[Viridian]

Hi Everyone,

I haven't posted much here but I sure have learned a lot from reading about different people's projects. I have been using vintage 12" coaxes for my main baffles and have been loving the sound. But.... I need more bass. I've pretty much been experimenting empirically, trying differing baffle shapes and sizes. Without measuring ability I know I could easily be chasing my tail. Add my general ignorance to the mix and I have the potential to have all kinds of fun!

So, I've read a bit on the different OB bass options and understand that a few of them cause there to be some efficiency loss to get lower response. Ultimately I would like to accomplish 2 things:
1. integrate passively with my 103dB/W main drivers @ approximately 135Hz XO.
2. provide a platform for my main baffles to sit on.

#2 is probably not a big deal. I see the advantage being that with some absorbent material I can acoustically isolate the bass from the main baffles while still being in basically the same plane.

#1 is probably not possible, but I would like to at least try before biamping with active XO. The vintage woofers i have are Pioneer 15" Alnicos which came from a sealed multiway box rated 99dB/W and they should be good to 30Hz. I don't have a rating for the woofers themselves, so do I lose efficiency in taking them out of a sealed box? Are they still 99dB in OB @ 1kHz? I understand that beneath Fequal I'm losing sensitivity fast.

Here is what I was thinking. On a straight OB two woofers(if rated 99dB/W for one) would be 102dB/W wired parallel for 4ohm load. Factor in an electrical gain of 3dB/W(4ohm woofer load vs. 8ohm coax load. Is that right?) and we have a woofer 105dB/2W for every watt my coaxes use at 103dB/W. Am I missing anything?

I was initially wanting to use the GR Research W frame plans adapted for 15" drivers. Compact and not too tall. The plans are here:
http://www.gr-research.com/pdf/obsub.pdf

So, what do you think would work best for my application? H, U or W frame? Or something else? I remember reading Martin King's papers saying that H frame loses you quite a bit of sensitivity to get more low end.

If I have to, I have perhaps 2 other options I can think of. I have another pair of vintage 12" drivers rated 103dB/W @ 16ohm. And a pair of 8ohm 8" vintage FR's rated 100dB/W. I mention this in case it might enable me to accomplish the passive integration with my woofers.

I was just about to go outside to start cutting the pieces for the W frame but thought that I really should ask the folks here for some input.

So, can I be helped?

Thanks,
Erik

[Viridian]

Pics of the woofers, I have 4 total:

[mcgsxr]

Any ability to measure the specs of the drivers?

Would be good to know FS, QTS and XMAX in order to optimize a design, I would think.

[Viridian]

Darn it, no. I am unable to do measurements. That makes it pretty much shooting in the dark, I know. I am guessing just looking at them, pretty low Qts and low xMax. Fs should be 35-40Hz or lower. If I utilize Graham Maynard's TBass, he recommends both low Qts and Lower Xmax in his OB bass implementations. And the Tbass gives another 6dB near resonance if memory serves, by flattening the impedance peak. Of course that means going active as I would need a pretty stout sand amp at that point to drive the woofers.

[Piek]

Hi,

I would estimate around 85dB/1W/1m for two 15" below 100Hz in a 40cm Open Baffle.
The numbers written on Loudspeakers of this era are often just marketing, there is no way these drivers will do 30Hz with more than 90dB (not at all) in a resonably sized sealed enclosure when measured correctly.

You can measure tsp with ARTA (Limp) look at the manual, it is not that complicated.

My Advice would be: read the manual, get some measurement gear, go active.

Regards, Micha

[Viridian]

Thanks for the reply Micha. Well, that is a little disheartening. I'm obviously not qualified to know at this point. You are right, I need to get some measurement software.

But.... My 12" coaxes are rated very efficient and from the Fr graph I can see a natural fall off of response around 100Hz. So, in my 50 cm L baffle(not counting the back leg) I only use 6dB bass boost and it seems to balance the music. I estimate that I am getting usable 40's Hz out of them And I can start to shake the walls with 2.5 watts. So, that is what makes me wonder about using these huge magnet Alnico 15"s. And I think that Pioneer didn't play tricks like Coral and others in measuring from only 0.5M for 1watt for their sensitivity ratings.

Anyway... I'll probably just make a test baffle to include 2 woofers and a coax per side. (I love these coaxes, they sound so big and have such slam.)

But... I was rereading MJK's article on U and H frames and in the last paragraph he says: "A few concepts not explored in this study are the impact of an unsymmetric H frame and tapered or expanding U or H frame geometries."

Now that seems like something interesting to look into further. Perhaps you can get the best of both worlds with this kind of geometry. Lower bass response and maintaining SPL. Almost like a short horn.

Erik

I did read Linkwitz and what he says about monopole vs dipole below Fequal. Monopole needs to move 4X's more air for every halving of frequency below Fequal and that Dipole needed to move 8X's more air to keep flat response. Pretty crazy.