Tracking The Trends

With that in mind, let’s continue the original story.

Loudspeakers are very likely to exhibit some degree of phase shift in their frequency response. Negative slopes in a phase trace indicate frequencies arriving late with respect to the reference signal, positive slopes indicate frequencies arriving early and no slope (horizontal tangent) indicates frequencies arriving in time with the reference signal (Figure 6).

These trends are not limited to any particular part of the frequency spectrum. More slope (regardless whether it’s negative or positive) for the same frequency range means more time offset, less slope means less time offset.

In Figure 7, we “scrub” with the delay locator of an analyzer through the IR of a subwoofer.

If you literally think of its IR as a track in a DAW (refer back to Figure 3), then depending on the current playback position (present), some frequencies will already have been heard (to the left of our current playback position = past) whereas the remaining frequencies (to the right of our current playback position = future) have yet to be heard.

As you can see, this “song” takes time and the phase response that describes time over frequency confirms this — otherwise it would be ruler-flat like a microphone cable (without any phase shift).

Regardless of where we are in the song, there’s no visible range of phase trace without a change in slope. Only at a bending point, from negative to positive slope or vice versa, is the tangent horizontal.

Here we observe a very narrow frequency span where measurement (output) and reference (input) are in sync.

In addition, the IR clearly shows that polarity is a moving target. The oscilloscope or DAW shows alternating current with alternating polarity. Some frequencies are in phase whereas others are out of phase.

However, this does not affect important decisions regarding, e.g., crossover alignment.

In such cases, the critical factor is matched phase responses in the overlapping region.