Jim -
What is your philosophy regarding Time alignment and Phase Coherency? Given the following simplistic definitions:
1) Time-alignment means that sounds start at two different drivers at the same instant will reach your ears at the same time.
2.) Phase-alignment means that there are no phase errors between drivers: the same frequency waveform, for example, produced by two drivers (in their overlap region) will be entirely in phase with each other (and thus completely reinforcing).
Paul Hales, Jim Thiel, Sean Casey (Zu Audio) and Roy Johnson of Green Mountain Audio are well respected speaker designers and feel very strongly about this topic. Jim I was interested to get your take on this topic.
Hi I believe Jim is burning rubber en route to RMAF. We don't want him texting while driving. Are you asking what we think of each condition--1 and 2? If a speaker satisfies condtion 1, it automatically satisfies condition 2. But the reverse isn't true. If two drivers start at the same time, and reach your ears at the same time, they will be phase coherent (condition 2). But you can have phase alignment without condition 1. Since I'm not driving, I'll give my opinion. Jim Thiel (may he rest in peace), Vandersteen, and others believe condition 1 confers audible benefits in terms of imaging and transparency. But it's a very difficult goal to achieve, generally requiring complex crossovers to ensure 1st order acoustic slopes over 2 or 3 octaves, sloped cabinets to align the acoustic centers of the drivers, a very robust tweeter (almost no ribbons need apply) that can stand up to the gentle slope at the bottom end, and very smooth drivers that don't run out of response too early to maintain the 1st order slopes (forgeddabout Seas magnesium woofs or mids). And then the goal will be maintained only on axis. Move up or to the side, and you lose condition 1. Plus, the wide overlap between drivers will cause interferrence patterns that can be quite audible. But is there magic to the sound when listening on axis? I've never heard it, and I've participated in A-B demonstrations with active crossovers that switch instantly between condition 1 and the more common 4th order acoustic slopes, and I couldn't hear any advantage. But that's not settled. Suffice it to say that condition 1 is hard to achieve, and greatly restricts your choice of drivers. And it's strictly a sweet spot sort of thing if it exists. Condition 2 can be achieved by any properly designed Linkwitz-Riley 4th order crossover, which is what all of the Salk speakers use. I go to great pains to achieve a high degree of phase alignment at the listening position, which I target at 3-4 meters. That ensures that the drivers will sum flat, but requires a lot of futzing since Jim uses flat baffles, and the driver acoustic centers are not aligned to start with. But---that doesn't mean the drivers are playing exactly the same part of the program material at the same time. The woofer and tweeter (in a 2-way) will be exactly one cycle apart. The + and - parts of the wave will coincide, so there won't be any destructive interferrence, but they won't be at exactly the same point in the music. Can the ear tell that? I don't think so, but I'm not on a crusade. We have to go with what we understand and believe, and I personally don't think Condition 1 is worth the cost and restrictions. I do think that phase alignment is important, and I like to think that's why a lot of people find the midrange on Salk speakers very transparent. But that's very controversial. I'm just going with what I've found to work. Other approaches may work as well.
