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Our /audio/merge Robot

Merge audio files into one

🤖/audio/merge overlays several audio files on top of each other.

Warning: This Robot is designed for overlaying audio files, not for concatenating them. If you need to join audio files end-to-end, use 🤖/audio/concat.

This Robot can merge an almost infinite number of audio files.

Usage example

If you have a form with 3 file input fields and wish to overlay the uploaded audios, instruct Transloadit using the name attribute of each input field. Use this attribute as the value for the fields key in the JSON, and set as to audio:

  "steps": {
    "merged": {
      "robot": "/audio/merge",
      "preset": "mp3",
      "use": {
        "steps": [
          { "name": ":original", "fields": "first_audio_file", "as": "audio" },
          { "name": ":original", "fields": "second_audio_file", "as": "audio" },
          { "name": ":original", "fields": "third_audio_file", "as": "audio" }
      "ffmpeg_stack": "v6.0.0"


  • use

    String / Array of Strings / Object required

    Specifies which Step(s) to use as input.

    • You can pick any names for Steps except ":original" (reserved for user uploads handled by Transloadit)

    • You can provide several Steps as input with arrays:

      "use": [

    💡 That’s likely all you need to know about use, but you can view Advanced use cases.

  • output_meta

    Object / Boolean ⋅ default: {}

    Allows you to specify a set of metadata that is more expensive on CPU power to calculate, and thus is disabled by default to keep your Assemblies processing fast.

    For images, you can add "has_transparency": true in this object to extract if the image contains transparent parts and "dominant_colors": true to extract an array of hexadecimal color codes from the image.

    For videos, you can add the "colorspace: true" parameter to extract the colorspace of the output video.

    For audio, you can add "mean_volume": true to get a single value representing the mean average volume of the audio file.

    You can also set this to false to skip metadata extraction and speed up transcoding.

  • preset

    String ⋅ default: "mp3"

    Performs conversion using pre-configured settings.

    If you specify your own FFmpeg parameters using the Robot's ffmpeg parameter and you have not specified a preset, then the default "mp3" preset is not applied. This is to prevent you from having to override each of the mp3 preset's values manually.

    For a list of audio presets, see audio presets.

  • bitrate

    Integer ⋅ default: auto [?]

    Bit rate of the resulting audio file, in bits per second. If not specified will default to the bit rate of the input audio file.

  • sample_rate

    Integer ⋅ default: auto [?]

    Sample rate of the resulting audio file, in Hertz. If not specified will default to the sample rate of the input audio file.

  • duration

    String ⋅ default: "longest"

    Duration of the output file compared to the duration of all merged audio files. Can be "first" (duration of the first input file), "shortest" (duration of the shortest audio file) or "longest" for the duration of the longest input file.

  • loop

    Boolean ⋅ default: false

    Specifies if any input files that do not match the target duration should be looped to match it. Useful for audio merging where your overlay file is typically much shorter than the main audio file.

  • volume

    String ⋅ default: "average"

    Valid values are "average" and "sum" here. "average" means each input is scaled 1/n (n is the number of inputs) or "sum" which means each individual audio stays on the same volume, but since we merge tracks 'on top' of each other, this could result in very loud output.

FFmpeg parameters

  • ffmpeg_stack

    String ⋅ default: "v5.0.0"

    Selects the FFmpeg stack version to use for encoding. These versions reflect real FFmpeg versions. We currently recommend to use "v6.0.0".

    Supported values: "v5.0.0", "v6.0.0".

    A full comparison of video presets, per stack, can be found here.

  • ffmpeg

    Object ⋅ default: {}

    A parameter object to be passed to FFmpeg. If a preset is used, the options specified are merged on top of the ones from the preset. For available options, see the FFmpeg documentation. Options specified here take precedence over the preset options.

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