Radiation efficiency for long-range vocal communication in mammals and birds.

Long-distance vocal communication by birds and mammals, including humans, is facilitated largely by radiation efficiency from the mouth or beak. Here, this efficiency is defined and quantified. It depends on frequency content of vocalization, mouth opening, head and upper body geometry, and directionality. Each of these factors is described mathematically with a piston-in-a-sphere model. While this model is considered a classic, never before has the high frequency solution been applied in detail to vocalization. Results indicate that frequency content in the 1-50 kHz range can be radiated with nearly 100% efficiency if a reactance peak in the radiation impedance is utilized with adjustments of head size, mouth opening, and beam direction. Without these adjustments, radiation efficiency is generally below 1%, especially in human speech where a high fundamental frequency is a disadvantage for intelligibility. Thus, two distinct modes of vocal communication are identified, (1) short range with optimized information transfer and (2) long range with maximum efficiency for release of acoustic power.