How to Diagnose and Fix Comb Filtering

Symptoms

Comb filtering produces a hollow, phasey, or nasal sound quality. The effect changes as you move your head, and speech sounds "tinny" or like it is coming through a tube. On measurement, the frequency response shows a series of evenly-spaced dips that resemble the teeth of a comb, hence the name. The spacing between nulls directly corresponds to the time delay between the original and reflected signals.

Common Causes

Any situation where two copies of the same signal arrive at the listening position with a time offset creates comb filtering. Strong early reflections from nearby hard surfaces are the most common cause. A reflective floor between a speaker and the listener creates a floor bounce comb filter. Two speakers reproducing the same signal but placed at different distances from the listener create speaker-to-speaker comb filtering. A monitor wedge reflecting off the drum riser creates a stage comb filter. Digital signal routing that splits and recombines with unequal path delays creates comb filtering in the signal chain itself.

Measurement Procedure

1. Open SonaVyx in Transfer Function mode with 1/24 octave or no smoothing.
2. Measure the system at the affected listening position.
3. Look for periodic nulls — evenly spaced dips across the frequency range.
4. Calculate the delay: delay in seconds equals 1 divided by the frequency spacing between adjacent nulls.
5. Convert to distance: distance in meters equals delay times 343 (speed of sound).
6. This distance tells you where to look for the reflection or delayed source.

Interpretation

Nulls spaced 1kHz apart indicate a 1ms delay (0.34m path difference). Nulls spaced 500Hz apart indicate a 2ms delay (0.69m path difference). Short delays (under 2ms) usually indicate nearby surface reflections. Longer delays (5-20ms) suggest speaker-to-speaker interference or distant reflections. The SonaVyx problem detector automatically identifies comb filter patterns and calculates the implied delay, saving you the manual calculation.

Solutions

The best fix is to eliminate the delayed copy. For surface reflections, add absorption at the reflection point — a 2-inch acoustic panel on the reflective surface eliminates the comb filter above 500Hz. For floor bounce, angle the speaker downward or raise the listening position. For speaker-to-speaker interference, adjust speaker placement so listeners are not equidistant between two speakers reproducing the same signal. Add delay to the nearer speaker to align arrival times if both speakers must cover the same area. For signal chain comb filtering, trace the routing to find the duplicate path and remove it.

Verification

After correction, the transfer function should show the periodic nulls reduced or eliminated. Compare before and after traces in SonaVyx. Remaining comb filtering below 500Hz is expected if the cause is a surface reflection — thin absorption does not control low-frequency reflections. If the comb filter persists after treatment, verify you identified the correct reflection point by temporarily covering suspicious surfaces with absorptive material and remeasuring.

Open Transfer Function to identify comb filtering