Posted by Lynn Olson (M) on November 10, 2005 at 23:15:21

In Reply to: Re: The new Apollo posted by Robert Bastani on November 9, 2005 at 15:56:14:

The sudden up-and-down of the phase graphs in the Klangundton.602 review is an artifact of how phase is plotted when it goes through a 360 degree rotation.

Although a 360-degree phase rotation sounds alarming, all you have to do is get a slight miscalibration in the test gear, or have a very gradual departure from minimum-phase behaviour, and you'll see the phase start to spin around and around. The test gear has no idea where the acoustic center of radiation is - that's up to you to tell it that - if you're a little bit off in your guess, you see that kind of phase rotation.

Sound travels about 344 meters/second, or 13,543 inches/second. Looking at it another way, a full wavelength at 13.5kHz is an inch long. A 90-degree phase shift - a quarter wavelength - at 1kHz is only about 3 and 3/8" long, not that much when you consider the measuring distance was probably 2 meters (80 inches).

The assumed center of radiation is typically the junction of the cone and dustcap, but in the real world it may be an inch or so behind that, depending on the HF response of the cone and the way it departs from minimum-phase behavior at high frequencies. Since the cone is no longer radiating as a piston, but instead breaking into little sub-cones, the center of radiation will move back and forth a little bit.

By the way, almost all drivers do this at the top of their working range. Rigid drivers (aluminum, magnesium, Kevlar, or carbon-fiber) do break up, and break extremely violently - that's the source of the 10dB peaks you see in the response curves for these drivers. These peaks are not easy to filter out of crossovers - there's only so much a notch filter can do, and it has some coloration of its own.

Gradual, controlled breakup with a paper cone is the more traditional approach (dating back to the Thirties), and this involves ribs on the cone, selective treatments, etc. The trick is to get subjectively smooth response while keeping distortion due to cone flex reasonably low - this is where using a big prosound driver comes in handy, since distortion is low to start with.