Onset Detection

An onset is the moment a musical event begins — a note struck, a drum hit, a consonant sung. Finding onsets is the root of the whole rhythm family: tempo, beats, and even section boundaries all grow from onset information. This page expands the timing section of MIR Overview.

Onset vs. transient vs. beat

These three are easy to confuse:

• An onset is the start of any sound event, whether or not it lands on a beat.
• A transient is the short, high-energy burst at the very front of a sound (the stick hitting the drum). Onsets are detected from transients and other spectral changes.
• A beat is a perceived pulse of the music. Beats usually coincide with onsets, but not every onset is a beat (a fast drum fill has many onsets between two beats).

So the pipeline is: spectral change → onsets → tempo/beats.

The onset-strength envelope

libsonare does not just emit a list of onset times. It first computes an onset-strength envelope: a continuous curve that rises wherever the spectrum changes abruptly.

The curve is built by measuring frame-to-frame increases in spectral energy, a spectral flux idea. When new energy appears, the curve spikes. During a sustained note, it stays low.

Peaks in this envelope are the candidate onsets.

The envelope matters because the shape of the curve, not just the peak list, is what tempo and beat algorithms analyze. A clean, peaky envelope gives confident tempo estimates; a smeared one does not.

Why this drives tempo and beats

A steady groove makes the onset envelope periodic — peaks recur at roughly even spacing. Tempo estimation looks for that period; beat tracking then places a pulse train onto the peaks. Both read the same envelope, which is why improving onset detection improves tempo and beat accuracy together.

DETECTOR · ONSET / BEATIDLE

Onsets vs beats — from attacks to a pulse

Onset detection marks every attack in the audio; beat tracking distils those into the steady pulse you would tap along to. Switch the view, then press play to watch each marker fire as the playhead reaches it.

Detect

How libsonare computes onset strength

libsonare derives the onset-strength envelope from STFT magnitudes by summing positive frame-to-frame spectral differences, optionally per mel band, then aggregating into a single curve. The envelope feeds the tempogram (for BPM) and the beat-tracking dynamic program, and it is also exposed for visualizers that want onset-driven motion. Because it is built on the shared STFT, requesting onsets alongside other features does not recompute the spectrogram.

Related: MIR Overview, Tempo and BPM, Beats and Downbeats, Spectrogram and STFT