Load Impulse Response Files in FFmpeg AFIR Filter

This article provides a straightforward guide on how to load and apply impulse response (IR) files using the FFmpeg afir filter. You will learn the correct command-line syntax for combining your source audio with an IR file, handling sample rate mismatches, and adjusting filter parameters like gain and wet/dry mix to achieve optimal convolution results.

The Basic Command Structure

To apply an impulse response to an audio file, you must feed both the main audio track and the IR audio file into FFmpeg. Because the afir filter requires two distinct input streams, you must use filter_complex to route them correctly.

Here is the standard command template:

ffmpeg -i input.wav -i impulse_response.wav -filter_complex "[0:a][1:a]afir[out]" -map "[out]" output.wav

In this command: * -i input.wav is the main audio source (Stream 0). * -i impulse_response.wav is the IR file (Stream 1). * [0:a][1:a]afir takes the audio from the first input and the audio from the second input, feeding them into the afir filter. * -map "[out]" maps the processed output to the final file.

Handling Sample Rate Mismatches

The afir filter requires both the input audio and the impulse response file to have the exact same sample rate. If they do not match, FFmpeg will throw an error. To prevent this, resample the impulse response stream before passing it to the filter:

ffmpeg -i input.wav -i impulse_response.wav -filter_complex "[1:a]aresample=async=1:min_comp=0.01:comp_delta=200:max_soft_comp=100000000[ir];[0:a][ir]afir[out]" -map "[out]" output.wav

Alternatively, you can use a simpler resampling chain if you know the target rate:

ffmpeg -i input.wav -i impulse_response.wav -filter_complex "[1:a]aresample=resampler=soxr[ir];[0:a][ir]afir[out]" -map "[out]" output.wav

Controlling Gain and Volume

Convolution often increases the overall signal level, which can cause digital clipping. The afir filter includes a gain (g) parameter to manage this.

• Automatic Gain Reduction: Set g=minmax to automatically analyze the IR and apply volume attenuation to prevent clipping.
• Manual Gain: Set a specific decibel value, such as g=-6dB or a linear scale value like g=0.5.

ffmpeg -i input.wav -i impulse_response.wav -filter_complex "[0:a][1:a]afir=g=minmax[out]" -map "[out]" output.wav

Adjusting Wet and Dry Mix

By default, the afir filter outputs only the wet (processed) signal. If you are simulating a space (reverb) and want to keep some of the original, unaffected audio, you can adjust the dry and wet parameters. The values are specified in decibels:

ffmpeg -i input.wav -i impulse_response.wav -filter_complex "[0:a][1:a]afir=dry=-6dB:wet=-12dB[out]" -map "[out]" output.wav

Setting dry=0dB keeps the original audio at full volume, while wet=-12dB adds a subtle layer of the impulse response effect.