Time coherence - how important and what speakers?

I just came off of 2 physics websites regarding the speed of sound versus frequency. Both the Cornell physics dept. and the Nasa physics website stated that the speed of sound in air is constant, and does not vary with frequency. The only variable in the speed of sound is the density of the medium(air) which will cause variations according to altitude, and humidity, or barometric pressure. In any case, the speed of sound will be constant for all frequencies at any given condition of the air at the time.

So this idea that the highs travel faster than the lows is not borne out by fact. It seems to be an "urban legend" that grew up around the "mysterious" functions of loudspeakers, that people did not understand.

As I previously stated, the time-alignment is done to simulate a single plane radiation source.

As Karls, stated, the single driver systems, including e-stats which may use a single panel, are inherently time and phase aligned, regardless of what some are reading into some "impulse charts". Any undesirable activity shown on the impulse charts are characteristics of the particular speaker being measured, and not a function of the speed of sound vs frequency.

Twl, I'm in over my head here, but I won't let that stop me. Is it not true that for a speaker to be properly "phase-aligned" by default it almost demands a 1st order crossover?

Supposedly, that's yet another reason why the Vandersteen Model 5's are so highly rated. Time alignment being another reason.

Correct me if I'm wrong.

Stehno, the 1st order crossover alignment, which consists of a single capacitor between drivers is a 90 degree phase alignment. It allows the bass driver to run full range, to its natural limits, and blocks the tweeter from operating below the frequency specified by the capacitor value. This protects the tweeter from harmful high power that may be present at the lower frequencies, and keeps intermodulation distortion to a minimum in the tweeter. The tweeter will be rolled-off at 6db/octave, from the crossover point downward, in this configuration.

The main advantage in this alignment, is that there is the fewest number of crossover parts to screw up the signal. And the presence of crossover inductors is eliminated, which is a major cause of phase shift. For this alignment to be successful, it requires careful matching of the drivers, so that there is not a "bump" in the response where the tweeter crosses in, and the woofer does not cross out. The woofer should be naturally rolling-off on its own at around 6db/octave, right at the crossover point, in order for this alignment to be optimal. Even though this is considered a 90 degree alignment, there is much to be said for the simplicity and signal preservation of this design. There are going to be phase-shifts in any crossover, and preserving the signal integrity is a valiant thing to do, considering the necessity of having a crossover in a multi-driver system. Having no crossover in a multi-driver system, with the drivers having matching natural roll-offs at a certain point would seem to be optimal, but the protection of the tweeter is vital, and intermod is present in unacceptably high amounts, as the tweeter vainly tries to respond to low frequency information that it cannot reproduce. So the single capacitor which functions primarily as a tweeter protector is a good compromise, if you can match the drivers well enough to keep the response curve smooth. If you want to avoid the crossover phase shift altogether, you must go to single-driver, which presents its own set of difficulties.

Twl, only first order filters are phase coherent, no exceptions. First order usually refers to the electrical filter, not the inherent slope of the driver's response. Therefore a first order filter consists of one reactive element, either an inductor or capacitor, in series or parallel with the load (the driver). And the phase shift caused by the reactive element varies with frequency, as a function of the ratio of reactive impedance to load impedance. In an "idealized" first order filter, there is a woofer inductor and a tweeter capacitor, each of which has a 45 degree phase shift at the crossover frequency, but in opposite directions. Over the rest of the frequency spectrum that each driver is intended to cover, the phase shift caused by the reactive elements is less than 45 degrees. Above the woofer's crossover frequency, and below the tweeter's crossover frequency, their phase shifts gradually approach 90 degrees, again in opposite directions.

Higher order filters add 90 degrees of phase shift per order. For example, a second order filter will have a 90 degree phase shift on both woofer and tweeter at the crossover frequency, approaching 180 degrees in each driver at the extremes. The two 90 degree phase shifts at the crossover frequency in opposite directions gives a net 180 degree phase shift between the drivers, meaning that if the drivers are wired in the same absolute phase, there will be a null at the crossover frequency. This then necessitates that the input to one of the drivers be run in inverted phase, meaning that throughout the entire range covered by the two drivers, they are operating in opposite phase. Talk about screwing up the integrity of the waveform!

Twl is right, there is no variation in the speed of sound relative to frequency. Each driver has a different "acoustical center", which is the point at which sound can be said to originate. It is not a physical property but an acoustical one, although a good approximation for any given driver is to use the point where the voice coil former meets the cone or dome.

Karls, thanks for the clarification. I was under the impression that the capacitor or coil shift would be additive to the reactive element, and not be cancelled by it, in a first order configuration. Is this fully proportional, or are there some varying degrees of shift that are not cancelled ?