 x 1200w (2ohms) or 2 x 650w into 4 ohms. For about the same price here in Australia, I could get two plate amps with only 350w into a 4 ohm load and the only real advantage is convenience.

You have a few choices here.

1) Follow some plans like www.speakerplans.de or the lab sub
2) Design your own vented sub (pretty easy)
3) Design a bass horn to blow them away (more difficult to design and build, bigger but more impressive)

You might also look at www.billfitzmaurice.com
search the forum for the lab sub
and consider Wayne Parham's 12pi bass horn at Pi Speakers