THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

238 10. Physiology of Hearing and Psychoacoustics100,000FREQUENCY RANGE, HERTZ10,0001,000100100BAT CAT DOG DOLPHIN GRASSHOPPER HUMANTYPE OF ANIMALFigure 10.9. Range of hearing in animals. Many animals hear a much wider range offrequencies than the human ear can sense.The term echolocation refers to an ability that odontocetes (and a few otheranimals such as bats) possess that enables them to locate and discriminate objectsby projecting high-frequency sound waves and listening for the echoes. The hearingof bats spans a 10 Hz–100 kHz range. Big brown bats (Eptesicus fuscus) producethe frequency range of 10–100 kHz for sonar or for acoustic social communicationand they also hear those ultrasonic frequencies. They have a lower frequency regionof auditory sensitivity from 10 Hz to 5 kHz and may use these lower frequenciesto detect insect prey by passive hearing of the insect’s own sounds. The hearingis tuned to 0.7–1.3 kHz indicating that some specialization of the auditory systemmay underlie the capacity to hear these lower frequencies.An Indian elephant is sensitive to low-frequency tones and could hear as lowas 16 Hz at 65 dB. However, the elephant is insensitive to high-frequency tonesand it generally could not hear above 12 kHz. The high-frequency hearing abilityis the poorest of any mammal yet tested and the failure of the elephant to hearmuch above 10 kHz demonstrates that the inverse correlation between the headsize (i.e., the interaural distance) and high-frequency hearing acuity is valid evenfor the largest of terrestrial mammals.Psychophysical investigations in a number of avian (bird) species over the pastthree decades have added significantly to the knowledge of hearing capabilitiesof this vertebrate group. Behavioral measurements of absolute auditory sensitivityin a wide variety of birds show a region of maximum sensitivity between 1 and5 kHz, with a rapid decrease in sensitivity at higher frequencies. Data accumulatedto date suggest that, in the region of 1–5 kHz, birds show a level of hearing