This is the first in a three-part series on phase alignment. Additional segments of the series are available here.
FFT based field measurement systems have made it possible for us to do phase alignment at fixed installations as well as at live events, where every venue demands a different approach.
This is particularly important these days, since mid-high boxes are often flown and subwoofers remain on the ground, meaning that phase differences at the listener location can be very significant.
Given the interest in the subject, and the remarkable improvement it can bring to a system, it seems like a good idea to write an article that describes the measurement process step by step. Before doing that, however, let’s go over the concept of phase.
Polarity and Phase
Polarity only has two discreet values: positive and negative. Polarity does not change with frequency, and may be accidentally inverted when the speaker cables are connected the wrong way, or when a signal cable is repaired and pins 2 and 3 get inverted, or when one of the bands has its polarity changed on the signal processor by mistake.
Other times polarity is purposely reversed, such as when a passive crossover filter is used.
Phase may have any value in degrees: values are continuous. To find out what the phase response is for a given speaker, we need to measure it.
Throughout this article we’ll see measurements performed using SATLive. Phase curves are shown on the bottom part of the image, while the magnitude frequency response will be at the top of the image. Figure 1 one shows a typical subwoofer phase response in blue, with polarity reversal in green. A 180º shift can be clearly observed at all frequencies.
Figure 1: Comparison of two phase curves, one with opposite polarity.
Running a phase measurement on our systems, and storing it for reference, can be useful for checking for correct polarity after maintenance.
Since we are only looking for a comparison, any measurement position that can be repeated easily would suffice.
For instance, the microphone can be placed directly in front of the speaker, in its centre, very close to the grille. This provides an easily repeatable measurement position and a clean measurement with no contamination from the environment.
What makes phase vary?
a) Any variation on a system’s magnitude frequency response will have an effect on the phase response.
For instance, the phase curve changes when equalization is added. Figure 2 shows the result of adding a bell-type filter centered at 5.04 Khz, with a width of 0.42 Oct and +10dB gain, to the high frequency band on a processor. Phase rises just before the centre frequency and falls just after it.
Figure 2: The green trace shows the effect of a bell-type filter on the phase response.
Since equalization affects the phase response, outputs should never be EQed once phase alignment has been done, especially around crossover frequencies.
Otherwise we would modify the phase of the output being equalized, hence affecting the relationship between phase responses, which is what we are trying to adjust for when adding delays.
EQing the input (on the processor itself or on a graphic equalizer or the mixing desk) will not affect phase alignment, since it happens before the crossover.
b) Adding delay to a band, or physically displacing it backwards (such as moving a subwoofer away from the measurement microphone) will have the same effect on the phase response.