Phase and Delay

Phase differences and delay cause comb filtering, changing timbre and stereo image.

Related

PhaseComb FilteringDelayMonitoring

The problem

Two mics on one source sum and the bass disappears; a wide synth collapses in mono. Phase and delay have changed the spectrum.

Physics or room

Phase is position within a cycle; delay is a shift in time. Two identical signals separated by τ cancel at odd multiples of a half cycle, producing comb filtering. Polarity inversion is 180 degrees, not any phase rotation. Sound travels 2.9 ms per metre.

Practice and judgement

Check polarity first, then time. Align mics and drivers with a click and measure. Keep parallel paths in sync, or a few milliseconds will scoop the bass. When an instrument vanishes, look for phase before adding width.

What to listen for

Press mono and hear whether bass and vocal thin out; flip one mic's polarity. On a drum hit, hear if the attack softens. With delay, decide whether the comb is colour or a hole.

Common misunderstandings

Phase and polarity differ; a 180-degree flip is a special case. Inverting is not always better; it depends on distance and source. Delay is not always the enemy; widening and echo rely on it.

Sources

• Everest, F. Alton，Master Handbook of Acoustics（McGraw-Hill）— 相位与梳状滤波章节核对，未引用
• Howard, David M. & Angus, Jamie，Acoustics and Psychoacoustics（Focal Press）— 延迟与听觉章节核对，未引用

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