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Old 22nd December 2014, 12:29 PM   #1
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Default can someone help me choose a crossover frequency

hey guys, im trying to design a crossover for a 6" SB17MFC35-4 and a SB29RDNC-C000-4

I have been doing lots of research on crossover but i just can decide on a frequency and type of crossover. from what iv worked out a 2000hz 4th order butterworth would be the best, but someone else is telling me a 4th order butterworth at 4000hz would be better, but i just can comprehend why the best i could work out is that its because our ears are less sensitive above 3000hz.

cheers guys.
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Old 22nd December 2014, 01:29 PM   #2
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Hi Mauds,

i would aim for 2k ...2,3k effective acoustical crossover frequency.

But when going passive 4th order is way too high IMO even for practical reasons.

At the lower XO frequencies you will have a more uniform dispersion, without the woofer beaming too much, as you have to expect it with a higher XO like 4k (way too high).

Vibrational modes will also be less a problem, with the woofer crossed lower.

About 2 octaves above Fs of the tweeter (600Hz) should be still "sane" when using a 2nd order Filter.

That tweeter will need at least some electromagetic damping as Qms is quite high:

If you use an L-Pad, a rather low resistor in parallel (e.g. 6Ohms) can provide at least some EMK-damping even in the region below the highpass filter's cutoff.

Alternatively you can compensate the resonant peak using an LCR series resonator in parallel to the tweeter.

Compensation of the impedance to be almost flat for both drivers in the "wider crossover region" will improve your filter.

Important:
You should optimize baffle shape and mounting positions of both drivers using e.g. "Edge" from Tolvan Data or any other well behaved "baffle diffraction software".


Edit: A higher crossover frequency is only recommended, if want to use that speaker (2-Way?) to be driven at higher levels: Then a higher crossover frequency would reduce voice coil travel and temperature for the tweeter.

But if you aim for low coloration in a 2-way, which seems the case when looking at your driver choices, i would go the "low fc" way as mentioned above.



Cheers
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Last edited by LineArray; 22nd December 2014 at 01:52 PM.
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Old 22nd December 2014, 01:44 PM   #3
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Listen to the perfomance of tweeter and bass at different x-over points and orders?
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Old 22nd December 2014, 01:50 PM   #4
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... one can do that, surely.

But unecessary IMO, because this is kind of "standard configuration" stuff.

Lower fc will yield the better speaker, it is as simple as that.

And: One has to design for a certain fc from the beginning, which also affects the baffle design and positions of the drivers.

So "designing" is (by far...) better than "trying" at that point IMO.




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Old 22nd December 2014, 03:48 PM   #5
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Try 2.2 KHz with simple crossover network like this one.
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Old 22nd December 2014, 04:14 PM   #6
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Hi tuantran,


as a simple starting point maybe ...


But this crossover will provide

- too less electrical damping for the tweeter

- will be influenced in slope too much by the impedance peak of the tweeter

- has too less steepness in the cutoff region for the woofer

Target should be 12dB/octave acoustical rolloff for both drivers IMO
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Last edited by LineArray; 22nd December 2014 at 04:18 PM.
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Old 22nd December 2014, 04:26 PM   #7
jReave is offline jReave  Canada
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Both of those drivers are are fairly easy to work with.

Chart 1: For the SB17MFC35-4, it looks like the -3dB point at 45* off-axis will be somewhere at about 2500Hz, so that should be your upper limit for the woofer in terms of directivity.

Chart 2: Looking at the 8ohm version which should have pretty similar performance, harmonic distortion of all kinds remains low all the way up to 3000Hz, so that's fine too.

Chart 3: Cone resonances also start to increase above 3000Hz, so overall I would stay below about 2500Hz or so for the woofer.

Chart 4: In terms of the tweeter, 3rd order harmonic distortion starts to increase below about 2500Hz although at 2000Hz, it's still below -70dB which is still fantastic. So given the low Fs, I'd look at crossing somewhere around 2000-2500Hz.

Personally, I go for 4th order LR acoustic slopes, which you may be able to achieve with 2nd order electrical but you may need to go to 3rd order electrical on one or both to do that and to keep phase in good alignment. I've had good success in simulations with that tweeter with values in the neighborhood of 8-10uF/.2mH/15-25uF depending on baffle size and driver placement.

What are you using to design the xo btw?
Attached Images
File Type: gif SB17.GIF (52.9 KB, 217 views)
File Type: gif SBAcoustics-SB17MFC35-8-HD.gif (22.7 KB, 216 views)
File Type: gif SBAcoustics-SB17MFC35-8-CSD.gif (24.7 KB, 215 views)
File Type: gif SBAcoustics_SB29RDCN-C000-4-HD.gif (17.3 KB, 214 views)
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Old 22nd December 2014, 04:33 PM   #8
jReave is offline jReave  Canada
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Give this a read. It might help you.

Jeff Bís Thoughts on Passive Crossover Design
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Old 22nd December 2014, 04:53 PM   #9
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Hi Jreave,

we don't seem to disagree on this ... LR 4 with -6dB crossover point
seems fine. Also there is agreement concerning the crossover frequency.

Nevertheless this is my "minimum Option" in contrast to the pic posted above (post#5) ...

http://www.sbacoustics.com/files/551...35-4-chart.gif

http://www.sbacoustics.com/index.php...29rdnc-c000-4/
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File Type: jpg XO_2W_01.jpg (464.4 KB, 74 views)
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Last edited by LineArray; 22nd December 2014 at 05:17 PM.
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Old 22nd December 2014, 05:16 PM   #10
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In all honesty, you really can't guess about the crossover. It begins as a measurement issue. Much later you can season to taste.

The first order of business is to get/measure the 1) on axis frequency response of the components 2) phase response, 3) non-linearities (2nd and 3rd harmonic distortion are good candidates) and 4) dispersion at the ballpark frequency of the crossover. Folks will weight these factors differently, but this is a good start.

Last edited by WithTarragon; 22nd December 2014 at 05:40 PM. Reason: ...
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