Chris Corben’s Anabat system popularized this method for analyzing bat calls. Chris deserves considerable praise for advancing the endeavor of acoustic recognition of bats. In fact, Chris accomplished a programming tour de force to enable Anabat to operate successfully on pre-Pentium laptops.
Sounds, and other vibrations, oscillate around an equilibrium, or zero point. Zero-crossing is a speedy algorithm for extracting the primary frequency information from such an oscillating signal. To enable its speed, the zero-crossing algorithm only keeps track of when a signal crosses the zero position. However, this speed comes at the sacrifice of information as zero-crossing disregards the magnitude of the oscillation (amplitude), and any other smaller magnitude oscillations that do not reach the zero position, e.g. harmonics.