How to Diagnose and Fix Echo Complaints

Symptoms

Listeners report hearing a distinct repetition of speech or transient sounds. Clapping produces a clear second clap rather than a smooth decay. Speakers hear their own words coming back to them with a noticeable delay. The echo is usually directional — listeners can identify which direction the reflected sound comes from. Echo is distinct from reverberation: reverberation is a smooth, continuous decay, while echo is a discrete, identifiable repetition. Echo complaints in large rooms typically involve the rear wall or distant parallel surfaces.

Common Causes

Echo requires two conditions: a strong reflection (low absorption at the reflecting surface) and sufficient delay (long path difference between direct and reflected sound). The minimum delay for a reflection to be perceived as a distinct echo rather than coloration is approximately 50ms, corresponding to a 17-meter additional path length. Common echo sources include flat rear walls in auditoriums and churches that are more than 8-9 meters from the source. Concave surfaces (domed ceilings, curved walls) that focus reflected sound into concentrated spots. Parallel walls that create flutter echo — a rapid series of reflections bouncing between two parallel surfaces. Balcony fascias that reflect sound back to the stage.

Measurement Procedure

1. Open SonaVyx and navigate to the RT60 measurement tool.
2. Capture an impulse response using the log sine sweep method.
3. Examine the impulse response waveform for discrete reflections.
4. Early reflections (under 50ms) contribute to room character. Late reflections (over 50ms) cause perceptible echo.
5. Note the delay time and relative level of any late reflections.
6. Calculate the reflection path: delay (ms) times 0.343 equals additional path length in meters.
7. Use this distance to identify which surface is responsible.

Interpretation

A reflection more than 10dB below the direct sound is generally not perceived as echo even if it arrives late. A reflection only 6dB below the direct sound arriving after 50ms will be clearly audible as echo. The combination of delay time and relative level determines whether a reflection is problematic. Flutter echo between parallel walls appears as a rapid series of equally-spaced reflections in the impulse response, each decaying by the absorption coefficient of the surfaces.

Solutions

For rear wall echo: add absorption panels to the rear wall. A 4-inch thick absorption panel covering at least 50% of the rear wall surface eliminates the reflection above 250Hz. For concave surface focusing: add diffusion panels (QRD or similar) that scatter the reflected energy rather than focusing it. For flutter echo between parallel walls: add absorption or diffusion to one or both walls, or angle one wall by at least 5 degrees to prevent the back-and-forth reflection path. For balcony fascia reflection: angle the fascia surface or add absorption to redirect reflected energy away from the audience. In all cases, diffusion is preferred over absorption when the goal is to reduce the echo while preserving the room natural reverberance.

Verification

After treatment, recapture the impulse response. The late reflection should be reduced by at least 10dB, ideally to 15dB or more below the direct sound. Listen for the clap test — a single hand clap should produce a smooth decay without a distinct repetition. Measure RT60 to verify that the absorption has not over-dampened the room.

Capture impulse response to find echo reflections