What is a Sound Spectrogram?
A sound spectrogram, like a musical score, is a visual representation of sound. As in musical notation, the horizontal dimension corresponds to time (reading from left to right), and the vertical dimension corresponds to frequency (or pitch), with higher sounds shown higher on the display. Frequency is measured in Hertz (Hz), or cycles per second; and in kilohertz (kHz) or thousands of cycles per second. The relative intensity of the sound at any particular time and frequency is indicated by the color of the spectrogram at that point.
A spectrogram provides more complete and precise information than a musical score because it is based on actual measurements of the changing frequency content of a sound over time, typically made by specialized computer software or hardware, such as Raven.
The illustration below is a sound spectrogram of a banded wren (Thryothorus pleurostictus), a common and vocally active species that inhabits the tropical dry deciduous forest of the Pacific slope of Central America. This song was recorded in Costa Rica during a research project studying song-type sharing, and contains a wide variety of song elements, which can be distinguished visually in the spectrogram.