How to Diagnose Comb Filtering in a Sound System
TL;DR
Comb filtering creates a series of evenly-spaced notches in the frequency response, caused by a delayed copy of the signal combining with the original. Common causes: floor reflections, nearby wall reflections, and misaligned speakers. SonaVyx's problem detector automatically identifies comb filtering patterns and calculates the implied delay.
What Is Comb Filtering?
When a signal combines with a delayed copy of itself, constructive and destructive interference creates a pattern of peaks and nulls. The nulls occur at frequencies where the delay equals an odd number of half-wavelengths: f_null = (2n-1) / (2 × delay). The spacing between nulls is 1/delay Hz. A 1 ms delay creates nulls every 1000 Hz; a 5 ms delay creates nulls every 200 Hz.
Step-by-Step Diagnosis
- Open Problem Detection at /tools/problem-detection. The comb filter detector runs automatically on any captured signal, looking for periodic null patterns in the frequency response.
- Alternatively, use Transfer Function mode at /measure?mode=tf with low smoothing (1/24 octave or off). Comb filtering appears as a regular washboard pattern of peaks and nulls. With heavy smoothing, the comb pattern is averaged out and invisible.
- Identify the null spacing. Measure the frequency gap between adjacent nulls. If nulls appear at 500, 700, 900, 1100 Hz (spacing = 200 Hz), the delay is 1/200 = 5 ms. This corresponds to a path difference of 5 ms × 343 m/s = 1.72 m.
- Find the reflection source. The path difference tells you the extra distance the reflection travels. Look for reflecting surfaces at half the path difference distance from the direct sound path. For a 1.72 m path difference, look for a surface 0.86 m from the direct line between speaker and mic.
- Verify by blocking the reflection. Hold an absorptive panel (even a thick blanket) between the suspected reflecting surface and the measurement mic. If the comb pattern disappears or reduces, you found the source.
- Check the impulse response. Switch to IR mode. Comb filtering from a reflection appears as a distinct secondary spike after the direct sound. The time gap between the direct spike and the reflection matches the calculated delay.
- Apply the fix. Options: (1) absorb the reflection with acoustic treatment, (2) move the speaker to change the angle, (3) move the listening position, (4) use directional speakers to reduce energy hitting the reflecting surface. EQ cannot fix comb filtering because the nulls are position-dependent.
Common Causes
| Cause | Typical Delay | Solution |
|---|---|---|
| Floor bounce | 1-3 ms | Carpet, speaker angle, mic height |
| Nearby wall reflection | 2-10 ms | Acoustic panels, speaker aim |
| Ceiling reflection | 3-8 ms | Ceiling clouds, speaker downfill |
| Lectern/pulpit reflection | 0.5-2 ms | Absorptive cover, mic repositioning |
| Two speakers at different distances | 1-20 ms | Time alignment, level tapering |
Common Mistakes
- Trying to EQ comb filtering. The nulls shift with position. EQ at one position makes another position worse. Fix the cause (the reflection), not the symptom.
- Using too much smoothing. 1/3 octave smoothing hides comb filtering. Use 1/12 or finer to see the pattern.
- Confusing room modes with comb filtering. Room modes cause isolated peaks/dips at low frequencies. Comb filtering causes regularly-spaced patterns across the spectrum.
Tool Bridge
Open the SonaVyx Problem Detector for automatic comb filter detection, or use Transfer Function mode with fine smoothing to see the pattern visually.
Try It Now
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Last updated: March 19, 2026