Couple Simple Dipole Question...

[santiu]

I'm building a pair of H-Frame dipole woofer with some budget 10" drivers I picked up.

From pictures I've seen it looks like the wires pass through the baffle. I just want to make sure okay to drill a small hole in the baffle to pass wires through.

Also, I'm wondering about the active EQ. I'm using drivers that have a fairly low Qts. Linktwitz mentions that for the Peerless 830500 (which also has a low Qts) you need extra equalization beyond the 6db/oct but doesn't mention what. Is it on his site somewhere, or am I supposed to buy the MT1 PCB & documentation to find out? Do any of you out there know what sort of EQ I might need.

thanks a bunch

[Davey]

1. Yes, you can drill a small hole to pass wires through.

2. There's no way to answer your second question until you identify the Qts and response of your particular driver. The Q-correction equalization for the 830500 is 8Hz-80Hz (20db gain.) This equalizes the Q to approximately 0.5 from 0.2. If your drivers Qts is similar then you're probably in the ball park.

Cheers,

Davey.

[santiu]

Is that 20db independent of frequency?

Here's the driver I'm using:
Fs 26.6 hz
Re 3.2 ohm
Qts 0.23
Qes 0.24
Qms 5.7
Vas 2.2 ft3
Sd 346 cm2
Xmax 13 mm (p-p)

Thanks again for the help!

[Davey]

No, it's not independent of frequency. The 8-80Hz I mentioned for the 830500 is specific to that driver. Yours will be different (higher in frequency.) You'd need to measure to verify.

The 10" driver you listed with Xmax of 6.5mm will have limited output capability in a dipole configuration relative to some other 10" drivers that have twice that much. You're going to use up that Xmax very quickly when you apply both Q-correction and dipole correction to the system. You SPL capability will be pretty low.

I take it you're aware of this, and this is a get-your-feet-wet project?

Cheers,

Davey.

[santiu]

Yeah, I realize the limited output. But I got these 4 drivers for $100 shipped to my door, so I figured I'd give it a shot just to hear what this dipole bass is all about. I tend to listen at fairly low levels anyways.

WHOA... never mind on that X-max number though. When I bought them, the person told me 12mm xmax, and I assumed that was p-p. But I'm just reading the manual now and it says the xmax is 2.4cm! So i guess 12mm is just one way! Yippee, extra travel I've attached some pictures of the driver for your viewing pleasure.

Also, I when I said frequency dependent. What I meant to ask is, is that 20db just a constant gain applied, or does it has some sort of slope (sorry, I'm still learning about EQs, so I'm a bit confused). Or is it 20db constant gain on top of the 6db/octave standard

Thanks again

[santiu]

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[santiu]

one last one with the rubber magnet cover removed.

[exipnos]

where did you get those drivers? website? link?

[Davey]

Santiu,

The 20db gain is a 6db/octave slope from a (6db/octave equals 20db/decade) 8-80Hz shelving filter. The 8Hz and 80Hz define the 3db points where the shelf begins and ends. (At 80Hz the response would be +3db and at 8Hz the response would be +17db.)

The dipole correction will also be a 6db/octave shelving filter, but it may not "line up" over the same frequency range you need for the Q correction. This will depend upon the shape/size of the H-baffle you create. A nominal value for baffles similar to the Linkwitz-sized ones would be 20Hz-320Hz which is a four octave range which means the 6db shelving filter would have a gain of 24db.

To implement this you'd build two shelving filters one 8-80 (20db gain) and one 20-320 (24db gain) and cascade them together to achieve the final result. At low frequencies you'd be boosting 44db, and somewhere in the middle of those two filters the slope would be 12db/octave...you can see where the very large excursion requirements come from.

The surround on those drivers looks fairly small to me.
I would hook those up in free-air initially and play a 20-30Hz tone through them at excursions near the rated Xmax to see if they make any aerodynamic noise. Drivers that are acceptable for dipole usage are very quiet with this test.

Cheers,

Davey.

[santiu]

exipnos, I bought these from an individual, so sorry, I don't have a source for you. They are made by NEXT in germany, which I believe is a company owned by FIAMM. Here's some more details on their products: http://www.fiamm.com/en/3/prod.php/22;224 (too bad the guy didn't have any of the AL woofers. Those frames look nice and open)

Davey, thanks again for all your help. Yeah, you are right, the frames look a bit constricting. Luckily, with only $100 invested in drivers, I'm not worried if I get less than optimal results. I guess I'll just deal with having limited excursion (and consequently levels) due to any aerodynamic noise I might get. Hopefully, I can get some non-ugly sound out of them at 30.87hz.

Hopefully I can get these things built before the end of the year (things just always get so busy around the holidays).

Anybody have any suggestions for free or cheap speaker measuring software for Macs (preferably something that runs under OS X)? I think I want to build the microphone listed on Linkwitz site and try to measure the response I'm getting with these uber cheap dipole woofers.

Thanks again for all your help I'll keep you all posted as I make progress