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Old 4th March 2011, 03:59 PM   #1
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Default Partial Dipol Idea

Hi Guys!

Here is an idea of a partial dipole arrangement, which my brother wants to realize within the next few month.
It will consist of a 25cm (10") woofer in a ported cabinet, two 17cm (6,5") mid-bass drivers in an open baffle and a conventional 25mm (1") dome tweeter. All drivers have 8R0 nominal impedance.
So, actually only the midrange will be dipole, while the low and high frequencies will be covered by a conventional approach.

The thing is, we want to use two midrange drivers to get a bigger imaging and soundstage and to cope with the higher-than-usual cone excursion due to the lack of a sealed cabinet.

The crossover should be as simple as possible. We want to start with a simple 1st order crossover without (!) a highpass filter for the midrange drivers. This would lead us to:
- 1 inductor for the 10" woofer
- 1 inductor for the two 6,5" mids
- 1 capacitor for the tweeter

Therefore it wouldn't be a good idea to just wire the mids in parallel, because together with the impedance of the woofer at low frequencies this would result in a quite low impedance in the bass region of approx. 2R7. The solution is to connect the mids in series, that would give us 16R for the mids and together with the woofer about 5R3 in the bass region, which is quite ok.

Two questions about this design:

1. Let's assume all three drivers have the same spl of 90dB/W/m. How would the spl of the midrange driver vary relatively to the other drivers when wired in series and working in an open baffle as dipoles?

2. Has anyone experiences with a design like this? How predictable are the results? (For instance: How critical is room placement?)

Thanks a lot!
Martin
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Old 4th March 2011, 04:04 PM   #2
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Btw., these are the drivers:

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Old 9th March 2011, 01:21 PM   #3
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The general rule of thumb would yeild a sensativity of -6dB when connecting two drivers in series. (+6dB in parallel). In reality, your net loss may be somewhat less than that, but it will still be a loss in some sensativity, plus remember to also factor-in the additional early dipole roll-off, which should only impact the mids in your example.

I use a Morel MDT-28 which is identical to your tweeter selection and am very happy with it in a single point source/dipole orientation. Instead of (2) 6.5" mids, I use a pro sound 8" coaxial coupled to the morel tweeter. I believe that has certain advantages as well. I also use a 10" woofer in a dipole configuration, not sealed. Typically pro sound drivers will be more sensative, and be able to keep up with your tweeter with less compensation.

I would re-think your desire to use a single coil on the woofer. In reality you will probably want a steeper roll off there. I have experimented with many crossover slopes where dipoles are concerned, and always gravitate back to a 12dB or 24dB per octave slope on the low end. Think about using a seperate plate amp for your low end, with a built-in steep adjustable filter.

Ed
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