Overlapping driver freq range for Baffle step - problems? - diyAudio
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Old 11th November 2004, 03:00 PM   #1
dvdwmth is offline dvdwmth  Canada
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Default Overlapping driver freq range for Baffle step - problems?

I'm using TB w871s with a 12" thats supposed to have a usable freq response up to around 1200 hz. I run it up to about 500hz 1st order in order to provide some baffle step compensation. I let the TB driver run full range.

This combination seems to work quite well. The sound is quite abit fuller and warmer then the tb is alone and everything sounds natural. Imaging is good.

What I'm wondering about though is if overlapping the two drivers can introduce problems that might not be obvious to the ear alone. I tried a baffle step circuit on another speaker that was hooked up to a small amp and it had a negative effect on the loudness capabilities of the speaker. So why don't more speakers deal with baffle step the way I've described above. Is there a drawback that I haven't thought of?
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Old 11th November 2004, 03:09 PM   #2
RJ is offline RJ  United States
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Good question!!!!
I not try only overlapping, I also try underlapping.
The only problem I see here is the phasing of the speakers and the different cone material & transient response at the frequency's they're blending with. Aurally I have no way of telling.
If it sounds better - GREAT!!!!
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Old 11th November 2004, 03:27 PM   #3
markp is offline markp  United States
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Imaging can suffer greatly from too much overlap of different sized drivers. The 12" is much slower than the tb in the top part of the overlap near 500hz. It will give you a warm, tubby sound that is no very accurate(but might sound 'nice'). The higher is freq the overlap the worse, you ideally want a pointsource not two sources far apart.
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Old 11th November 2004, 03:35 PM   #4
dvdwmth is offline dvdwmth  Canada
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why do you think transient response might be negatively affected.

I have never really wrapped my head around phase. Why would this be an issue.

To me it sounds good, but I dont have a better reference so it might not sound as good as it could.
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Old 11th November 2004, 03:44 PM   #5
dvdwmth is offline dvdwmth  Canada
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Quote:
Originally posted by markp
Imaging can suffer greatly from too much overlap of different sized drivers. The 12" is much slower than the tb in the top part of the overlap near 500hz. It will give you a warm, tubby sound that is no very accurate(but might sound 'nice'). The higher is freq the overlap the worse, you ideally want a pointsource not two sources far apart.

Aren't first order XOs supposed to have the best imaging. Why is this the case if driver overlap messes up imaging. You'd think that a higher order XO would be better.
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Old 11th November 2004, 03:55 PM   #6
markp is offline markp  United States
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Quote:
Originally posted by dvdwmth



Aren't first order XOs supposed to have the best imaging. Why is this the case if driver overlap messes up imaging. You'd think that a higher order XO would be better.
He is overlapping the tb and the 12" up to 500hz so there are two sources of sound from 500hz on down. If the tb was also crossed in at 500hz the overlap would be much less. The 12" should not be used that high as it is too heavy and slow and also the large diameter is not good for dispertion at higher freqs. 1st order xovers in general have fewer phase problems than higher order xovers. Phase shift is 90degrees per order of xover, so 3rd order is 270degrees phase shifted. If that is crossed to a 2nd order there is 90degrees between the two drivers(assuming that they were in phase as the xover freq without a filter on them).
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Old 15th November 2004, 10:36 PM   #7
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Hi

What you have described isn't all that uncommon in older designs, and may sound good.

If you want to test the effectiveness of the first order filter, just run a test CD like Stereophile II and use a Radio Shack sound meter (corrected, if possible), and unhook each driver in turn while playing test tones. Start with the 12" and run tones up to 1000 Hz to see if the driver is rolling off at the desired rate.
With the TB, reverse the procedure, start at the higher freqs and work downward, measuring as you go.

Of course, there is no substitute for real measurements.

Ordinarily, a 6dB filter is not enough to attenuate the unwanted break-up at frequencies higher in the response of most 12", unless they are well damped to begin with. Could be that you got lucky, but it is unlikely.

Also, as the TB will continue trying to reproduce a signal well below the filter, and a 6dB first order cut may not provide much protection if driven hard, or attenuate sufficiently to prevent overlap. When the two drivers overlap, you can get some cancellation from the comb-filtering that occurs, creating a rough response.

Most people would use a sealed box tuned to the approximate x-over frequency which would help damp the response mechanically below the tuning. Another way is to mount the TB unit on a small open baffle which would serve the same purpose, which is to roll off the low end output.

Another problem, besides phase, is the dispersion at the crossover, which can effect imaging. With the 12", the sound is gradually retreating up toward the center of the cone with increased frequency, narrowing the field as it goes. Ideally, you want the two drivers to match dispersion angles at the crossover.

Just a few thoughts... BTW, the bass speed thing is mostly BS. How well the two drivers integrate at the crossover is the determining factor, which allows the high frequency information to remain intact to perserve bass "speed" along with the low.
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