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

444 16. Ultrasonicscatching small insects at the rate of one every 10 s for as long as half an hour. Theirability to discriminate between objects such as food and raindrops or foliage canbe described as being nothing less than phenomenal; and yet when a large numberof bats fly in close proximity to each other in a potentially confusing backgroundof ultrasonic noise, they continue to locate prey and avoid collisions with eachother.The Noctillio bat of Trinidad catches small fish by dipping its feet below thesurface of the water, after emitting a series of repetitive pulses. It has been conjecturedthat the characteristic ripples created by the fish are being detected by the batrather than echoes from an object beneath the water surface. This is particularlyremarkable in view of the fact that the sound must penetrate a barrier with a veryhigh-reflection coefficient. As with bats, the echolocation of porpoises appears tobe unaffected by the presence of interfering noises or jamming.Moths, a prime target of bats, use ultrasonics for self-defense. When a moth detectsa sonic pulse from a bat, it immediately takes evasive action through zigzaggingin its flight and executing power dives. Moths can detect bats as far away as13 m, and their ears can detect the cries of approaching bats at this distance, butwhen the bats are moving away their ears stop registering at about this distance.It was also observed by Roeder and Treat that when a bat makes a straight-onapproach, it was observed to emit an uneven, sporadic signal; this was interpretedas an indication that the bat was counter-maneuvering by varying the intensity ofits sonar pings.Cetaceans constitute a group of sea mammals, which includes whales, dolphins,and porpoises. They are extremely intelligent as well as beautiful creatures, andare of the greatest interest to acousticians. Porpoise sounds have been describedas whistling, barking, rasping, repetitive clicks or pulses, mewing. The cetaceansemit signals for the purpose of echolocation and communication with each other.Signals as high as 170 kHz have been observed in the clicks of porpoises, whichvary in repetition rate from five clicks to several hundred per second. Extensiveobservations are being conducted to observe the emission patterns of differentcetacean species, which vary from location to location at different times of theyear even with the same group tracked on an almost continuous basis.At least two species of birds, Steatornia and Collocalla, are known to be echolocators.Ornithologists have reported that calls by various birds may be comprehendedonly by members of their own local flocks. Gulls or crows will alwaysrespond to calls from members of their own flocks, but they may or may notrespond to distress or assembly calls from gulls or crows from another region.The hearing of dogs extends well beyond the frequency range of human hearing.The “silent” dog whistles that generate ultrasonic output can thus be used to summona dog. Although dogs are generally endowed with especially keen senses ofsmell, hearing, and sight, there is no evidence that they make use of echolocation.Certain effects become more evident in wave propagation through a mediumas the acoustic signal extends into the ultrasonic range. The attenuation of thesignal’s amplitude occurs not only because of the spreading of the wave front, butalso because of the conversion of the acoustical energy into heat and scattering