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Old 4th May 2004, 06:24 PM   #1
MBK is offline MBK  Singapore
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Default crossover slopes other than 6dB/oct multiples?

Hi there,

do any of you have references pointing to crossovers with other slopes than 6 dB/oct and multiples thereof? I have a fullrange driver that decreases ca. 4 dB/oct on axis and would like to try a simple passive x-o to a tweeter. (Later on I plan to do this cleanly with active x-o and more standard slopes).
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Old 4th May 2004, 06:51 PM   #2
azira is offline azira  United States
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I don't think you're going to find an easy answer for this. But I don't think it's impossible either because there's always a digital dsp solution. It may be impossible in a passive solution though...

The 6db/oct magic number comes from the slope of the impedance rise of a cap/coil with respect to frequency.

Impedance wise, Caps are X = 1/jwc while coils are X = jwL. Since j and w are effectively not controllable, I think that in order to have a slope in betwen multiples of 6db/oct you would need a device that decreases capacitance with frequency or increases inductance with frequency or vice versa.
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Old 4th May 2004, 08:23 PM   #3
sreten is offline sreten  United Kingdom
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Years ago for a white noise to pink noise converter I built a
3dB/octave filter effective over the range 20Hz to 20KHz.

Its easily done with a series resistor and then a number of
parallel RC sections to ground, though much easier to work
out using a simulator rather than pen and paper.

I think I needed six RC sections for less than 0.5dB ripple.

Don't neglect BSC.

sreten.
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Old 4th May 2004, 08:58 PM   #4
Svante is offline Svante  Sweden
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Filters are built up of components whose impedances either go +6dB/oct (inductors) or -6dB/oct (capacitors). This means that any filter ultimately has a slope of n*6dB/oct, where n is an integer. In a transition region, the actual slope can be anything, and the pinking filter that sreten described is an extreme example on how to extend this transition region to the entire audible spectrum.
Simplified, the n is the order of the filter, and often synonymous with the number of Ls and Cs in the filter. This is why n is an integer. So, saying that a filter has a slope of eg 8 dB/oct might be true, but probably only for a small frequency region, and thus pretty useless IMO. Knowing that it is a second order filter would make me feel much better since it tells me something about the design. If I am interested in the details about the response, I'd like a response curve instead.
To return to your problem, it seems as if your speaker might need a dose of a "transition region" in a range. As sreten says, BSC should also be taken inte account. Maybe it is the BSC you see? To say anything about the details on this we would need to know more about the conditions for your measurements.

HTH
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Old 5th May 2004, 05:15 AM   #5
MBK is offline MBK  Singapore
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Actually in a way it is BSC.

The driver is a Diatone fullrange in dipole arrangement (open baffle). The baffle is 14" wide. The measurement was on-axis, in-room using Linkwitz's burst signals and his peak detector (DIY).

What I figure is that, in the lower range and above the baffle rolloff, I get a boost from random addition of front and rear waves. As F goes up, dispersion narrows, so that I am getting more and more the "real" monopole on-axis response because the phases don't add anymore.

So I just bought tweeters to improve on the radiation characteristics of the system. The Diatone by itself is great on-axis and images really well, however due to the narrowing dispersion it is certainly not power flat and that shows as a certain dullness off axis, and a lack of perceived dynamic punch. I thought of helping him out with a tweeter.

Now, if I do a classic crossover, I somehow negate the fullrange advantage, and in order to get a clean transition iI'd have to do it in a range where the Diatone's dispersion is still clean (say, 2 kHz). I am looking for a way to just add the lacking off axis radiation above 3-4 kHz while keeping the Diatone working fullrange. So that would mean, slowly phasing in the tweeter.

Another option I thought of is to simply add the tweeter off axis. Either top firing, or rear firing. That would ensure to make its output diffuse - if not achieving true off axis fill-in, it would help the power spectrum, and if I do it above 4 kHz, the effects on imaging and the lacking time alignment should lead to increased "spaciousness" without hurting coherence and imaging. It may just work by setting the correct x-o point, and to lower the tweeter level until we're out of the important frequency range, say, fix it until 10 or 12 kHz and let it fall where it may above that.

Or, I could sum the channels out of phase at line level, that gives me the reverberant component only, and use the tweeter to radiate just that (I have the tri-amp setup prepared already).

Or, I could add the tweeter on axis and put sort of an inverted phase plug in front of it to remove the on axis component.

Am I making my life too hard ?

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Old 5th May 2004, 06:07 AM   #6
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Hi MBK,


don't forget you have not only the BSC to keep in mind, but also -as you've already mentioned yourself- the radiation behaviour of your FR driver at the crossover frequency. Even at 2kHz, you will encounter a jump in the off-axis response due to highly different driver sizes (beaming of the FR, while the tweeter is happily radiating into 2pi w/ its own baffle step features...). So, even if on-axis everything looks neat, you might get into trouble off-axis. At what frequency the power response will jerk is actually dependent on the dipole response plus the 2 pi (or so) radiation of the tweeter. (As a matter of fact, you most certainly will get into trouble off-axis, in terms of what you measure - but this might nonetheless sound good.)

Also, watch out for a weird vertical interference pattern you might introduce by using filter slopes which are too flat (b/c the overlap will extend over a much too large frequency range); also the acoustical axis might change. This is especially critical if you go to really high frequencies. In turn, you can't go too low without compromising the tweeter (and the FR concept, of course). In the end, you might very well lose (much) more than you gain.

You could also try and play around with little, cone-shaped diffusors or acoustical lenses in front of your driver. But this will decrease on-axis SPL (and probably flatness of the response, too).

Cheers,
bk.
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Old 5th May 2004, 03:35 PM   #7
MBK is offline MBK  Singapore
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Hallo Brummkreisel,

actually the tweeter should be out of the baffle step problem range is crossed over > 2kHz. The dispersion needs to be addressed though. The Diatone is a 6.5 " driver and should be OK (omnidirectional) up to close to 2 kHz and due to the dipole cancellation effects off axis, even higher. I think it would be perfectly workable to use a steep x-o at 2-3kHz, however it would be nice if I had the best of both worlds, fullrange dipole with filled-in off axis response...
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Old 5th May 2004, 03:41 PM   #8
tiroth is offline tiroth  United States
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In some situations it should also be possible to use asymmetric crossovers to achieve intermediate slopes over a certain frequency range. I've thought of doing this in situations where a 1st order crossover is desirable, but you want to push the response down a bit more later on to avoid a resonance 4 octaves later.

The math would require some work, maybe best done empirically/by simulation.
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Old 5th May 2004, 03:46 PM   #9
MBK is offline MBK  Singapore
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One thought I had was to use a notch filter with an insanely wide Q. That should give a variety of HP/LP slopes. Just not to forget to add a second filter on the "unused side" of the notch.
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Old 5th May 2004, 03:54 PM   #10
markp is offline markp  United States
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Several filter types do not have a 6Db/oct roll-off per order. Bessels do not, for example. They are close, but not 6Db/per on higher orders.
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