This is meant to be a brief, non-technical introduction to interpreting sonagrams. A sonogram (synonyms: sonagram or spectrogram) is a representation of a sound with frequency (pitch) on the y-axis vs. time on the x-axis. Sonograms provide images that can be intuitively interpreted and directly measured and compared qualitatively and quantitatively. Many types of bird vocalizations are readily recognizable on sonograms, and it is easy to distinguish the different main types of vocalizations, e.g. whistles, trills, nasal notes, harsh calls, chips, etc. Some sound types have harmonics, which are consecutive multiples of the fundamental (main) frequency. The fundamental frequency is typically the lowest band of a series of harmonics; if the sonogram is in color it will be in the brightest color, while in a grayscale sonogram it will be the darkest and widest harmonic.
How to recognize different sounds on sonograms:
Screeches or hisses are broadband, indistinct ‘smears’. The harsher or harder the note the more distinct the pattern. Rattles are quickly repeated, short hard screech-type notes.
Nasal sounds such as quacks or caws appear as series of several closely spaced harmonics of similar power, such that it is often difficult to determine which is the fundamental frequency. Metallic sounds can look similar but typically have more widely spaced harmonics and often harsh components.
Whistles are thin mostly horizontal notes, with or without harmonics. If harmonics are present, the sound is typically richer than if they are lacking. The less discrete or “fuzzier” looking the whistle, the buzzier it sounds.
Chirps and chips are very short nearly vertical broadband notes. Trills are quickly repeated series of chirps or chips.
Warbles are complex mixes of quickly modulated whistles and sometimes other notes.
Not all qualities of bird sounds are readily apparent in sonograms, for example it is difficult to distinguish a “hoot” or a “coo” from a whistle, except by their typically low pitch.
- The waveform or oscillogram (the upper graph for each AVoCet recording) shows power or amplitude (loudness) vs. time. Waveforms are particularly useful in visualizing or analyzing pattern and rhythm. Waveforms will be clearest for recordings with little background or other noise. Most recordings show a much clearer waveform if the lowest frequencies are filtered (removed).
- The spectrum (the lower graph for each AVoCet recording) shows power vs. frequency, giving a profile of which frequencies exhibit the greatest amplitude. The pattern of occurrence and number of power peaks can be highly characteristic to species, and provides yet another way to view and compare vocalizations. In order to show a spectrum in Raven, a section of the recording must be selected, and this is shown in pale pink in the image files. The spectrum therefore will be highly dependent on which sector of a given recording is chosen, particularly for variable, non-repetitive sounds. It is also strongly influenced by other sounds in the segment of the recording selected, for example anthropogenic sound at low frequencies, and insect sound at high frequencies.
- Making your own sonograms:
- There are now many software packages that make sonograms, some of which can be downloaded for free, while most others require a license. We use Raven Pro (Interactive Sound Analysis Software; Cornell Lab of Bioacoustics Research Program; http://www.birds.cornell.edu/brp/raven/RavenOverview.html), which is a sound analysis program in which a limited amount of editing can be done. There are a variety of licensing options for Raven, including a free lite version.