A rising sweep is the easiest sound to sanity-check by eye: its ridge should climb steadily
across the heatmap, then flatten when the tone holds. That is exactly what renders here —
a 3 s linear chirp 200 → 4000 Hz plus a 0.5 s steady 1 kHz tail at sr=22050 goes through
feature.melspectrogram (n_mels=128) → convert.power_to_db →
specshow (magma), with the MFCC(13) matrix and waveshow below.
Before the tier-2 repair, specshow(melspectrogram(y)) threw
'cmap: data contains no valid finite values' on the package's own
Array<Float32Array> format (Array.prototype.flat does not flatten typed-array rows).
Numeric asserts were node-verified against dist before this page was written
(rho 0.9998, steady bin 38, mfcc 13×151, NoteFormatter A4).