How to Read a Sonogram

Hearing the Recorded Bat Sounds

Humans cannot hear bat calls, which are very high pitched and above the range of human hearing. A bat detector used in the field simply translates calls down to a frequency humans can hear.  The calls are so fast, however, that most of them sound alike.  The bat sounds you can hear on this website enable us to distinguish among the calls because the calls are slowed down ten times.  Sounds are the primary clue helping researchers tell the difference between Yuma Bats and Little Brown Bats, who often roost together and who look very much alike. Researchers make recordings of the sounds in the field, and then analyze them on a computer before making identifications of each species.

Looking at Sonograms—Pictures of Bat Calls

Pictures of bat sounds are called sonograms. They show the pitch of each call, how this pitch varies up and down over time, how long each part of the call lasts, and how intense each part of the call is.

On this site, all sonograms of local bats are shown side by side on one page for comparison. Click on a specific sonogram for an enlarged view. Here is how to read the sonograms:

1. The call is shown in two ways, as a bright green line at the bottom of the frame, which is the oscillogram, and as bright blue colored images within the area above, which are the sonograms.
2. Time is shown in milliseconds along the horizontal ruler at the bottom.  Read from left to right. This ruler tells time for both the green oscillogram below it and for the sonogram above it.
3. When the oscillogram’s green line gets thicker, the sound is getting louder.  When the green line gets thinner, the sound is getting softer.  A very thin green line indicates silence.
4. On the sonogram, the vertical ruler along the left side shows frequency in kHz (kilohertz) of the sound.  The higher up, the higher the sound.
   
   
5. The blue images show the sound frequency over time.  For the Hoary bat, the blue sound image follows a relatively straight line.  The vertical ruler shows that the sound starts at 20 kHz and stays at that level.  It gets louder part way along, indicated by the red splotch within the blue. The green histogram at the bottom also shows the loudness and how it tapers off.
   
6. The calls of the Yuma bat start very high at 100 kHz and descend in pitch, getting louder near the lower pitch.  You will notice that the red color lines up with the widest part of the oscillogram’s green line below—both indicate loud sound.  The blue splotch above the main sound indicates overtones produced at the same time.
7. A hunting bat, shown by the “Silver-haired Bat Feeding” sonogram, first detects possible prey.  Then it emits extremely rapid and short bursts of sound as shown on the sonogram until it homes in on its prey.  There is a tiny pause as it gulps the insect.  Then it resumes echolocation, shown by the louder, longer sound at the end. 

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