STILL / FIELD NOTES 08

Spectrogram vs spectrum: frequency over time

A spectrum shows how signal energy is distributed across frequency for a current analysis window. A spectrogram adds time, so repeated or changing frequency components become easier to follow. Still includes both views.

September 8, 2026 · Arve Tech, maker of Still

Read the axes before the colors

Check which axis represents time and which represents frequency. Color indicates the level mapping used by the display, not an independent safety classification. A bright region can be prominent relative to the selected display range without proving a hazardous exposure.

Use repeatable events

A source that cycles on and off may form a repeating pattern. Record the event timing, the selected frequency scale and the microphone setup. A graph cannot establish the source's identity or a legal noise threshold without additional evidence.

Try a repeatable approach

1. Choose spectrum for a frequency view or spectrogram for changes over time.
2. Keep display and microphone settings consistent.
3. Write what physically changed during the observation.

Source and scope

Brüel & Kjær: environmental noise measurement handbook

Measurement guidance is educational. Still is not a certified sound level meter and does not establish medical, legal or workplace-compliance conclusions. App-specific statements describe version 1.0.0, currently awaiting App Store review.