Phase and Delay  

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Mikekel
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30/05/2019 5:17 am  

This might be a silly question but I'm a noob so here we go anyway... Building a 2-way cross that consists of 2 mid woofers and a horn tweeter, I've got my freq/spl graph looking nice and flat +/- 2db from 200hz-20khz, the impedance is right where I want it but I just cant seem to align the tweeter phase with the woofers. My question is, is there an easy way to sort this out and given that my freq response is good does it actually matter all that much if my phase is out??? My tweeter is set back about 200mm on the Z-axis to allow for the depth of the horn and that seems to be the main reason for the phase issue, is there a way to introduce a delay to the woofers in a passive circuit??? Thanks for the help in advance...


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Speaker Builder
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30/05/2019 10:17 am  

Driver phasing is probably less of an issue than most people think it is.

The only time you really need to worry about phasing is in the crossover region where both bass and treble drivers are noticeably active at the same time. If the phase error is great enough in this small region of frequencies you will get a drop off in sound level, due to cancellation.  

Outside of this region it is far less of an issue simply because the frequencies are unrelated. For example: It is not reasonable to expect a 5khz tone to maintain a phase relationship with a 1khz tone because they are not harmonically related. 

To diminish the problem within the crossover region your best bet is to sharpen the transition between drivers. This is usually done with 2nd or 3rd order crossovers producing 12 and 18 db per octave slopes respectively. You can also manipulate the frequencies slightly, increasing or decreasing the low pass point for the woofer and/or the high pass point for the tweeter to minimize the effect.

Also keep in mind that the coils and capacitors within a crossover cause phase shifting of their own and it could well be that in some designs the tweeter is actually out of phase with the woofers because of it... simply try reversing the wires on the tweeter and see what happens.

Hope this helps....

 

 


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Mikekel
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31/05/2019 4:03 am  

Thank you Douglas, that's music to my ears (pun intended)... My freq response is nice throughout the entire bandwidth, I had to sharpen up the roll off on the tweeter as you suggested and invert the polarity, however my phase graph looks like a 5yo has taken to it with a crayon... The woofer has one nice line in the middle but the tweeter graph has about twenty lines. I've been playing around trying to fix it and align the phase at the cross point but nothing I'm doing seems to make any difference... Having said that, there is no evidence of any cancellation at the cross point so can I just forget about the phase and call it good??? 🤔 


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JonnyAudio
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31/05/2019 7:53 am  

I use a 3-Way Active Crossover on my system & have found no problems with the Bass to Mid to Treble responses, I have mono wired the active for the bass to get a bridged effect to the sound were the bass has increased a lot from the same amp sources, is that not the effect of bass timing being different in stereo bass ?

As for Mid / Tops after years of study, I have always found passive crossover to be weak (as Douglas Blake explains) & cause a lot of those missing, over-lapping frequencies, if you can use a 3-Way active Crossover will be able to tweak your system a lot better - but the choice is yours really ?

Hope my input help also !


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Speaker Builder
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31/05/2019 11:29 am  
Posted by: Mikekel

I've been playing around trying to fix it and align the phase at the cross point but nothing I'm doing seems to make any difference... Having said that, there is no evidence of any cancellation at the cross point so can I just forget about the phase and call it good???  

First... are you working with software like REW?  If you are the microphone could be picking up room echos so the tweeter phasing might not be nearly as bad as you think.  Phasing in speakers is a freakshow to begin with, room effects only make it worse.

This is where you let your ears be your guide. Set up a listening session with source music you are very familiar with. Listen quietly, where your ears are most discerning, just above conversational levels.  If the music sounds right... I'd say you've got 90% of what you need.  The rest might be fixable with speaker placement, rotation etc. in your listening room.

Quite frankly I wouldn't worry about phasing unless you are getting a drop off  or a swell near the crossover point.

Ideally the -3 db points for the woofer and tweeter should be the same frequency. That way the drivers add up to provide a flat response. If there is a phase issue short sweeps from half an octave below to half an octave above the crossover frequency will reveal it as a swell or a dip in output. At this point plus or minus 3db output won't make much audible difference, so don't let the perfectionist in you get carried away.

Of course there's always the "BBC Dip" which was inserted deliberately into some classic designs to minimize phasing issues... This is done by dropping the lows off a bit early and bringing the tweeter in a bit late to minimize the overlap between them. It leaves a slight dip, about 3 or 4 db, in output at the crossover frequency. Yes, it is an audible effect, but if managed correctly it won't produce any sense of lacking in the overall sound.

Again ... now that you've done all the metrics, it really is up to your ears... if it sounds right, it probably is right.

 


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Mikekel
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31/05/2019 6:30 pm  

Thank you for your comprehensive responses guys, your help is much appreciated 👍 


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Speaker Builder
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01/06/2019 10:06 pm  

Be sure to let us know how it turns out....


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bjaurelio
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24/08/2019 7:59 pm  

If you are using REW, you should be gating your measurements so they don't include reflections. It means less frequency resolution, but it's the only way to ensure you are capturing the minimum phase response of the speaker. Once you add reflections, it's no longer minimum phase. The only passe issue to worry about is the crossover region. As long as both drivers are have relatively parallel phase through the crossover region, they will sum properly.


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