Configure FFmpeg aecho Filter Delay Decay and Feedback

The FFmpeg aecho filter allows you to add a realistic echo effect to audio streams by controlling the direct and reflected sound. This guide provides a straightforward explanation of how to configure the filter’s core parameters—specifically delay time, decay factor, and simulated feedback—along with practical command-line examples to get the exact audio effect you need.

Understanding the aecho Syntax

The basic syntax for the aecho filter is:

aecho=in_gain:out_gain:delays:decays

• in_gain: Sets the input volume (0.0 to 1.0).
• out_gain: Sets the output volume (0.0 to 1.0).
• delays: A list of delay times in milliseconds, separated by the pipe character (|).
• decays: A list of decay factors (volume drop-offs) corresponding to each delay, separated by the pipe character (|).

Configuring Delay Time

The delay time determines how long it takes (in milliseconds) for the echo to be heard after the original sound.

• To configure a single delay of 1 second (1000 milliseconds): aecho=0.8:0.8:1000:0.5
• To configure multiple distinct delays (e.g., a short slapback echo followed by a longer echo), separate the values with a pipe (|). For example, 500ms and 1000ms: aecho=0.8:0.8:500|1000:0.5|0.3

Configuring the Decay Factor

The decay factor determines how much quieter each subsequent echo is compared to the original sound. It is expressed as a decimal between 0.0 (complete silence/no echo) and 1.0 (same volume as the original sound, which can cause clipping if not managed).

• A high decay factor like 0.7 creates a loud, lingering echo.
• A low decay factor like 0.2 creates a quick, subtle echo that fades rapidly.
• Each delay time mapped in the command must have a corresponding decay factor. For example, if you have two delays, you must define two decays: aecho=0.8:0.8:500|1000:0.6|0.3 (The 500ms delay has a 0.6 decay, and the 1000ms delay has a 0.3 decay).

Configuring Feedback

The aecho filter does not have a dedicated “feedback” parameter like some analog delay pedals. Instead, feedback—the repetition of echoes fading over time—is achieved by configuring multiple delay lines with progressively lower decay factors.

To simulate a dense feedback loop where the sound bounces repeatedly, stack multiple delay steps at equal intervals with descending decay values:

ffmpeg -i input.wav -af "aecho=0.8:0.8:250|500|750|1000:0.5|0.25|0.12|0.06" output.wav

In this example, the delay occurs every 250 milliseconds, and the decay factor is halved each time, simulating a natural, decaying feedback tail.