Many marine and terrestrial animals make sounds. Sound travels in waves through water or air. We can make measurements from the sound waves such as frequency and amplitude, which tell us about the pitch and loudness of the sound, respectively. It is important to understand these features of different animal sounds, as they can help us to estimate how far away we can record these sounds with microphones or hydrophones (underwater microphones). Sometimes we make recordings but we are not sure what animal produced the sounds, so all we can do is describe the signal.
An example of estimating how far away we can detect high frequency Blainville's beaked whale clicks:
Marques, T.A., L. Thomas, J. Ward, N. DiMarzio and P. L. Tyack. 2009. Estimating cetacean population density using fixed passive acoustic sensors: an example with beaked whales. Journal of the Acoustical Society of America 125: 1982-1994.
An example of estimating how far away we can detect low frequency right whale calls:
Marques, T.A., L. Thomas, L. Munger, S. Wiggins and J.A. Hildebrand. 2011. Estimating North Pacific right whale (Eubalaena japonica) density using passive acoustic cue counting. Endangered Species Research 13: 163-172.
An example of describing unknown signals (which we suspect to be blue whale calls):
Sousa, A. G. & Harris, D. 2015. Description and seasonal detection of two potential whale calls recorded in the Indian Ocean. Journal of the Acoustical Society of America. 138: 3, 1379-1388.