Phase Problems

Two microphones on one source produce time differences that cancel frequencies. Check phase before tone.

Related

PhasePhase CheckStereo Technique

The problem

When two microphones hear the same source, any difference in arrival time cancels some frequencies. It sounds like a hole cut into the EQ that no boost can fill. A single microphone hides it; the pair exposes it.

Signal chain

The time difference between the two arrivals creates a phase difference, and summing them produces comb filtering. Polarity inversion is 180 degrees at every frequency; time alignment shifts with frequency. They are not the same thing. A larger distance difference pushes the first cancellation lower and the notches closer together.

Placement and settings

Follow the three-to-one rule — the second microphone should be at least three times farther from the source than the first. Check polarity on the top and bottom of a drum, on two piano pickups and on close plus distant guitar mics. Fine-tune with sample delay rather than relying on the polarity switch alone.

What to listen for

Sum the two microphones to mono and hear whether the lows and low mids thin out. Move one microphone a few centimetres and listen for a sudden hollow tone. You can also invert polarity on one microphone and compare the two sums.

Common misconceptions

• Phase is polarity → polarity is only the 180-degree case.
• The polarity switch fixes every phase problem → comb filtering from a time difference needs delay.
• Phase only matters while tracking → multi-microphone material needs the same check at mixdown.

Sources

• AES Convention paper on comb filtering and microphone arrays
• Eargle, The Microphone Book, 3rd ed., multi-microphone phase chapters

Used for fact-checking only; all prose is original.