Infrasonic Calls and How They Work
Pedestrians can hear the thump of bass coming from an overamplified car stereo while remaining blissfully unaware of the higher pitches. We hear the rumble of distant thunder but can’t hear the crack of lightning unless it strikes close by. We hear the bass in these sounds but not the treble because powerful low-frequency sound travels long distances well. But the bass humans hear cuts off at about 20 Hz. Below that, a human can only feel the sound at close range, as it vibrates his or her chest. Elephants, whales, hippopotamuses, the okapi and rhinoceroses, on the other hand, appear to hear and produce sounds well below our range. And it’s not just the large animals that hear infrasound. Pigeons, guinea fowl, cod, cuttlefish, octopus, squid and the capercaille, a Eurasian grouse, all hear infrasound.
Sound, as vertebrates hear it, consists of waves of relatively high and low air pressure. (See discussions of bees and mosquitoes in previous chapters for other ways to hear.) When these waves reach the eardrum, they push it in and pull it out, setting in motion a vibration transmitted through the middle ear bones to the cochlea, where specialized cells produce nerve impulses. Our brains interpret these impulses as sound. The waves we hear best range from about a yard peak to peak down to tiny fractions of an inch. Infrasound waves range from tens of yards to miles in length.
These long waves travel through brush and trees relatively unimpeded because the ability of an object to reflect a sound wave depends on the ratio of the wavelength to the size of the object. Small objects such as grass stems, leaves and trees have no effect on very long infrasound waves. On the other hand, they reflect and scatter higher frequencies easily. Even the molecules in air absorb a good deal of high frequency sound, while leaving infrasound unaffected. This makes infrasound ideal for long-distance communication.
Sound travels from its source in all directions losing about 6 decibels for every doubling of distance. With no interruption, the sound will spread spherically. (Certain shapes can modify this spreading effect. A horn couples the sound sourcea trumpet player’s lips or the tiny speaker in a horn-loaded audio speaker systemwith the air. The larger opening then becomes the sound source, and sound spreads from there.) In a field study of infrasound in Africa, Langbauer found that environments such as bare ground, tall grass and woodland had little or no effect on sounds below 60 Hz projected from a custom-made Pachyderm 2 loudspeaker and recorded at four distances from about 10 to 125 yards from the speaker.
Even very large speakers can’t duplicate the power of elephant infrasonic calls, which have been measured at near-thunder levels about five yards from the elephant. The speakers used in field experiments, although huge by home stereo standards, can only produce volumes half that. Extrapolating from playback experiments, Langbauer estimates elephants can hear infrasonic calls at least 2.5 miles away.