Comb Filtering — Causes, Detection & Solutions
Comb filtering occurs when two coherent signals combine with a time offset, creating periodic peaks and nulls. The pattern resembles comb teeth with null spacing at 1/delay intervals. It is the most common frequency response degradation in multi-source audio systems and reflection-prone environments.
How Comb Filtering Works
When direct sound combines with a delayed copy at equal level, cancellation occurs at odd half-wavelength multiples. First null at f = 1/(2t) for delay t seconds. A 1 ms delay creates nulls at 500 Hz, 1500 Hz, 2500 Hz. Equal-level summation produces complete nulls; 6 dB difference limits depth to 7 dB.
Common Causes
Overlapping speaker coverage zones. Floor reflections below microphones. Parallel surface reflections in small rooms. Any scenario where the same signal arrives via two paths with different distances creates comb filtering proportional to the path length difference.
Detection Methods
Transfer function per AES-2id shows periodic ripple in magnitude and coherence dips at null frequencies. Null spacing = 1/delay reveals the time offset. The impulse response shows the reflection as a discrete peak following direct arrival. Time gap equals the comb filter delay.
Solutions
Time alignment eliminates the offset. Speaker splay reduces overlap. Absorption treats reflective surfaces. Microphone repositioning or polar pattern change reduces reflected pickup. Never use EQ: null positions shift with listener location.
Try It Now
Detect comb filtering with transfer function analysis