time allign or not?

[dallaire]

I'm sure that I have asked a variation of this question before but I was wondering if there is a benefit to allign driver voice coils vertically on the baffle if one is using a 24db per octave electronic constant voltage crossover? I think if a read things correct, that things would only be in phase at the cross frequency and not at any other frequencies, if this is correct, than is there anything to be gained by alligning drivers on baffle???
Thanks,
dallaire

[Conrad Hoffman]

I use a 24dB active crossover, and find great benefit from keeping the tweeters back around the plane of the midrange drivers. I have separate boxes, so it's easy to slide them around. I don't know if it's "time alignment" or just how the lobes are directed, but I consider it a useful tuning tool and don't bother putting a name to it.

[Iain McNeill]

I think it is more important to compensate the crossover for each drivers magnitude and phase response. The phase error due to using a driver near it's low frequency roll off (highpass) is much more significant / perceptible (in my mind at least) than the error due to aligning acoustic centers.

[dallaire]

Iain, I have not read anything about this type of phase error. When you say stay away from crossing at the frequencies near the low end roll of, how close are you talking? using a 24db per octave slope? and then would this phase problem you speak of not effect most "two way" designs out there??? I mean they are all crossing over at pretty much the limits of the two drivers right? this is where a three way setup I think is so nice, as long as "I" am not using and designing a passive crossover, I could never get it right, from what I have read, it hard enough for the pro designers!
dallaire

[Iain McNeill]

A 24dB/octave slope means that both drivers are contributing to the combined acoustic output over a 2 octave band. When a driver is 25dB down, it's contribution to the total output is less than 1dB.

Over this 2 octave band, each driver has to acoustically output the correct magnitude & phase response to properly combine. The acoustic output is the sum of the magnitude & phase responses of both the electrical filter and the driver in cabinet and if the driver-in-cabinet response isn't flat magnitude and 0° phase then the electrical filter isn't defining the acoustical output.

I think this is why a lot of 2- and 3- ways with off-the-shelf crossovers sound so-so.

From my simulations, the driver-in-cabinet phase response due to the high-pass roll off causes more significant problems. One way of mitigating this is to bias the crossover frequency up a bit but the best way is to design the filter to complement the driver/cabinet.

As you say, this can be hard when all you have are R, L & C's.

What are you using for your XO?

[bzfcocon]

Originally posted by Iain McNeill
I think it is more important to compensate the crossover for each drivers magnitude and phase response.

I'll second on that.
Blind "time-alignment" disregarding the phase relationships between drivers is useless - that's why "generic" crossovers like active with "book" slopes just won't work.

People usually think of drivers as having linear phase, but, as already pointed out, roll-offs have associated phase shifts that need to be considered.

[forr]

Hi Dallaire,

---benefit to allign driver voice coils vertically on the baffle if one is using a 24db per octave electronic constant voltage crossover?---

The theory behind crossovers supposes all drivers to be verticaly aligned. Your question should be : "What do I lose in my drivers are not strictly aligned ?"

In his book, Dickason shows response variations for offsets of 1" and 2" at 1000 Hz for an LR 24 : about 0.25 dB and 1.25 dB.

With digital crossover, it is easy to nullify the offset.


About the resonance of the high-passed driver, a long time ago, Marshall Leach wrote an AES article about its influence on the overall response. I have been intrigued that nobody seems to have done further work on the subject so I decided to do simulations myself. I found it was quite easy. I published some results in other threads but they did not retain much attention.

If you provide
- the frequency, kind and order of crossover (here LR24),
- the current vertical offset and the resonance frequency and Q of the high-passed driver,
I can do a simulation to determine what response variations can be expected.

[F1 FAN]

Interestingly Linkwitz seems to think time alignment is important .http://www.linkwitzlab.com/filters.htm#4 Although his statement is in regards to active crossovers it would equally apply to passives.

[dallaire]

Well guy's, I am using the "Marchand XM1 crossovers" I have twelve position rotary switches for tweet and mid level control. power supply is also Phil's design. I have chose Peerless six inch woofer HDS Nomex, four inch HDS PPB mids/woofers, and Scan Speak 9500 tweeter. I was thinking of perhaps 350hz to mid and 2500-3000 to tweeter. Nothing is built yet, still in design phase, all this input is wonderful!!!
Thanks,
vince

[F1 FAN]

Those drivers and active xovers should produce stellar results.


I using an active 3way with LR4 slopes at 300 and 2 k.