The latter category can be quantified as ambient noise, which is variable between sites depending on localized sources such as shipping traffic, and biologics. To detect a deliberate signal, it must be detectable above ambient noise. In other words, ambient noise acts to mask a signal. Noise most effectively masks a signal when the frequencies of the two sounds closely match. The level a signal has to be above ambient noise in order to be detectable is referred to as a Critical Ratio.
Critical ratios are species specific.
Outside of a particular range of frequencies, sounds may not be audible to a species at all. The below text and downloads are taken from the program's website, with permission. If you need the .pdf viewer, visit here . Digital Representation of Sound provides a brief explanation of how sound is represented digitally. A Biologist's Introduction to Spectrum Analysis , provides some conceptual background for making and interpreting spectrograms and spectra with Canary.