There is no difference in principle between the infrasonic and ultrasonic
sounds which are inaudible to humans (or other animals) and the sounds that we can
hear. In all cases, sound is a wave of pressure and particle oscillations propagating
through an elastic medium, such as air. This chapter is about the physical laws that
govern how animals produce sound signals and how physical principles determine the
signals’ frequency content and sound level, the nature of the sound field (sound
pressure versus particle vibrations) as well as directional properties of the emitted
signal. Many of these properties are dictated by simple physical relationships between
the size of the sound emitter and the wavelength of emitted sound. The wavelengths of
the signals need to be sufficiently short in relation to the size of the emitter to allow for
the efficient production of propagating sound pressure waves. To produce directional
sounds, even higher frequencies and shorter wavelengths are needed. In this context
‘short’ is measured relative to the size of the sound source. Some sound sources, such
as dipoles and pistons, are inherently directional, whereas others, such as monopoles,
are inherently omnidirectional.
Keywords: Bioacoustics, ka product, Sound production, Sound source.
