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Multi-Way Conventional loudspeakers with crossovers

Crossover Design - When to use driver offset? (Dz)
Crossover Design - When to use driver offset? (Dz)
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Old 31st July 2018, 05:44 PM   #1
PedroDeMio is offline PedroDeMio  Brazil
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Question Crossover Design - When to use driver offset? (Dz)

Hi guys

Pretty simple question, when I need to change the Z location of one of the drivers in a 2 way crossover?

For the crossover I'm currently working the measurements resulted in a 33 mm offset between the midbass and tweeter at the crossover frequency

What doesn't make sense to me is that since the microphone and speaker are completely still across all measurements, wouldn't this offset already be included in the phase measurement?

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Old 31st July 2018, 09:11 PM   #2
wolf_teeth is offline wolf_teeth  United States
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If you are measuring on one axis, you can optimize for that axis without the Z offset. If you take independent axis measurements, and want to look at off-axis responses, then you have to use the X/Y/Z offsets.

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Wolf
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Old 31st July 2018, 11:05 PM   #3
Tromperie is offline Tromperie  Australia
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Quote:
when I need to change the Z location of one of the drivers in a 2 way crossover
As long as you securely hold your head in a vice, driver offset is worth it, otherwise if you move at all, it all changes.
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Old 31st July 2018, 11:57 PM   #4
wintermute is online now wintermute  Australia
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Crossover Design - When to use driver offset? (Dz)
Whether or not the offset is in the measurements or not depends on the measurement method/software.

You either need to do dual channel measurements or have a way of locking time zero and do all measurements with the mic in the same positon to get measurements with the offsets (and hence phase data) baked in.

Tony.
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Old 1st August 2018, 01:42 AM   #5
eriksquires is offline eriksquires  United States
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Quote:
What doesn't make sense to me is that since the microphone and speaker are completely still across all measurements, wouldn't this offset already be included in the phase measurement?
It is, when you play both at the same time, the interference patterns are horrible.

OK that was kind of a joke. The problem is that phase is relative. So if you measure a tweeter in an anechoic room at 1 meter, and again at 3, the phase should be identical (excluding variations due to room interactions). There is no relative distance in the measurement with most tools.

So we use interferometry to measure the relative distance between them to a very high resolution. NOW we can measure relative phase, and proper crossover design.



Best,

E
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Old 1st August 2018, 01:47 AM   #6
PedroDeMio is offline PedroDeMio  Brazil
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Thanks

Yes, I have measured off axis at 10 intervals

But, even if I consider only the on axis response, there is a bit of difference between including the offset or not

With offset (the crossover was optimized using the offset):

Click the image to open in full size.

Without offset:

Click the image to open in full size.
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Old 1st August 2018, 02:51 AM   #7
eriksquires is offline eriksquires  United States
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Invert a driver in your sims, and see.
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Old 1st August 2018, 02:56 AM   #8
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Crossover Design - When to use driver offset? (Dz)
Which program did you use to do the measurements? Did you follow Kimmo's recommended procedure using dual channel measurements?

If you followed his procedure then the Z offset should already be included, and should NOT be put into the driver z offset. If you for instance converted all of the measurements to minimum phase, then you would need to put in the X Y and Z offsets.

If you need to accrurately work out the Z offset then Jeff Bagbies procedure should give it to you. https://www.google.com/url?sa=t&rct=...Xj6hirNa7YYlU9

edt: actually if you measured each driver on axis (doing the dual channel) then the Y offset should also be entered (and X offset if the drivers are not all in line vertically), if you measured all drivers on the same axis (eg the tweeter) using dual channel, then NO offsets should be entered.

edit2: The best way to check is to do the measurements and also take measurements with a simple crossover in place (same setup) then check with overlays whether your simulated response matches up with the measured response of that simple crossover. If they don't (within the tolerances expected of the components) then there is something not right in your sim inputs or your measurements.

Tony.
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Last edited by wintermute; 1st August 2018 at 03:02 AM. Reason: add comment on X and Y axis
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Old 1st August 2018, 03:30 AM   #9
PedroDeMio is offline PedroDeMio  Brazil
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Quote:
Originally Posted by eriksquires View Post
Invert a driver in your sims, and see.
Yeah, I had tried that, without the offset I get the deep notch, with I get a notch but way shallower

Quote:
Originally Posted by wintermute View Post
Which program did you use to do the measurements? Did you follow Kimmo's recommended procedure using dual channel measurements?

If you followed his procedure then the Z offset should already be included, and should NOT be put into the driver z offset. If you for instance converted all of the measurements to minimum phase, then you would need to put in the X Y and Z offsets.

If you need to accrurately work out the Z offset then Jeff Bagbies procedure should give it to you. https://www.google.com/url?sa=t&rct=...Xj6hirNa7YYlU9

edt: actually if you measured each driver on axis (doing the dual channel) then the Y offset should also be entered (and X offset if the drivers are not all in line vertically), if you measured all drivers on the same axis (eg the tweeter) using dual channel, then NO offsets should be entered.

edit2: The best way to check is to do the measurements and also take measurements with a simple crossover in place (same setup) then check with overlays whether your simulated response matches up with the measured response of that simple crossover. If they don't (within the tolerances expected of the components) then there is something not right in your sim inputs or your measurements.

Tony.
No, I used REW and since I have an UMIK I can't do two channel measurements

I measured both drivers with the microphone in the same position, rotating the speaker on a jig that I build, them measured near field for the midbass and passive radiators and used the merger to arrive in the full range response

The procedure I used is basically the one you posted, measuring each driver and the sum and finding the Dz using any crossover software (Time-Alignment)

Thanks for the tip about using the crossover, I still have to order the parts for this one but I have an old one that was used with the same drivers in another box, the response will not be perfect but as you said will shine some light if what I'm simulating actually happens
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Old 1st August 2018, 03:56 AM   #10
wintermute is online now wintermute  Australia
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Crossover Design - When to use driver offset? (Dz)
OK You might want to consider using holm impulse to do the measurements. It will give you relative phase if you use the "lock time zero" option (without dual channel, but you need to do a sanity check to make sure the latency hasn't changed during the measurements, just do the same measurement as the first and make sure they still line up for time zero). However, I found that I did get much better results with my off axis measurements once I changed to using REW dual channel.

There were some oddities in the sim with vituixcad with my holmimpulse time zero locked measurements that went away with the REW dual channel. However, There was a second variable, in that the holm meausrements were taken all on the tweeter axis (with one measurement for boths M's in my MTM) and the REW measurements were taken on axis with each individual driver, so I can't definitively say whether REW dual channel works better than Holm impulse time zero locked.

I'm not sure if REW has a way to effectively set time zero with single channel measurements. It always (as far as I can tell) sets time zero to the peak of the impulse... Just checked and it has two methods, one as I stated, and the other it estimates it. However I do not believe that the estimate will give you accruate Z offset information.

I'm also not sure whether with time zero set to the impulse peak that the offset will work properly either.

another sanity check you can do (which may be even better than using measuring a crossover) is to Measure each driver individually and then measure both playing together (do it twice once inverted)..

Then use the measurements of both drivers running together as an overlay and just do a crossover with both drivers connected to source with no other components and check if it matches.

I found that I got similar results in the sim when using Holm impulse time zero locked meaurements (with no offets entered for drivers) and minimum phase versions of the same measurements with all driver offsets entered.

Tony.
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Last edited by wintermute; 1st August 2018 at 03:58 AM. Reason: add without dual channel comment.
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