How to Batch Process Audio Files for Post-Production

Post-production rarely receives audio in exactly the shape it needs. A shoot comes back at the wrong sample rate, a client wants MP3 references of every take, an editor asks for mono files instead of stereo pairs. Each change is simple on its own. The problem is doing it forty times, identically, without missing a file or overwriting the wrong one.

Batch processing applies one set of settings to a whole group of files. Done well, it gives you consistent output and frees you for work that actually needs ears. Done carelessly, it multiplies a single wrong setting across an entire delivery. This article covers both sides.

Why batch processing matters in post-production

The value of a batch is less about speed than about consistency. When every file goes through the same settings, you know the whole folder shares one sample rate, one bit depth and one naming rule. That is what editors, sound designers and delivery specs care about: predictable files that import cleanly and line up with each other.

Manual repetition is where errors creep in. Somewhere around file twenty-three, someone picks 44.1 kHz instead of 48 kHz, or saves over a source file. A batch removes that category of mistake, provided the settings were right to begin with.

When one-by-one processing stops making sense

• A full shooting day recorded at 96 kHz that needs to match a 48 kHz edit timeline.
• Music or sound library files at 44.1 kHz going into a video project.
• A folder of mixes or takes that a director wants as lightweight MP3 references.
• WAV deliverables that need to become AIFF for a Mac-based workflow or a client's spec.
• Stereo recordings where each side is actually a separate microphone and should become two mono files.
• Any repeated operation where the correct result is known in advance and nothing needs to be judged by ear.

Prepare the folder before you start

Most batch problems are caused before the batch runs. A few minutes of preparation saves re-running the job later.

• Work on copies, never on the only copy of original production sound.
• Group files that genuinely need the same treatment. Do not mix 96 kHz location files with 44.1 kHz music if they need different handling.
• Remove files that should not be processed: slates, test tones, partial recordings you do not intend to deliver.
• Decide on a naming rule for the output so converted files are never confused with sources.
• Choose an output folder that is separate from the source folder.

Check channel structure, sample rate and bit depth first

Never assume a folder is uniform. One file recorded on a different device, or exported with different settings, can behave differently from the rest. Open a few representative files in the Audio Inspector to confirm the channel count, sample rate, bit depth and duration before choosing any settings.

Channel structure matters most. A batch that expects stereo input should not silently receive a six-channel poly file. Sample rate and bit depth tell you whether a conversion is actually needed, and whether your chosen output would reduce resolution. If you are unsure what those numbers mean for your project, the article on sample rate versus bit depth explains the difference.

Batch sample rate conversion

Sample rate conversion is the most common batch job in video work, because picture workflows are built around 48 kHz while music, libraries and some recorders use 44.1, 88.2 or 96 kHz. Converting properly means resampling the audio, not relabelling the header: the waveform is recalculated at the new rate.

With the batch sample rate converter you choose a target rate once, keep or change the bit depth, and every file in the queue is converted to match. Resampling changes the audio data, so treat it as a one-way step and keep the originals.

Converting multiple WAV files to MP3

MP3 is for listening copies, not for editing. It is a lossy format, so use it for review links, client approvals or quick references, and keep WAV as the working format. A batch WAV to MP3 conversion is ideal when a director wants to hear every take from a day without downloading gigabytes of uncompressed audio.

MP3 is also a stereo-oriented format. Files with more than two channels are refused in the batch rather than folded down to stereo, because a batch should never change channel structure without you noticing. Split or remap multichannel files first if you need MP3 versions.

Converting multiple WAV files to AIFF

AIFF carries the same uncompressed PCM audio as WAV in a different container, historically favoured on macOS and by some music and library workflows. Moving a folder from WAV to AIFF with batch WAV to AIFF conversion keeps the audio uncompressed; what changes is the container and, therefore, which metadata it can carry.

Splitting stereo WAV files into mono

Many stereo recordings are not stereo images at all. A recorder may put a boom on one side and a lavalier on the other, or two separate microphones on a two-track device. In that case you want two independent mono files, not a summed mono mix.

Splitting several stereo files at once with batch stereo to mono extracts each channel exactly as recorded, without downmixing. The left channel becomes one file, the right channel another, with no change to the samples.

Multichannel and channel preparation

Poly WAV files from production recorders and surround stems add another layer: which physical channel holds which signal. Before any batch touches multichannel audio, confirm the channel order. Track names in iXML metadata can help, but they are not guaranteed, and a missing or mismatched layout is common.

When channels need to be reordered, removed or renamed, the Channel Mapper handles that for an individual file, so you can fix the structure deliberately before anything is converted in bulk.

Metadata considerations

Metadata is where batch conversions surprise people most. Not everything survives a change of format, and it is better to know that in advance than to discover it in the edit.

• Compatible descriptive metadata, such as title, artist or comments, may be carried across where the output format supports it.
• Production metadata, including BWF timecode, iXML scene and take information and markers, is specific to WAV workflows and should not be expected to survive every conversion, especially to MP3 or AIFF.
• If your edit relies on timecode or scene and take data, keep the original WAV files alongside any converted copies.
• Always check a converted file in the software that will receive it before processing the entire folder.

Browser-based, local processing

Audio Tool Kit processes audio in your browser on your own computer rather than on a remote server, so there is no upload step for large production files. Files in a batch are handled sequentially, one after another, which keeps memory use manageable even for long recordings. Speed depends on your machine and the size of the files.

When not to batch process

• When files need individual decisions: levels, noise reduction, edits or EQ belong in a DAW.
• When the folder mixes formats, channel counts or sample rates that need different treatment.
• When you have not yet checked a test file in the receiving software.
• When the source files are your only copy.
• When a spec requires loudness measurement or sign-off rather than a mechanical conversion.

A practical post-production workflow

• Copy the delivery and keep the originals untouched.
• Inspect a few files to confirm channel count, sample rate and bit depth.
• Sort files into groups that need identical treatment.
• Fix channel order or structure on the files that need it.
• Run one test file through the intended settings and check it in the target software.
• Run the batch, writing output to a separate folder with a clear naming rule.
• Spot-check several outputs, including the first and last, before handing them on.