Comb Filtering in Live Sound: Causes and Solutions

Comb filtering creates alternating peaks and nulls in the frequency response, producing hollow or phasey sound quality. This phenomenon occurs whenever two copies of a signal combine with a time delay between them. Understanding comb filtering enables minimizing its negative effects in live sound applications.

How Comb Filtering Forms

When a signal combines with a delayed copy of itself, certain frequencies reinforce while others cancel. The reinforcement and cancellation pattern depends on the delay time.

At frequencies where the delay equals half a wavelength, the signals are 180 degrees out of phase and cancel. At frequencies where the delay equals a full wavelength, they align and reinforce.

The pattern repeats at multiples of the fundamental cancellation frequency, creating the characteristic comb-shaped response curve.

Common Causes

Reflections from hard surfaces: Sound reaching a microphone directly and after reflecting off a floor, wall, or ceiling creates comb filtering.

Multi-microphone bleed: Sound from one source reaching two microphones at different distances creates comb filtering when the channels combine.

Speaker boundary interference: Sound from a speaker combining with its floor reflection at listener positions creates comb filtering.

Monitor wedge interaction: Sound from wedge monitors reflecting off nearby surfaces before reaching the performer’s ears.

Identifying Comb Filtering

The characteristic sound is hollow, thin, and phasey. Moving slightly changes the affected frequencies, creating a swirling quality.

Frequency analysis shows the comb pattern—evenly spaced peaks and nulls across the spectrum.

Comparing the sound with and without the suspected combining source reveals the comb filter effect.

Minimizing Reflections

Absorptive materials reduce reflections that cause comb filtering. Rugs on hard floors, panels on reflective walls, or moving away from boundaries all help.

Angling reflective surfaces so reflections do not return directly to microphones changes the path enough to reduce audible effect.

Close microphone placement reduces the ratio of reflected to direct sound. The direct sound dominates when the microphone is very close to the source.

Multi-Microphone Strategies

Following the 3:1 rule reduces microphone-to-microphone comb filtering. Microphones should be at least three times further apart than their distance to their sources.

Gating reduces bleed between sources. When a source is not playing, its microphone is closed, eliminating bleed that would cause comb filtering.

Strategic microphone choice—directional patterns that reject off-axis sound—reduces bleed contribution.

Speaker Placement Considerations

Elevating speakers reduces floor bounce comb filtering at audience positions. The reflection path changes, shifting the comb filter pattern.

Angling speakers downward directs less energy at the floor, reducing the reflected component.

Moving speakers away from walls prevents wall reflections from combining with direct sound at listener positions.

Time Alignment Approach

If comb filtering comes from a delayed source that cannot be eliminated, time-aligning the sources can shift the comb filter pattern to less audible frequencies.

Delaying the earlier signal to match the later one converts the comb filter into a level change (the delay difference becomes zero).

This approach works for controlled situations but may not be practical when the delay source is environmental reflection.

When to Accept Comb Filtering

Not all comb filtering can or should be eliminated. Some amount is inherent in any multi-source acoustic environment.

Addressing major problems while accepting minor ones represents practical approach. Obsessing over every comb filter source wastes time.

The audience rarely notices mild comb filtering. Obvious hollowness warrants attention; subtle effects may not.

Specific Scenarios

Snare top and bottom mics: The two mics create potential comb filtering when summed. Time alignment or polarity reversal addresses this.

Drum overheads: Unequal distances from snare to overheads creates comb filtering on the snare image. Equidistant positioning helps.

Vocal mic near floor wedge: The wedge’s sound reflecting off the floor before reaching the mic creates comb filtering. Positioning changes help.

Lectern with reflective podium surface: The hard surface creates reflection that combs with direct sound. Absorptive treatment or positioning changes address this.

Measurement and Analysis

Real-time analyzers show comb filter patterns when present. The characteristic evenly spaced peaks and nulls appear on the frequency display.

Transfer function measurement comparing input to output reveals comb filtering introduced by the acoustic environment or signal chain.

Impulse response analysis shows the time delays causing comb filtering. Professional system engineers use these tools for precise diagnosis.