All echolocating animals face an acoustic challenge. Only a tiny fraction of each echolocation cry returns as a faint reflection: most of the initial call is dispersed through the air. But, as an animal approaches its target, the volume of reflected cries can increase dramatically as the acoustic losses decline. Ulrik Nørum, Signe Brinkløv and Annemarie Surlykke explain that the intensity of a returning echo can increase by four orders of magnitude as a bat closes in from 5 m to 5 cm. However, to overcome the challenge of interpreting the reflections, bats dramatically reduce the volume of their echolocation cries during the final approach, and the pattern that was thought to best match this decline was a logarithmic decrease. However, the trio point out that the logarithmic model fails to accurately predict the bat's volume during the early stages of an approach, suggesting that remote bats produce cries that are much louder than is physically possible. So, Nørum and his colleagues developed another equation – where the volume starts from a maximum and decreases exponentially during an approach – to predict how inbound echolocating animals modulate their echolocation volume and tested the new equation's accuracy by comparing it with the precisely recorded echolocation cries of five species of bat exiting their roosts and hunting (p. 3045).
Successfully recording 53 echolocation sequences, the team found that the reduction in the bats' volume was better matched by the new exponential equation than by the earlier logarithmic model. In addition, the new equation also allows scientists to predict the animal's top volume and the point at which they begin to reduce the volume of echolocation calls as they approach an object. The team suggests that their new model could help us to understand how animals modulate their volume while communicating over a wide range of distances.