To Shout or to Whisper? Strategies for Encoding Public and Private Information in Sound Signals

Abstract

To make sound communication public or private is a question of making the active space of the emitted sound signals large or small. A sender can only encode the sound signals within the parameter space defined by its own anatomy and physiology but, in addition, it may choose acoustic behaviors that will help to increase or decrease the active space. Both signal encoding and behavioral choices are limited by the acoustical properties of the noisy environment that changes the propagating sound signal. To make the sound signals public by increasing the size of the active space, the sender must try to overcome the limitations of the environment, whereas to make the sound signals private the sender could make use of these limitations to reduce the size of the active space. Signal encoding is also limited by the auditory physiology of the receiver whose auditory sensitivity and critical ratio relative to the ambient sound level determine the distance, over which a sound signal can be received. The best documented and most important parameter for changing the size of the active space is the source level of the emitted sound, which may be divided into high-amplitude broadcast sounds and low-amplitude soft sounds. In addition, sound signal frequency parameters may help to improve reception of public sound signals and to reduce reception of private signals, whereas other potential active space regulating parameters such as signal duration only follow predictions for some species.