# Audio Loudness Normalizer

> Measure LUFS and true peak, then normalise to Spotify, podcast or EBU R 128 targets. Measures integrated loudness (LUFS), true peak (dBTP) and loudness range (LRA) with ffmpeg's EBU R 128 meter, then normalises the file in two passes to a streaming, podcast or broadcast target with a true-peak limit. Shows the measured values before and after, with a player and a download.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://audioloudnessnormalizer.usefulweb.site/ (interactive tool at https://audioloudnessnormalizer.usefulweb.site/tool)
- JSON API: POST https://audioloudnessnormalizer.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://audioloudnessnormalizer.usefulweb.site/api/v1/tool)
- OpenAPI: https://audioloudnessnormalizer.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://audioloudnessnormalizer.usefulweb.site/mcp - tool `audio_loudness_normalizer`

## Inputs
- file (file, required): Audio file - MP3, WAV, FLAC, Ogg/Opus, M4A, AIFF; max 100 MB and 45 minutes. Processed on our server in a private temporary folder that is deleted afterwards; the result can be downloaded for about 2 hours.
- preset (select): Target
- targetLufs (number): Custom integrated loudness - Only used with "Custom target".
- truePeak (number): Custom true-peak limit - Only used with "Custom target".
- lra (number): Loudness range target - Higher keeps more dynamics. If the source range is larger, ffmpeg compresses it.
- format (select): Output format
- bitrate (select): Bitrate (lossy formats)

## Method
- Pass 1 measures the file with ffmpeg's loudnorm filter (ITU-R BS.1770 / EBU R 128 meter): integrated loudness (LUFS), true peak (dBTP, oversampled), loudness range (LRA) and the gating threshold.
- Pass 2 feeds the measured values back into loudnorm with linear=true: if the whole file can reach the target with one gain change without exceeding the true-peak limit, only the gain changes (linear). Otherwise loudnorm switches to dynamic mode, which compresses and limits — the result reports which one happened.
- loudnorm works at 192 kHz internally; the output is resampled back to the original sample rate. The podcast-mono preset also mixes to mono.
- Pass 3 measures the finished file again, so the "after" values are real measurements of the download, not the targets (for files over 15 minutes the normaliser's own output report is used instead, to stay within the time limit). Files up to 45 minutes are accepted.
- Spotify (−14 LUFS, true peak below −1 dB) and EBU R 128 (−23 LUFS) are published targets; YouTube ≈ −14 and Apple ≈ −16 LUFS are widely used measured/recommended values, not contractual specs. Podcast −16 LUFS stereo / −19 LUFS mono are common industry recommendations.

## Limits
- Very short files (under about 3 seconds) give less reliable integrated loudness, because the meter gates in 400 ms blocks.