Phase Issues in Live Sound: Understanding and Correcting

Phase issues occur when related signals arrive at different times, causing frequency cancellations and reinforcements. Multi-microphone setups, speaker systems, and even single microphones in reflective environments experience phase effects. Understanding phase enables corrective action that improves sound quality.

Understanding Phase

Sound waves have positive and negative pressure components. Two waves perfectly aligned reinforce each other; waves 180 degrees out of phase cancel each other.

Real-world phase relationships are rarely perfect alignment or perfect cancellation. Partial phase offset causes frequency-dependent reinforcement and cancellation patterns.

Phase issues manifest as thin, hollow, or unnatural sound. Frequencies that should be present are reduced or absent; others may be unnaturally emphasized.

Polarity vs Phase

Polarity reversal flips positive and negative—a 180-degree inversion. This is a fixed relationship independent of frequency.

Phase shift varies with frequency and time. Different frequencies experience different phase shifts, creating complex interactions.

Polarity switches on mixers (often labeled “phase” or with the symbol ø) provide 180-degree inversion. This corrects polarity errors but does not address time-based phase issues.

Multi-Microphone Phase Issues

Multiple microphones on the same source capture sound at different times based on their positions. The acoustic path length differences create phase differences.

Snare drum with top and bottom microphones: The bottom mic receives the snare sound slightly later than the top mic. Summing them creates cancellations at specific frequencies.

Drum overheads: If the overheads are not equidistant from the snare, the snare arrives at different times. This causes comb filtering on the snare in the overhead image.

Guitar amp with two microphones: Different positions capture different parts of the speaker with different timing. Combining them may create phase problems.

The 3:1 Rule

The 3:1 rule minimizes phase interaction between microphones on different sources. Microphones should be at least three times further apart than the distance from each to its source.

If a vocalist is 6 inches from their microphone, the next microphone should be at least 18 inches away.

Following this rule reduces the level of bleed sufficiently that phase cancellation is less audible.

Correcting Polarity Issues

Use the polarity/phase switch to flip one of two microphones and listen for improvement. If the sound improves, keep the switch engaged.

Trial and error works—flip the switch, compare, decide. The better-sounding position is correct for that setup.

Bottom snare mic is commonly polarity-inverted to align with the top mic. The membranes move opposite directions when the drum is struck.

Time Alignment

Time alignment addresses phase issues by delaying the earlier signal to match the later one. This restores coherent timing.

Digital mixers may include channel delay for time alignment. Delaying the closer microphone aligns its arrival time with the more distant one.

Delay amount calculation: Sound travels approximately 1.1 feet per millisecond. A 2-foot path difference needs about 1.8 ms of delay on the closer source.

Speaker System Phase

Main speakers and subwoofers may have phase issues at the crossover frequency. The electronic crossover creates filter-related phase shift; physical positioning adds timing differences.

Subwoofer polarity adjustment aligns the subwoofer output with main speakers. Trial and error—flip polarity and listen for the tightest, fullest crossover region.

Subwoofer delay compensates for position differences. If the subwoofers are further from the listener than the mains, delay the mains to align.

Comb Filtering

Comb filtering creates a series of peaks and nulls in the frequency response, resembling a comb on an analyzer display.

Reflections from hard surfaces create comb filtering. Direct sound combines with reflected sound, creating the comb pattern.

Multi-microphone bleed creates comb filtering. The same source reaching two microphones at different times sums to create the comb effect.

Addressing comb filtering involves reducing reflections, repositioning microphones, or accepting the coloration as part of the sound.

Listening for Phase Issues

Thin, hollow sound often indicates phase cancellation. Low frequencies cancel more noticeably than highs.

Checking mono compatibility reveals phase issues. If stereo content loses impact or definition when summed to mono, phase problems exist.

Solo two related channels and flip polarity on one. The setting that sounds fuller has better phase alignment.

Practical Approach

Not all phase issues require correction. Some are inherent in multi-microphone recording and part of the sound.

Address obvious problems: thin snare, weak bass from the kick mic, hollow guitar sound. These symptoms suggest correctable phase issues.

Accept what cannot be easily fixed. Obsessing over minor phase interactions consumes time that could improve other aspects of the mix.