OB system... Questions before beginning...

[Godzilla]

Hi all! I am beginning a new project. It will be my first OB system. I have been listening to BIBs for a few years and feel like trying something different.

Using this chart as a reference:

http://members.myactv.net/~je205d/obsize-vs-freq.jpg

Does it apply to drivers of different diameters? For example, i see OB systems comprising of small Fostex 3 and 4 inch drivers and a 15 inch driver as well as 8 inch drivers with 15 inch. In general, will the larger 8 inch drivers sound fuller on an OB than smaller drivers? Does the chart consider Qts of the driver or is it just a basic estimation of what a driver might do on a given baffle size?

I am leaning towards 8 inch drivers like the Betsy or even the Pioneer B20 to get my bearings on how an OB system can sound. I am concerned about Qts but maybe overly so if the 15 inch woofer will cover approx 150Hz down. Any comments would be appreciated.

Godzilla

[MJK]

Godzilla,

Please, analyze and design your system. The MathCad worksheets will do everything for you including the passive crossover design. You can vary driver locations, baffle size/shape, and include even a little of the room interaction. In a couple of hours you can optimize the OB system and build it once to get the best performance possible.

[just a guy]

The chart you linked to has nothing to do with drivers at all. It simply shows how wide a baffle needs to be to support a certain frequency, below which you need either a high q driver or bsc to remain flat. On it's own, that chart is pretty useless unless you already have a solid idea of how a driver with any given fs and qts will act. That chart is here. EDIT - you have to click the following link to get to the homepage and then click "7 Lautsprecherwahl" to get to the chart. http://dipolplus.de/frameset.htm But software is easier to use and more accurate.

If you already own MJK's worksheets I would heartily recommend that you use them. If you don't own them and don't want to for any reason you can get most of the functionality of the OB worksheet from "XLBaffle" and "The Edge". Both programs are super easy to use but you have to use both of them and somehow average and splice the results together, since XLBaffle can't compute offset drivers and won't do system response for multiple drivers. This is how I was doing it until recently.

Now that MJK has changed the licensing policy to a one time fee I've finally purchased it and I'm currently learning to use it. Now that I'm getting a feel for it I'm starting to like it alot and I highly recommend it.

[MJK]

Not trying to beat a dead horse, but take a look.

http://www.quarter-wave.com/Back_Doo...ve_8_22_09.pdf

I am admittedly biased. The worksheet handles the open baffle generated hump, two drivers, and allows you to input a mix of 1st to 4th order passive crossovers to optimize the full range resposne of your two way OB speaker system while taking into account the nearby room boundaries. I do not know of any other software that does all of these things.

[ra7]

Hello godzilla,

i'm planning to build some OB too... I have the AN 8s and the Jordan 92s... keep us informed on how it pans out.

Martin,

Which woofer would you recommend of the lot that have passed under you so far?

I mean which one is will give you most bang for buck?

Thanks.

[MJK]

I can easily recommend the Eminence Alpha 15A, the quality is high and the price is reasonable. In an OB it can get you near 40 Hz, in an H frame you can get close to 30 Hz. Need more efficiency then use two per side.

I have looked at some of the Goldwood 15" (and used the 18" in my H frame) woofers with Qts values near 1.0 and they also look good. The Goldwood drivers are probably cheaper but may not have the same quality as the Eminence Alpha 15A.

My strong recommendation is to look for a woofer with a Qts value greater than 0.9 so you can count on close to flat SPL response down to fs. Crossover between 100 and 200 Hz. Don't spend a lot of money on a fancy woofer, spend the money you save on a better full range or midrange driver.

That is my recipe,

[Nelson Pass]

Originally Posted by Godzilla

I am leaning towards 8 inch drivers like the Betsy or even the Pioneer B20 to get my bearings on how an OB system can sound. I am concerned about Qts but maybe overly so if the 15 inch woofer will cover approx 150Hz down. Any comments would be appreciated.

Bigger is better, within limits. I favor baffle widths up
to 36 inches, and heights of 60 inches. The B20 actually has
a suprising amount of bottom end in such a baffle, although
it does need a tweeter of some sort from 10 KHz up.

[Nelson Pass]

Originally Posted by MJK

My strong recommendation is to look for a woofer with a Qts value greater than 0.9 so you can count on close to flat SPL response down to fs. Crossover between 100 and 200 Hz. Don't spend a lot of money on a fancy woofer, spend the money you save on a better full range or midrange driver.

With all due respect, I have tried the Alpha, Beta, and Delta
in my 36" X 60" OB, and I don't like the Alpha nearly as
much as the Beta, with or without EQ. To me, the Alpha
comes across as a little boomy and with less punch.

Of course, that's just me....

Here's my near field curves on the three.

[human.bin]

Mr.Pass, MJK, you both use very large baffle on your OBs. now it seems from more sources, like Linkwitz and Gainphile here on the forum (Multi-way guy doing tries and tries) that a large baffle on the mid-highs will quite certainly make the sound less natural...

any comments? have you ever tried smaller baffles following the size of the FR not the one of the Woofer (baffles within the thumb-rule gainphile and others are pointing out)?

is it an issue only for tweeters?

[MJK]

Nelson,

I think the bass response has a lot to do with the baffle size, I use smaller baffles so the Alpha 15A bass is a little more rolled off. My favorite baffle size is 20" to 24" wide by 40" tall. I have not noticed any boom, I might find the Beta 15A to be a little bass light on my baffle. Integrating the woofer properties and the baffle size is key to dialing in the bass.