The spectrum analyzer above gives us a graph of all the frequencies that are present during an audio recording at a given time. The resulting graph is understood as a spectrogram. The darker areas are those where the frequencies have very low intensities, and therefore the orange and yellow areas represent frequencies that have high intensities within the sound. You’ll toggle between a linear or logarithmic frequency scale by ticking or unsticking the logarithmic frequency checkbox.

In some ways, this demo is analogous to the Virtual Oscilloscope demo, but there’s an important and really important difference. Within the oscilloscope demo, the plot shows the displacement of an audio signal versus the time, which is named the time-domain signal. This demo shows the signal represented during a different way: the frequency domain. The frequency spectrum is generated by applying a Fourier transform to the time-domain signal.

The demo above allows you to pick variety of preset audio files, like whale/dolphin clicks, police sirens, bird songs, whistling, musical instruments and even an old 56k dial-up modem. Each of those has unique and interesting patterns for you to watch. Additionally, you’ll upload your own audio files. to look at the spectrogram, choose your sound input, then click the play button and therefore the graph will appear on the screen, moving from right to left. You’ll stop the motion by clicking the pause button on the audio player.

The violin recording especially clearly demonstrates the rich harmonic content for every note played (this appears on the spectrogram as multiple higher frequencies being generated for every fundamental frequency). This is often in contrast to the whistling recording which features a very strong fundamental component, and has just one additional harmonic, indicating that a person’s whistle is extremely on the brink of a pure wave.

Here are some links to sites that have interesting sound files with which you’ll generate your own spectrograms.

Xenon-canto – a really large database of bird calls from round the world

Underwater sounds recorded in Glacier Bay