Crossover Experts! Hole in 12dB/octave, (both sides), response? Help!! - diyAudio
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Old 15th July 2002, 04:29 AM   #1
Wizard of Kelts
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Default Crossover Experts! Hole in 12dB/octave, (both sides), response? Help!!

Years ago., the loudspeaker books were telling us that if you have a 12 dB/octave slope on both sides, there will be a hole in the response right at the crossover frequency.

Therefore, they recommended that the higher frequency speaker-either the midrange or the tweeter-reverse it's polarity when hooked up to a 12 dB/octave crossover.

I am told that this solution is little used today.

Two questions, then.

A) Is there in fact a hole in the middle of the response at the crossover frequency in a 12 dB?octave, (both sides) crossover?

B) If so, and they don't reverse the polarity of one of the speakers, then what methods do they use to deal with it?
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Old 15th July 2002, 10:16 AM   #2
yeti is offline yeti  Germany
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They use a Linkwitz-Riley Filter that is composed of two Butterworth filters in series with the same x-over frequency,so it gives -6db and -180 degrees phase.
On axis you'll get a flat response because the two drivers are in phase and add 3db.
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Old 15th July 2002, 12:32 PM   #3
f4ier is offline f4ier  Australia
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A) Yes.

B) I'm not a crossover expert, but I would imagine that the next best thing to reversing the tweeter's polarity is to introduce an approriate amount of delay to either driver. Delay network(s) or offsetting either driver can introduce the needed phase difference. I would imagine a delay somewhere around 180/(360*fc) is a good starting point. But this delay seems to introduce ripples around and above the crossover point, fc.

You could probably offset either driver. Personally, I'd rather reverse the tweeter's polarity

hth
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Old 15th July 2002, 12:57 PM   #4
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Yeti:

Thanks for the response. I should have mentioned that I was talking about passive crossover networks. With all the emphasis on active crossover networks today, I should have made that clear.

A) Is what you are talking about-2 Linkwitz-Riley networks in series-attainable in a passive crossover?

B) If the two drivers together add 3 dB, should we then tailor our crossover so that the respnse of each driver is 3 dB down at the crossover point? I know that seems obvious, but I thought I would make sure.

Finally, I would like to invite you to add your thoughts to the related thread here:
Positive Reinforcement When Driver Centers Are One Wavelength Apart At Crossover?

PS: Am going to Linkwitz's website right now.
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Old 15th July 2002, 01:12 PM   #5
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F4ier:

So there is a hole! Thought so.

Okay, suppose I wanted to dodge this by keeping the woofer a 12 dB/octave slope but making the tweeter:

A) A 6 dB/octave network. Does this eliminate the hole? If so, how many dB should the tweeter be at crossover?

B) An 18 dB/octave slope. Does this eliminate the hole? If so, how many dB should the tweeter be at crossover?

I have read some loudspeaker designers are less than enthusiastic about having one driver's slope even order and the other driver's slope odd order, but it seems to me a nice way to dodge a problem. Have you heard of any big problems with making one slope odd and the other even?

I fully realize that there is no such thing as a perfect passive crossover. If you want an advantage in one direction, you have to give up something else. Everything is an uneasy compromise.

I'm just looking for reasonably flat on-axis and off-axis response. I really don't care that much about a 1 dB ripple here or there. That is not really hearable to a normal person, and the drivers have ripples themselves.

Finally, I would appreciate any thoughts you might have on this related thread:
Positive Reinforcement When Driver Centers Are One Wavelength Apart At Crossover?
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Old 15th July 2002, 03:29 PM   #6
f4ier is offline f4ier  Australia
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A, B) The null would be reduced. But you'd really have to experiment with actual measurements rather than theoretical data to know for sure.

I'm not sure why it would be bad to have unequal order filter slopes, but if it gets the job done, why not Experienced builders might have more to say on this The example filter I applied to the DIY2000 FRD files can be seen at Crossover Simulator's site. As you can see, a 2nd-order LP filter with a 3rd-order HP filter produces a reasonably flat system response (though I should mention the actual woofer delay of 70uS was not considered in the above example filter set).

As much as I'd like to recommend xoversim, it is not yet finished. Have you tried Speaker Workshop for your current 2-way project?
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Old 15th July 2002, 04:48 PM   #7
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F4ier:

Thanks for the response. I eagerly await the release of Crossover Simulator.

I've tried to get Speaker Workshop going on my computer but it always seems to be more involved than I have time for. This might be a good time to give it another crack.

I have done some measurements that lead me to believe that there is positive reinforcement between woofer and tweeter when the centers of the drivers are one wavelength apart at the crossover frequency. The measurements did not outright prove it, but they did suggest it.

I have also read some mention of positive reinforcement at the wavelength and negative reinforcement at the half wavelength .

I did not see any mention in Crossover Simulator about the space between the centers of the drivers you are trying to cross over. Do you plan to take this phenomenon, (if it exists), into account?

I would think that might be important.
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Old 15th July 2002, 05:12 PM   #8
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Kelticwizard,

One aspect of the complete system that your overlooking is that the woofer response is attenuating at about 6db/octave near your proposed crossover point. This will add a 1st order slope and phase shift to any xover you use. So if you use a 2nd order, 12db/oct xover the woofer response will have a 3rd order, 18db/oct lowpass cutoff slope.

I'm not sure what this will do to the total response, but it won't be the theoretical response of the symetrical 2nd order butterworth filter.

Rodd Yamashita
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Old 15th July 2002, 06:16 PM   #9
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Quote:
Originally posted by kelticwizard

B) If the two drivers together add 3 dB, should we then tailor our crossover so that the respnse of each driver is 3 dB down at the crossover point? I know that seems obvious, but I thought I would make sure.
[/B]
This is already compensated for when you follow the crossover calculations. The crossover frequency is -3db.

Cheers, Dan
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Old 16th July 2002, 01:28 PM   #10
yeti is offline yeti  Germany
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A) It is posible to obtain passive Riley-Linkwitz filters by adding two identical butterworth filters,the resulting filter is a 4th order high or low pass.
B) The high and the low pass is down -6db at crossover frequency(because the 2 butterworthfilters they consist of have -3db each),one branch has +180 degree phase,the other -180 degrees,so you don't have to invert your tweeter.
2 times -6db add up to -3db because they are in phase,the fact that both drivers work in phase add another +3db's ,it's the ideal crossover for D'appolito speakers.
You'll get a maximum flat response on axis.
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