TimeAligned1

Time aligned, Phase coherent, Phase aligned, Transient accurate, What's up with that?

Often in the discussion of loudspeakers terms like "time aligned", "phase coherent", "phase aligned" and "transient accurate" come up. Also quite often individuals are confused by what these terms mean and what can be expected from a speaker system that has one of more of these properties. Part of this confusion lies in the observation that some manufactures and speaker designers actually use these terms incorrectly. For example I have seen the claim of "phase coherent" applied to speakers that at best can be called "phase aligned". Part of the confusion lies in the observation that there does not seem to be consistent definition of all of these terms. Part of the confusion stems from the fact that some of the terms are actually trademarks owned by manufactures. So what do, or should, these terms convey about a loudspeaker? To begin let me define several terms that will be needed in the discussion:

1) Acoustic center or AC:

As I shall refer to it, the acoustic center of a driver is the position, on the driver axis, relative to which the driver exhibits a minimum phase response. This position is usually measured with reference to the mounting surface of the driver. For a tweeter this would be the face plate. For a woofer it would be the mounting flange on the basket. The minimum phase acoustic center (AC) is located approximately at the point where the dome or cone of the driver is attached to the voice coil. Thus for a tweeter the AC may be positioned just several mm from the face plate surface. For a woofer the AC may be several cm behind the mounting flange. When defined this way, the acoustic center is a fixed point in space.

2) Relative offset:

The term relative offset is used to indicate the difference between the ACs of the woofer and tweeter. For example if it is stated that the relative offset between the woofer and the tweeter is 23 mm this would typically mean that the woofer AC is located 23 mm behind the tweeter.

3) Target response:

The target response refers to the frequency response function, both amplitude and phase, that one attemps to match the acoustic output of a speaker system or a crossover filter/driver combination to. For example, if one were to use a 2nd order, high pass, Linkwitz-Riley transferfunction for a tweeter crossover target function then one would expect the acoustic output of the tweeter to match the LR target function when connected to the crossover filter.

4)Standard crossover filter:

When I refer to standard filters or crossovers I am referring to the odd order Butterworth crossovers, 1st, 3rd, 5th, etc, or the even order Linkwitz-Riley crossovers, 2nd ,4th ,6th, etc.

5) Time aligned:

When a speaker system is said to be time aligned it usually refers to the ability of the speaker to produce a sharp, single rise in acoustic output, followed by an exponential decay (with some oscillation) when subject to a step change in input signal. Non time aligned systems typically produce a distorted rise, consisting of multiple pulses associated with the woofer and tweeter seperately. More simply put, in a time aligned system the initial woofer and tweeter signals, in response to a step change in input, reach the listener at the same time. Of course, this only happens on the design axis. Time aligned system may or may not be phase coherent, phase aligned, or truely transient accutate.

6) Phase coherent:

A speaker system that is phase coherent should retain, in its acoustic output, the same phase relationships between different frequencies as was present in the input signal. A speaker system that has flat response and is phase coherent is, by definition, transient accurate. Similarly, a truely transient accurate speaker system is phase coherent.

7) Phase aligned:

When someone claims their speaker is phase aligned you should inquire, "With respect to what?" Without additional information the term "phase aligned" has little meaning in my mind. Any crossover which accurately follows some predefined theoretical target functions could be said to be "phase aligned". Surely correctly designed crossovers of the standard type employed in systems where there is no relative offset between the drivers are all phase aligned.

8) Transient perfect;

This is perhaps the easiest term to define. It simply means that the output of the speaker will be a faithful acoustic representation of the input in both amplitude and phase. A trasnient perfect crossover is one for which the crossover introduces no transient distortion of the wave form on the design axis.

Of course all of these terms only apply with reference to a specific design point or design axis. Move off the design axis and alignment of any type is lost simply because the path length to the different drivers will change. And, perhaps more importantly, with the exception of transient perfect, any of these terms can easily be missapplied. Finally, while I believe that these definition are reasonable and sensible representations of what each term should convey about a speaker system, they are in no means universally agreed upon by the speaker building community. Nor is there even agreement on the audible effects these various design considerations impose on the final speaker system.

With these definitions in plase let's move on to look at some differnt speaker design approaches and the effect on system performance.