Spectrogram Analysis FAQ: What is a spectrogram? A spectrogram is a graphic representation of the frequency and intensity of a sound as it travels through time. In other words, a spectrogram is a picture of a sound. Spectrograms can be generated for almost any sound or vibration, regardless of the medium it travels through, be it air, water, cement, or soil. Spectrograms are powerful tools for studying the complexity of animal sounds. What information does a spectrogram show? Time: The x-‐axis of a spectrogram represents time, usually in seconds (s). It shows how long a given sound lasts—that is, its duration—as well as the rate of sound, or how many sounds occur in a given time frame. For example, if you recorded one person clapping her hands quickly and another clapping his hands slowly, the spectra of the person clapping quickly would appear closer together on the spectrogram than the slow clapper’s spectra. Frequency: The frequency of sound is the number of sound wave cycles that are transmitted per second and is measured in a unit called “Hertz” (Hz). We interpret frequency as pitch. High-‐pitched sounds are composed of high frequency waves, and low-‐pitched sounds are composed of low frequency waves. In a spectrogram, the vertical (y) axis represents frequency. The spectrogram below shows how even an apparently “pure” sound, like the snap of the finger, actually spans many different frequencies. Fig 1: Six snaps, each spanning a 20kHz frequency range. Note the slightly wider frequency bands at 3kHz indicating a longer duration at that frequency. Amplitude: The amount of energy being carried by a sound wave corresponds to the height, or amplitude, of that sound. What we interpret as the volume of a sound depends on the amplitude of the sound wave—for example, louder sounds consist of waves with higher amplitudes. In a spectrogram, amplitude is represented by the contrast between the background color and the color of the sound spectra: the louder the sound, the more its spectra will contrast with the background. How can I make my own spectrogram? Though there are many freely available programs for sound analysis, we suggest using Raven Lite, from the Cornell Lab of Ornithology’s Bioacoustics Research Program. Raven Lite is “a free software program that lets users record, save, and visualize sounds as spectrograms and waveforms. Raven Lite is intended for students, educators, and hobbyists, and can be used for learning about sounds, as an aid in birdsong recognition, and in musical instruction.” You can download it here. For a more complete discussion of spectrographic sound analysis and for information on how spectrograms are generated and interpreted, try this set of guides from The Cornell lab of Ornithology Bioacoustics Research Program
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