A spectrogram shows the frequency content of audio changing over time. Harmonics appear as horizontal lines, transients as vertical lines, vibrato as wavy lines, and noise as diffuse color. Spectrograms reveal: sibilance (bright vertical streaks at 5–10 kHz), room resonances (persistent horizontal lines), and reverb tails (gradual fading of all frequencies).
iZotope RX's spectral editor lets you literally see and erase unwanted sounds in the spectrogram — removing a cough from a recording, eliminating string squeaks, or cleaning up dialogue. Most EQs now include a real-time spectrogram behind the frequency curve.