Much of sound editing and analysis, particularly for animal sounds, is done using spectrograms. Spectrograms are graphs that show the frequency (or pitch) on the y-axis and time on the x-axis. The loudness of the sound is indicated by the intensity of the color. They read somewhat like a musical score, except that time is measured more continuously on the x-axis. But like a musical score, higher notes appear higher on the y-axis.
In addition to using the visual representations of the sounds to fine-tune editing or help identify species, the sound images are just beautiful and fascinating. For example, last summer I recorded a couple of species of bush crickets (katydids) in my backyard, which were broadcasting in the ultrasound (> 20 kHz; humans hear from roughly 20 Hz to 20,000 Hz (or 20 kHz), although very few adults can hear above 15 kHz). The recording was made about 11 pm.
A short snippet of it looks like this:
The scale on the left goes from 0-100 kHz and the scale on the bottom cover just over 13 seconds. Sounds below 10 kHz were filtered out (traffic, dogs barking, etc.). The image shows two different crickets calling – one almost continuously at 20-25 kHz, and the other intermittently broadcasts syllables from 20 to about 50 kHz. These sounds are stridulations, made by the crickets rubbing their outer wings over a file located on the cuticle (like rubbing your finger over the teeth of a plastic comb).
This is what is sounds like (slowed down to make it audible, recording covers a longer time period than the spectrogram above, and includes the feeding calls of a bat near the end).
Can you hear the difference between the two crickets? If we zoom in on the syllable on the left, so that the time scale covers 6 seconds, we can see more detail:
And if we zoom in even further, so that the time scale is only 3 seconds, you really start to see the intricate nature of the syllable:
Again, all of these sounds are produced above our range of hearing!
Bird songs can be even more remarkable. Birds, of course, are master singers, even capable of producing more than one note at the same time. Luckily for us, most of their songs are produced in the range at which we hear the best – 2 to 5 kHz. Some say that this is not coincidental; our hearing is adapted to bird song, as birds are a fantastic early warning system of danger in the area. Here is a short snippet of birds in my front yard:
And here is what part of the spectrogram looks like:
In this case, the pitch scale on the left only goes to 24 kHz, and the time scale is about 20 seconds. Note that, in general, each bird occupies its own little acoustic space and minimizes overlapping with other species in either pitch or time. Bernie Krause refers to these separations as Acoustical Niches, and believes they are fundamental in shaping the acoustic environment of each habitat, giving each soundscape a unique signature.
References:
Krause, B. 2012. The Great Animal Orchestra. Little, Brown, and Co.
Sales, G. and D. Pye. 1974. Ultrasonic communication by animals. Chapman and Hall.
Recording notes: bush crickets recorded with a Dodotronics Ultramic 200K and Acer Iconia W500 tablet. Birds recorded with a Sony PCM-M10 and Audio-Technica AT2022 with FEL SK3.5 pre-amp. Recordings analyzed in Adobe Audition and Raven Lite. Labels added to bird spectrogram in Adobe Photoshop.
A longer version of the recording of the birds in my yard appears in the album, Sonoran Desert Spring.
Facinating!