LUFS loudness meter — measure integrated loudness vs. streaming targets

Loudness measurement happens at the most sensitive point in the pipeline

Checking Integrated LUFS is a pre-release step. The file on the table is the current best master — the mix that will go live. An online tool that receives this file for analysis gets your most polished work at its most final stage.

Loudness numbers also carry structural information about the audio. A wide LRA signals a dynamic, acoustic-style recording; a narrow LRA with high Integrated LUFS is typical of heavily compressed electronic music. The measurement result alone reveals something about the nature of the work — and retaining it on a server means retaining that characterisation.

What happens when you send your master to a free online loudness checker

Server-side loudness checkers need the full audio payload to compute anything — loudness is an average over the entire duration. That means ‘just checking the LUFS’ and ‘uploading the master file’ are the same event. The operational feeling (no transformation, just a number) understates the data exposure.

Terms of service for free tools frequently permit use of uploaded content for service improvement. Whether that applies to audio files in practice, and what ‘service improvement’ covers, is rarely spelled out. For a master that hasn’t been released, the window between upload and distribution is exactly when you most want control over where the file has been.

K-weighting biquad and OfflineAudioContext — ITU-R BS.1770 inside the browser

The tool decodes audio with decodeAudioData, resamples to 48 kHz via OfflineAudioContext, then applies a two-stage K-weighting filter: a high-frequency shelving filter and a DC-block filter, as specified in ITU-R BS.1770-4. Instantaneous power is collected in 400 ms gating blocks; relative gating (-70 LKFS absolute threshold then -10 LU relative) produces the Integrated Loudness figure. LRA comes from the spread of the short-term loudness (3-second window) distribution.

The entire computation lives in browser memory. Open DevTools Network and measure a file: no audio-related requests after the initial load. The source — including the biquad coefficient calculation — is on GitHub.

Building a loudness check into your pre-release routine

Running a file through loudness analysis before export catches the mismatch between your intended sound and what streaming platforms will normalise to. A master pushed significantly above -14 LUFS gets pulled back by Spotify and YouTube; the relative spaciousness and dynamics you mixed for change in ways that weren’t auditioned. Catching this in the browser, without the file leaving your machine, is the lowest-friction version of that check.

Pair LUFS measurement with audio-true-peak for a complete pre-delivery assessment. Sample Peak can read clean while the reconstructed waveform (inter-sample) goes above 0 dBTP after MP3 / AAC encoding. Checking both before uploading to a distributor avoids the clipping artefacts that only appear post-transcode. If the integrated value misses the target, audio-volume applies a static gain in seconds; audio-waveform and audio-spectrum are useful side-checks when you want to see what the meter is responding to.

Inside ITU-R BS.1770 — why K-weighting beats plain RMS

Loudness measurement is not plain RMS — it operates on power that has first been routed through K-weighting, a perceptual frequency curve. The BS.1770 K-weighting filter is two biquads in series: (1) a stage 1 RLB filter that high-passes around 38 Hz to suppress DC and very low-frequency rumble that contributes nothing to perceived loudness, and (2) a stage 2 high-shelf that adds about +4 dB above 1.5 kHz. The shelf reflects how the ear weighs the 2–4 kHz band (the consonant range) more heavily than lows or extreme highs, so bass-heavy material can’t game the meter by piling on inaudible low energy.

The Integrated Loudness algorithm collects K-weighted power in 400 ms rectangular blocks, then applies two-stage gating: (a) drop blocks below the -70 LUFS absolute threshold (silent regions), then (b) drop blocks below -10 LU relative to the running mean (quiet passages). Loudness Range (LRA) comes from the spread of short-term (3-second window) loudness between the 10th and 95th percentile. Roughly: LRA = 3–5 is hyper-compressed EDM, LRA = 8–12 is mainstream pop, LRA = 15–20+ is classical or film. Stereo uses ITU channel weights (L=R=C=1.0, Ls=Rs=1.41); this tool supports the stereo case (L=R=1).

Platform targets and what normalisation actually does

The current major streaming targets are Spotify -14 LUFS, YouTube -14 LUFS, Apple Music -16 LUFS, Amazon Music -14 LUFS, Apple Podcasts -16 LUFS, TIDAL -14 LUFS, broadcast EBU R128 -23 LUFS, and US terrestrial ATSC A/85 -24 LKFS. A master exceeding the target (say -9 LUFS for Spotify) gets attenuated by -5 dB on the platform side. Hugely under the target (say -22 LUFS on Spotify with the Loud profile) can be auto-gained up, potentially pushing True Peak above 0 dBTP and clipping.

Pitfalls to know: (a) each platform’s normalisation is opt-in/opt-out (Spotify Premium’s loudness setting, Apple Music Sound Check), so listeners with normalisation off get the original level, (b) Album mode (Spotify, Apple Music) normalises to the album’s peak Integrated Loudness rather than per-track, leaving quiet tracks at their natural level inside the album, (c) podcast targets at -16 LUFS are quieter than music, so inserted music beds can feel disproportionately loud, (d) loudness normalisation does not guarantee True Peak ≤ -1 dBTP, so always pair LUFS with audio-true-peak. A typical routine: if the delta against your -14 LUFS target reads +3 LU, apply -3 dB with audio-volume and re-check True Peak before publishing.