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RE: Clarifications on Time Coherence

Roy said:
"Time delays in speakers are imposed by four things:
- the speaker's crossover-circuit design
- by non-pistonic drivers
- by the low-frequency tuning of any raw driver, woofer or tweeter
- by the locations of the drivers relative to one's ear."


You did leave out one very important cause of time delay that every designer has to account for in proper engineering so as to establish smooth driver integration at and through the crossover region in a multi way system :

Each driver's native or minimum phase response


Non pistonic and non point source behavior relative to cone breakup or beaming problems are usually dealt with by selecting the proper crossover point and driver for the intended function. It is not a significant player in establishing phase and time coherent speakers if the proper steps are taken up front. Not sure what you mean by "low frequency tuning" and its impact. However, as we all know, every driver (especially moving coil drivers) exhibit capacitive and inductive behavior when stimulated by time varying voltage (a music signal, for example). The resulting storage of electrical and mechanical energy in every transducer results in what is commonly referred to as a native or minimum phase response. If one is to achieve a well integrated pair of drivers at crossover, this inherent phase behavior must be taken into account - otherwise, achieving a substantial reverse null at crossover (on the order of 30-40 db indicating excellent driver integration) will be impossible.

Roy also said:

"More rapid rates of acoustic rolloff, such as 12, 18 or 24 dB/oct, inject time-delays that are also different at each frequency."

You seem to be implying that steeper roll offs result in "time delays" that can't be compensated for where this doesn't occur with first order crossovers??? If that is what you are trying to say, you're incorrect. All passive components in crossovers introduce acoustic phase changes that vary with frequency. The simple delayed discharge of current through a capacitor and resistor in series is ample evidence of that. The fundamental difference between 1st order and higher order crossovers is higher group delay at the crossover point and the level of energy storage contributing to an undamped or overdamped impulse response. More importantly, contrary to what you're implying, crossover induced "time delays" are a straw man problem. With any properly executed crossover, acoustic phase difference between drivers moving from the lowest frequency portion to the highest about a particular crossover point should be constant (in quadrature with odd ordered networks, and either 180 degrees or in phase with even ordered networks). Group delay and its audibility are another issue not really related to phase and time "coherence". All crossover networks - 1st order through Nth order - suffer from group delay. Widely acknowledged studies have been published discussing the degree of audibility with respect to group delay and frequency. That issue is separate and distinct from the traditional issues of time coherence and phase coherence in speaker design.

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