Adding an echo (or reflected sound) to an audio track is a great way to add more ambience or smoothness to a choppy playback. It’s a personal desired effect but for how simple it is to use, it’s a nice filter to have in the back pocket.
Here aecho uses gain, delays and decays make for a great airy echo effect that fades over time:
$ ffmpeg -i input.mp3 -af "aecho=in_gain=0.5:out_gain=0.5:delays=500:decays=0.2" output.mp3
In this filter the delay and decay arguments are plural because multiple delays and decays may be stacked using the | syntax for extra echo control, as seen below:
$ ffmpeg -i input.mp3 -af "aecho=in_gain=0.5:out_gain=0.5:delays=500|200:decays=0.2|1.0" output.mp3
Tip: The number of delays and decays must equal the same amount so if there are 2 delay numbers, there has to be 2 decay numbers.
aecho Indicates the name of the echo filter in_gain Indicates the input gain reflected signal (default 0.6) out_gain Indicates the output reflected signal (default 0.3) delays Indicates the a list of time intervals (in milliseconds) between the original signal and reflections (0.0 to 90000.0 with default 0.5) decays Indicates the list of loudness of each reflected signal (0.0 to 1.0 with default 0.5)
Tip: If this filter is applied to video input, a pixel format may be required, -pix_fmt yuv420p.