Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Nope.

Posted by Jon Risch on June 6, 2000 at 17:54:14

In Reply to: Re: Physically aligning drivers posted by Mike Bates on June 5, 2000 at 15:38:43:

This method does not align the driver to be time aligned or time coherent. All it does is FOR A GIVEN SUM OF TOTAL PHASE SHIFTS, is to create a null with those particular drivers and that particular crossover alignment. This may by sheer chance, happen to align the drivers, but more often than not, it will only align for a 'local null', and moving one of the drivers a full wavelength at the crossover frequency in one direction or the other will also achieve a null.

The acoustic roll-off of each dirver will affect the phase response in the crossover region, as will the order and alignment of the electrical portion of the system, so the sumn of the driver phase response and the electrical network response is what is being nulled out here, not the actual acoustic cneter or origin of the drivers.

The acoustic center of a driver is essentially that point where the waveform seems to originate from along an axial dimension. Typically, for most dynamic direct radiator speakers, your typical woofer, midrange or dome tweeter, the acoustic center will be even with the magnet systems backplate. The front of the plate for really widerange drivers, and the rear of the backplate (the rear of the magnet) for more typical bandlimited drivers. So if you want to line up a woofer, mid, tweet so as to start in a situation where the wave launch from each driver will reach you at the same time along the axis of mid driver (approx.), then lining up the rear of the magnets with each other will get you to within a fraction of an inch of the actual center. This is for those speakers without shielding cups and bucking magnets, and should reference to the primary (front) magnet on such systems.

This then allows the textbook crossovers to work as they seem they should, if the other speaker parameters are nominal and not to shabby, and the driver impedances are corrected for.

I know it would seem that at the least, the acoustic center would be in the plane of the voice coil winding, but due to the less than infinite bandwidth of any given speaker, the acoustic center is pushed back even further. It is an axiom that any bandlimiting causes an acoustical delay.

Jon Risch

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