Spectrograph / Spectrogram
Definition
Spectrograph / Spectrogram
A spectrograph (or spectrogram) displays the frequency content of an audio signal over time as a two-dimensional image, with frequency on one axis, time on the other, and amplitude represented by color intensity. This time-frequency representation reveals patterns invisible in standard spectrum or waveform views, including sweeping tones, intermittent noise, and modulation effects. SonaVyx includes a real-time spectrograph in its measurement workspace.
How It Is Measured
The spectrograph is generated by computing successive short-time FFTs on overlapping segments of the audio signal and mapping each spectrum as a vertical line of colored pixels. SonaVyx uses Canvas 2D rendering with a logarithmic frequency axis to match human hearing perception. Color mapping ranges from dark blue (quiet) through green and yellow to red (loud), with configurable history depth from 5 to 60 seconds.
Practical Example
During a venue sound check, the SonaVyx spectrograph reveals a horizontal bright line at 120 Hz that persists regardless of program material — indicating a ground loop hum (second harmonic of 60 Hz mains). The constant frequency and steady amplitude distinguish it from musical content. The engineer traces and eliminates the ground loop, and the 120 Hz line disappears from the spectrograph.
Time-Frequency Trade-off
The spectrograph inherits the FFT time-frequency trade-off. Longer FFT windows provide better frequency resolution but poorer time resolution (features smear in time). Shorter windows give better temporal precision but coarser frequency resolution. SonaVyx uses a 4096-point FFT by default, providing a good balance of 11.7 Hz frequency resolution and 85 ms time resolution at 48 kHz sample rate.
Pattern Recognition
Spectrographs reveal diagnostic patterns that other displays miss. Feedback appears as a narrow horizontal line at the ringing frequency. Sweeps appear as diagonal lines. Comb filtering shows as horizontal striping. HVAC noise appears as broadband energy concentrated at low frequencies. Speech appears as time-varying formant tracks. These patterns help identify problems quickly during live events or diagnostic sessions.
Envelope and Modulation
The spectrograph reveals amplitude modulation patterns — compressor pumping appears as rhythmic brightening and dimming across all frequencies. Power supply ripple shows as modulation at mains frequency (50 or 60 Hz). Flutter echo appears as rapid temporal repetitions in the mid-frequency range. SonaVyx spectrograph mode complements the standard RTA and transfer function displays for comprehensive system analysis.
Try It Now
View real-time spectrograph — free audio analysis in your browser