wimpysworld Audio Metrics Reference
Objective definitions of the audio metrics ffmpeg computes. Each entry states what the metric measures, how ffmpeg computes it, its units, scale, and source filter. Definitions only: no perceptual interpretation, no quality judgement, no threshold-to-meaning mapping.
Verified against FFmpeg filter docs (https://ffmpeg.org/ffmpeg-filters.html) and source on release/8.1 and master (libavfilter/af_aspectralstats.c, af_astats.c, af_loudnorm.c, doc/filters.texi). Where the fact-sheet flagged a derivation as medium-confidence, this skill marks it Confidence: medium.
aspectralstats - per-frame spectral statistics
Statistics run over the half-spectrum magnitude array magnitude[n] = hypotf(re, im) of length size = win_size/2; FFT output is pre-scaled by 1/win_size; scale = max_freq/size with max_freq = sample_rate/2. Metadata keys emit per channel as lavfi.aspectralstats.<N>.<key>.
| Metric | Measures | ffmpeg computation | Units | Range/scale | Confidence |
|---|---|---|---|---|---|
| mean | Arithmetic mean of magnitude bins | sum(mag[n]) / size |
magnitude (linear) | >= 0 | high |
| variance | Population variance of magnitudes about mean |
sum((mag[n]-mean)^2) / size |
magnitude^2 | >= 0 | high |
| centroid | Magnitude-weighted mean frequency | Sum(mag[n]*n*scale) / Sum mag[n] |
Hz | 0 .. max_freq | high |
| spread | Magnitude-weighted std-dev of frequency about centroid | sqrt( Sum(mag[n]*(n*scale-centroid)^2) / Sum mag[n] ) |
Hz | >= 0 | high |
| skewness | Third standardised spectral moment about centroid | Sum(mag[n]*(n*scale-centroid)^3) / (Sum mag[n] * spread^3) |
dimensionless | signed | high |
| kurtosis | Fourth standardised spectral moment about centroid | Sum(mag[n]*(n*scale-centroid)^4) / (Sum mag[n] * spread^4) |
dimensionless | >= 0 | high |
| entropy | Magnitude-weighted log-spread, normalised by log(N) |
-Sum(mag[n]*ln(mag[n]+eps)) / ln(size) |
dimensionless | unbounded; not a 0-1 probability entropy (magnitudes are raw, not normalised to sum 1) | medium |
| flatness | Geometric mean / arithmetic mean of magnitudes | exp(mean(ln(mag[n]+eps))) / mean(mag[n]+eps) |
dimensionless ratio | 0 .. 1 (linear, not dB) | high |
| crest | Spectral crest: peak bin / mean bin of the magnitude spectrum | max(mag[n]) / mean(mag[n]) |
dimensionless ratio | >= 1 | high |
| flux | L2 distance between this frame's and the previous frame's magnitude spectrum | sqrt( Sum(mag[n] - prev_mag[n])^2 ) |
magnitude (linear) | >= 0; per-frame; not normalised | high |
| slope | Linear-regression slope of magnitude vs normalised bin index | Sum(((n-m)/m)*(mag[n]-mean_mag)) / Sum((n-m)/m)^2, m = size*0.5 |
magnitude per normalised-bin | signed | medium |
| decrease | Relative spectral decrease from the first bin | Sum_{n>=1}((mag[n]-mag[0])/n) / Sum_{n>=1} mag[n] |
dimensionless | signed | medium |
| rolloff | Frequency below which 85% of cumulative magnitude lies | smallest n with Sum_{0..n} mag >= 0.85*Sum mag, returns n*scale |
Hz | 0 .. max_freq | high |
Source notes:
- entropy: ffmpeg applies
lnto raw magnitudes (not a normalised PMF) and divides byln(size); the result is not a conventional normalised Shannon entropy and is not bounded to[0,1]. - slope/decrease: docs are terse; the exact normalisation (
m = size*0.5, normalised index) is taken from source. - Division-by-zero guards return
1.f(centroid, spread, skewness, kurtosis, entropy) or0.f(flatness, crest, slope, decrease).
astats - time-domain level statistics
Samples are normalised to [-1, 1]; LINEAR_TO_DB(x) = 20*log10(x). Metadata key lavfi.astats.<N>.<Key>. The length option (default 0.05 s) sets the short window for Noise_floor, RMS_peak, and RMS_trough.
| Metric | Measures | ffmpeg computation | Units | Range/scale | Confidence |
|---|---|---|---|---|---|
| RMS level | RMS amplitude of samples | 20*log10( sqrt(Sum x^2 / N) ) |
dBFS | <= 0 (0 = full scale) | high |
| Peak level | Largest absolute sample | 20*log10( max(-nmin, nmax) ) |
dBFS | <= 0 | high |
| Crest factor | Peak amplitude / RMS amplitude (time domain) | max(-nmin, nmax) / sqrt(Sum x^2/N); returns 1 if RMS=0 |
linear ratio, dimensionless (not dB) | >= 1 | high |
| Dynamic range | Span between loudest and quietest non-zero sample | 20*log10( 2*max(|min|,|max|) / min_non_zero ) |
dB | >= 0 | high |
| Noise floor | Minimum local peak over the sliding window | 20*log10(noise_floor), min of per-window local peaks over length seconds |
dBFS | <= 0 | high |
| Flat factor | Run-length flatness at the min/max levels | 20*log10( (min_runs+max_runs)/(min_count+max_count) ) |
dB-scaled | >= 0 | medium |
| Peak count | Number of occasions (not samples) the signal hit Min or Max level | min_count + max_count |
count (integer) | >= 0 | high |
Source quirk (low impact): per-channel metadata Crest_factor numerator uses raw min/max while the console-log path uses normalised nmin/nmax; they agree for float/double formats. Both paths are peak/RMS linear ratios, never dB.
ebur128 - loudness
Logged values are labelled M, S, I, LRA, TPK/FTPK. Read-only metadata exports integrated, range, lra_low, lra_high, true_peak. Loudness uses K-weighting plus mean-square per ITU-R BS.1770; true peak uses libswresample oversampling.
| Metric | Measures | Computation / window | Units | Range/scale | Confidence |
|---|---|---|---|---|---|
| Integrated loudness (I) | Gated programme loudness over the whole input | BS.1770 K-weighted mean-square, two-stage gating | LUFS | typ. <= 0 | high |
| Loudness Range (LRA) | Statistical spread of short-term loudness | distribution of 3 s short-term values; lra_low/lra_high in LUFS |
LU | >= 0 | high |
| True Peak | Inter-sample peak on the oversampled signal | peak of the up-sampled signal via libswresample | dBTP | typ. <= 0 | high |
| Momentary (M) | Loudness over a 400 ms window | BS.1770 loudness, 400 ms sliding | LUFS (or LU if relative) | - | high |
| Short-term (S) | Loudness over a 3 s window | BS.1770 loudness, 3 s sliding | LUFS (or LU) | - | high |
Gating (BS.1770-2 onward, adopted by EBU R128 / Tech 3341): absolute gate -70 LUFS, then a relative gate -10 LU below the absolute-gated mean. (From the named standards, not the ffmpeg docs.)
loudnorm - EBU R128 normalisation (FFmpeg 8.1)
JSON stats print at filter teardown when print_format=json. Two measurement states: r128_in (raw input) and r128_out (post-processing output). Output is exactly these 10 keys; no gain/offset field exists in 8.1 (any blog or master claim of extra fields does not apply). 8.1 is byte-identical to master for options, struct, JSON, and decision regions.
| Field | Measures | I/O/Control | Units | Range/scale | Confidence |
|---|---|---|---|---|---|
input_i |
Integrated loudness of input | Input (measured) | LUFS | negative-going; %.2f |
high |
input_tp |
Max sample peak across channels of input, 20*log10(tp_in) |
Input (measured) | dBTP | typ. <= 0 | high (sample peak, not oversampled true-peak meter) |
input_lra |
Loudness range of input | Input (measured) | LU | >= 0 | high |
input_thresh |
Relative gating threshold of input | Input (measured) | LUFS | negative-going | high |
output_i |
Integrated loudness of processed output | Output (measured) | LUFS | negative-going | high |
output_tp |
Max sample peak across channels of output | Output (measured) | dBTP | typ. <= 0; %+.2f |
high |
output_lra |
Loudness range of processed output | Output (measured) | LU | >= 0 | high |
output_thresh |
Relative gating threshold of output | Output (measured) | LUFS | negative-going | high |
normalization_type |
Which path ran | Control (decision) | string | linear or dynamic |
high |
target_offset |
target_i - output_i: residual gap to target integrated loudness; feed back as offset on a second pass |
Control (residual) | LU | signed, ~0 | high |
loudnorm default targets and valid option ranges (AVOption table, 8.1; {default}, min, max):
I/i: -24.0, range -70.0 .. -5.0 (integrated loudness target, LUFS)LRA/lra: 7.0, range 1.0 .. 50.0 (loudness range target, LU)TP/tp: -2.0, range -9.0 .. 0.0 (max true peak, dBTP)linear: 1 (bool, default on) - linear normalisation attempted only if all fourmeasured_*values are supplieddual_mono: 0;print_format:none|json|summary
Method: K-weighted loudness per ITU-R BS.1770 / EBU R128 via the bundled ebur128. Dynamic mode (default unless linear preconditions are met) applies a per-frame Gaussian-smoothed gain envelope plus a true-peak limiter and forces the input to 192000 Hz internally. Linear mode applies a single static gain offset to all samples. normalization_type reports which ran. loudnorm's reported *_tp is sample-peak; the standalone ebur128 filter's True Peak is oversampled.
Disambiguations
- Spectral crest vs crest factor:
aspectralstats.crest=max(magnitude)/mean(magnitude)over the frequency-domain magnitude spectrum (per-frame spectral peakiness).astatsCrest factor =peak_sample/RMSin the time domain. Different domains; both are linear dimensionless ratios >= 1, never dB. Do not conflate. - Kurtosis baseline:
aspectralstats.kurtosisis Pearson kurtosis (4th-power moment /Sum mag * spread^4). No-3and no-0subtraction. It is not excess kurtosis. - Flatness scale: a 0-1 linear ratio (geometric mean / arithmetic mean of magnitudes), not dB.
1.0= flat spectrum, toward0= peaky. - Entropy scale/basis:
-Sum(mag*ln(mag+eps)) / ln(N), natural log, normalised byln(N)withN= bins. Magnitudes are raw (not a PMF), so this is not a conventional 0-1 normalised entropy and is not bounded to[0,1]. - Flux normalisation: L2 norm between consecutive frames, per-frame, not normalised by bin count or energy. The first frame compares against a zeroed previous frame.
- True-peak provenance:
ebur128True Peak is oversampled (per BS.1770 Annex 2).loudnorm*_tpis sample peak (20*log10(sample_peak)), not oversampled.
Standards and platform targets
External reference targets, not interpretation. Each row marks whether it is a formal standard or a platform norm.
| Target | Exact value | Tolerance / extras | Publisher | Classification |
|---|---|---|---|---|
| ITU-R BS.1770 | measurement method only (LKFS, dBTP) | 4x oversampling, K-weighting; defines method, not targets | ITU-R | standard |
| EBU R128 integrated | -23.0 LUFS | +/-1.0 LU (live); +/-0.5 LU non-live programme (since v3.0) | EBU | standard (Recommendation) |
| EBU R128 true peak ceiling | -1 dBTP | production max; meas. tol. +/-0.3 dB; meter per BS.1770/Tech 3341 | EBU / ITU-R | standard |
| ATSC A/85 (US TV) | -24 LKFS | +/-2 dB; CALM-Act-mandated for US commercials | ATSC | standard (US, legally mandated for commercials) |
| Spotify | -14 LUFS | playback ref (Loud -11 / Normal -14 / Quiet -19); mastering <= -1 dBTP | Spotify | platform-norm |
| Apple Podcasts | -16 LKFS | +/-1 dB; true peak <= -1 dBFS; per BS.1770-5 | Apple | platform-norm |
| Apple Music (Sound Check) | -16 LUFS | proprietary normalisation | Apple | platform-norm |
| YouTube | ~ -14 LUFS | observed normalisation reference; no formal spec | YouTube | platform-norm |
| ACX / Audible (audiobook) | -23 to -18 dB RMS | peak <= -3 dB; noise floor <= -60 dB RMS; submission gate (RMS/dBFS) | ACX (Amazon) | platform-norm (binding submission spec) |
| AES TD1004 / TD1008 (streaming) | -16 to -20 LUFS | max peak -1.0 dBTP; TD1008 (2021): -18 speech/mixed, -16 music | AES | recommendation (technical document) |
Provenance: formal standards are ITU-R BS.1770, EBU R128, and ATSC A/85 (A/85 is legally enforced in the US via the CALM Act for commercials). Platform norms are Spotify, Apple (Podcasts and Music), YouTube, and ACX. AES TD1004/TD1008 is an AES technical document/recommendation, not a formal standard.
Standards URLs:
- ITU-R BS.1770: https://www.itu.int/rec/R-REC-BS.1770/en
- EBU R128: https://tech.ebu.ch/publications/r128 (PDF: https://tech.ebu.ch/docs/r/r128.pdf)
- EBU Tech 3341: https://tech.ebu.ch/publications/tech3341
- EBU Tech 3342: https://tech.ebu.ch/publications/tech3342
- ATSC A/85: https://www.atsc.org/.../A85-2013.pdf
- AES TD1004: https://aes.org/.../AESTD1004_1_15_10.pdf
- Spotify: https://support.spotify.com/us/artists/article/loudness-normalization/
- Apple Podcasts: https://podcasters.apple.com/support/893-audio-requirements
- ACX: https://help.acx.com/s/article/what-are-the-acx-audio-submission-requirements
Producing the metrics
Canonical ffmpeg invocations. -f null - discards output and keeps the filter's printed stats.
# Per-frame spectral statistics (aspectralstats)
ffmpeg -i in.wav -af aspectralstats -f null -
# Time-domain level statistics (astats)
ffmpeg -i in.wav -af astats -f null -
# Loudness: integrated, LRA, true peak, momentary, short-term (ebur128)
ffmpeg -i in.wav -af ebur128 -f null -
# Loudness measurement as machine-readable JSON (loudnorm, measurement pass)
ffmpeg -i in.wav -af loudnorm=print_format=json -f null -
To collect per-frame aspectralstats or astats metadata for downstream parsing, route the metadata to a log with ametadata:
ffmpeg -i in.wav -af aspectralstats,ametadata=mode=print:file=spectral.log -f null -
ffmpeg -i in.wav -af astats=metadata=1,ametadata=mode=print:file=levels.log -f null -
Spectrogram image (showspectrumpic): renders the signal as a 2-D image. The x axis is time, the y axis is frequency, and magnitude is mapped to colour. scale selects the magnitude mapping (lin, log, sqrt, cbrt); fscale selects the frequency-axis mapping (lin or log).
ffmpeg -i in.wav -lavfi showspectrumpic=s=1280x480:scale=log:fscale=log spectrogram.png