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Multi-Way Conventional loudspeakers with crossovers

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Old 28th November 2013, 08:52 AM   #11
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Originally Posted by Juhazi View Post
The concept of a loudspeaker's acoustic center is a little vaque. The physical acoustic center of a transducer with some diameter, at d/2 distance =r) from the surface of the membrane. Well, most membranes are cones, some domes which makes this a littel difficult. AES PNW Meeting Report - The Acoustic Centre

Often it is said that tweeter has it acoustic center behind the membrane. But this is just a rule of thumb with the prejudice that a woofer's center is at it's membrane level.
Absolute acoustic centre of a driver is difficult perhaps impossible to measure with absolute certainty, and may not necessarily be constant with frequency at frequencies above the piston range of the driver when the driver is becoming large relative to the wavelength and going through breakup modes.

Relative acoustic centre offset of two drivers which are being crossed over is easy to measure though - measure the "excess group delay" on the design axis of the speaker at a sufficient distance.

The difference in minimum arrival time within each drivers pass band (well away from the crossover points, where the crossover itself will add some additional delay) can be read off directly in fractions of a millisecond and converted to a relative distance offset.

This must be a windowed reflection free measurement with a long enough window time for the phase response within the low frequency drivers passband to be sufficiently accurate.
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Last edited by DBMandrake; 28th November 2013 at 09:08 AM.
 
Old 28th November 2013, 09:21 AM   #12
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Originally Posted by blue_luke View Post
I don't know about slanted baffle, but one obvious side effect is the fact that at least the front and rear are not parallel, which helps avoid standing waves inside the enclosure. Now at the frequencies we are talking about, this is dubious! I am not to sure if it has any effect on this aspect.
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Unfortunately non-parallel walls reducing or preventing standing waves is a bit of an old wives tale - it doesn't prevent or reduce standing wave formation - it just alters the frequencies that they occur at somewhat.

If you have an enclosed space like a cabinet and not much damping you will have standing waves no matter what shape it is, even curved panels. (Same applies to listening rooms)
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Old 28th November 2013, 09:36 AM   #13
tvrgeek is offline tvrgeek  United States
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"vauge", "often it is said". Not really good for engineering practice. This is why we MEASURE the AC of the drivers. You don't get an absolute value, but a delta between two drivers. One could have some reference driver I guess like some planer mid-range.

I still use my old Sound Colony pulser. One of these days I will figure out a pure PC based system.
 
Old 28th November 2013, 09:57 AM   #14
Juhazi is offline Juhazi  Finland
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dsp crossover makes xo phase match/time delay/time alingment "easy".

However this "fine-tuned" alignment applies only to one spesific point in space! My speaker has M/T xo at 3000Hz. Delay is so critical that phasing turns 180¤ with only 0.06ms, which means 55,7mm distance.

"Design axis" is usually between mid and tweeter height and distance 1,5 - 2,5 meters. Spl is usually metered or normalized to 1m. A vertical shift must be quite big to make the phase match roll over, but small deviations come easily. These are masked by more severe phenomenoms like diffractions etc.

3000Hz wavelength is 114,3333...mm (4,5"), it means 360¤ phase rotation.
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Last edited by Juhazi; 28th November 2013 at 10:00 AM.
 
Old 28th November 2013, 10:11 AM   #15
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Originally Posted by Juhazi View Post
dsp crossover makes xo phase match/time delay/time alingment "easy".

However this "fine-tuned" alignment applies only to one spesific point in space!
Digital delay for driver acoustic centre offset will only be correct at one point in space - as you go off the horizontal axis your digital delay is now incorrect. Inverse square law falloff between the two drivers will not track either, if the offset is large enough. (For example a woofer and horn system with a large acoustic centre offset)

Only physical alignment of acoustic centres will keep the two drivers time aligned as you go off the horizontal axis. Of course there will still be an error in the vertical off axis response but maintaining a consistent response on the horizontal axis through the listening window is far more important (especially for imaging) than the vertical off axis response.
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Last edited by DBMandrake; 28th November 2013 at 10:14 AM.
 
Old 28th November 2013, 11:59 AM   #16
Juhazi is offline Juhazi  Finland
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"Only physical alignment of acoustic centres will keep the two drivers time aligned as you go off the horizontal axis."

Yes, I can confirm that. But the change is small until we go more than 45¤ off. I measure my dipoles 0-180¤ and phase shift can be seen. At 90¤ the physical distance difference is 0 and dsp-delay turns phase 180¤ -->reversing the phase at 3000Hz! Even between W/M xo a 900Hz suffers but only ~30¤ shift!

It is actually rather difficult to make a 3-way speaker with perfect physical time-alignment, because of the wave length of lowest xo. At least I have not seen one. This phenomenom is the only strong point of single fullrange louspeaker!
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Old 28th November 2013, 12:31 PM   #17
tvrgeek is offline tvrgeek  United States
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Too bad no real full range exists.
 
Old 28th November 2013, 02:29 PM   #18
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Which is why the correct term is, and always has been, widebander
 
Old 28th November 2013, 02:47 PM   #19
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Quote:
Originally Posted by DBMandrake View Post
Unfortunately non-parallel walls reducing or preventing standing waves is a bit of an old wives tale - it doesn't prevent or reduce standing wave formation - it just alters the frequencies that they occur at somewhat.

If you have an enclosed space like a cabinet and not much damping you will have standing waves no matter what shape it is, even curved panels. (Same applies to listening rooms)
can you provide further support to your statements?

non parallel walls are used in cabinets and by studio engineers to design studios. If it didn't work why go to all the trouble?
 
Old 28th November 2013, 02:53 PM   #20
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