Moving Targets

Now let’s get to the tough reality about loudspeakers. They have lots of phase shift and phase delay. A good quality active loudspeaker can be engineered to keep the range from 500 Hz on up within ±60 degrees of phase shift (less than 1 ms of phase delay). By 100 Hz we can expect 5 ms of phase delay, rising rapidly below that.

This is important because in the LF range, the loudspeaker can’t be characterized as having a single arrival time. It has arrival times that span a very large range. For example, a 2-way system that reaches down to 70 Hz might have 10 to 15 ms of phase delay at the bottom (Figure 1).

A subwoofer covering from 30 Hz to 100 Hz typically shows more than 30 ms of phase delay between its upper and lower range, with a continuous range of values in between (Figure 2). We kid ourselves when we say the sub energy arrived at the mix position at 100 ms because it actually arrives spread over a 30 ms range around that.

The mains, by contrast, have widespread agreement about arrival time, with their upper six octaves all within 0.5 ms (at least the good ones do). This is why you see that beautiful impulse spike.How can we time align a thing that has 30 ms of slop factor over a course of two octaves? If you ever wonder why your analyzer has a hard time finding an impulse on subwoofers, think about the fact that the energy is spread over time – a lot of time. The ambiguous impulse response reading of the analyzer is the result of the temporal ambiguity of the system being measured, not the analysis method (even if you stretch out the measurement window and restrict the measured bandwidth).

The causes of LF phase delay accumulation are a complex mix of electroacoustic behaviors and filters (maybe another article, another day), but the trend is observable on virtually any loudspeaker. Therefore our main array has phase shift (phase delay) that increases in the low end.

Matched models have matched phase delay, which means they have no phase offset (and hence no time offset). Such a matched pairing is inherently phase compatible. Any phase offset between them would have to be the result of time offset (such as a longer path) and would be remedied (if desired) by time alignment (Figure 3).

A marriage between 2-way and 3-way systems is a classic phase compatibility challenge. All-pass filters are the most typical tools for phase alignment of incompatible models.A different loudspeaker model may accumulate phase shift at a different rate over frequency, and therefore we will find phase offset begin to accrue between the pair.

This is like the relay race where one team is faster than the other. If the amount of phase offset is small, we would classify the pairing as “phase compatible,” which means time alignment is still an applicable tool. If the offsets are large, then we will need to use both time alignment (to synchronize the compatible areas) and phase alignment (to reduce phase offset in the incompatible areas).