Bat sounds
Many textbooks will tell you that you can hear from 20 Hz to20 kHz. Don’t believe them: if you are over twenty, you are probably deaf to sounds above 17 kHz. The high-frequency limit of our hearing declines so significantly and so predictably with age that a youth-repelling generator of higher-frequency sound called the Mosquito has been used by irate shopkeepers since 2009, as only teenage ears (and those of children and babies) are still capable of detecting the annoying tones the device produces. However, it is probably fair to say that we who cannot hear anything about17 kHz are not inconvenienced by the fact, showing that it is of little evolutionary benefit to hear such high notes, otherwise we would have developed more robust hearing systems.
Nevertheless, we are missing out—not because the ultrasonic soundscape is an especially rich one, but because we cannot exploit some handy physical properties of sounds that are negligible at those frequencies that we can hear. The bat, however, is not so handicapped.
The sophistication with which bats employ ultrasound is astounding. In pitch darkness, an Egyptian fruit bat (Rousettus aegyptiacus) with an 80 cm wingspan can easily ffy between vertical rods just 53 cm apart without touching them. To achieve such feats, and to hunt, bats use echolocation—generating ultrasounds and timing the delays until they hear the echoes—which inform them of the distances of nearby objects.
Sightless humans approximate this, judging distances to walls from echo delays, but what they end up with is not a vision of the world. It cannot be, at the wavelengths they can hear: imagine a jetty standing in a calm lake. A light breeze blows, forming ripples about a centimetre in wavelength, which bounce ofl the jetty’s columns in circular patterns (Figure 21). Later, a gale sets in, making much longer waves, which roll past the columns as if they’re not there. For waves of sound, the same principle applies: they are only aflected by (and hence can only detect) obstacles larger than they are.