Visualizing sounds with spectrograms

The graphs you see next to each of our sound clips are spectrograms: a visual aid to understanding the sound you’re hearing. Frequency (or pitch) is graphed on the vertical axis; time is on the horizontal axis. Darker parts of the graph are louder.

The scream call 

As a sound plays you can follow it across the graph, left to right. Listen to the five right whale “scream” calls in this clip. Each one starts high and then drops quickly, just like the comma-shaped figures on the graph. If you listen closely, you can notice fine details, such as how the second call drops off immediately, without the brief, level introduction of the other four calls.

The moan call

Now look at this right whale “moan” call. It lasts a lot longer than the screams, and it stays on a nearly level pitch, with just a slight rise in the middle. The three separate lines indicate that the eerie, foghorn-like sound is actually a combination of three pitches, though the sounds are difficult for most people to separate.

A series of right whale up-calls

Spectrograms are a great way to dissect a sound. The graph lets you isolate parts of a recording, just as you might zoom in on the pixels of a digital photograph. By analyzing attributes of the pixels in a spectrogram, the auto-detection software can uncover a right whale’s quiet up-calls hidden in a blanket of noise.

Here’s an example: Listening to the sound alone in this clip, it’s difficult to pick out the 11 up-calls--but on the spectrogram the calls are plain to see.

More about spectrograms--Bioacoustics Research Program site