Isolation Ward From ebony pucks to magic foil, mystical and controversial tweaks.

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RE: Absolute polarity . . .

Posted by caspian@peak.org (A) on September 14, 2009 at 08:57:00

In Reply to: RE: Absolute polarity . . . posted by unclestu52 on September 13, 2009 at 20:56:16:

"But again look at the impulse test results in Stereophile. The woofers are often many milliseconds behind the tweeters and often out of polarity.

Some will claim that it can not be achieved, to which I simply point out that even the cheaper Vandersteen speakers achieve relatively accurate time and polarity alignment (not saying they are perfect as they have issues of their own)."

Time and phase alignment (not the same thing) are noble goals, that some multi-way speaker designers strive for even at the cost of other serious compromises. The usual strategies -- stepped or tilted-back baffles, combined with first order electrical filters (as in Thiel and Vandersteen) -- may result in a pretty impulse response, but ONLY on a very narrowly defined vertical design axis. Raise or lower the measurement mic (or your head) by a couple of centimeters, and watch (hear) that beautiful impulse response go to hell. Add to this the lobing in the vertical polar response, typical of shallow crossovers, and you will measure/hear response dips around the crossover frequency, caused by phase cancellation, just a little bit above or below the primary lobe of the vertical design axis. (The shallow highpass slope may also allow too much energy to reach the tweeter below crossover frequency, resulting in audible distortion even at moderately loud SPLs).

These are inherent problems with ALL multiway speakers where there is vertical and horizontal separation between the drivers -- particularly mid and tweeter, where wavelengths are quite short in the crossover region. A coaxial mid/tweeter assembly is a better solution, and has been under constant refinement for years. The new Thiel unit, where the flat midrange ring radiator does not horn-load the tweeter, looks quite promising. Distance between woofer and midrange matters less, since the wavelengths around crossover frequency are so much longer. The multi-axial Cabasse "eyeball" drivers eliminate even this separation.

Even so, a perfect impulse response is likely ever to be seen ONLY with single driver speakers, be they dynamic or planar.

Phase tracking between drivers in a multiway system is a separate, but related issue. Look at the phase vs. frequency trace of ANY speaker driver made, and you will see that the phase goes through several 360 degree rotations over the operating range of the driver. When you're crossing one driver to another, the trick is to get the two phase responses to line up through the crossover region. Generally speaking, if you get excellent phase tracking, you will get a ruler-flat summation in the frequency response through the crossover region, and vice-versa. But the SYSTEM phase will still rotate with rising frequency. So the absolute phase of the speaker's acoustical output will still only match that of the electrical input signal at certain frequencies.

It's all compromise, and there's no free lunch.

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