Infrasound is sound at frequencies lower than the low frequency end of human hearing threshold at 20 Hz. It is known, however, that humans can perceive sounds below this frequency at very high pressure levels. Infrasound can come from many natural as well as man-made sources, including weather patterns, topographic features, ocean wave activity, thunderstorms, geomagnetic storms, earthquakes, jet streams, mountain ranges, and rocket launchings. Infrasounds are also present in the vocalizations of some animals. Low frequency sounds can travel for long distances with very little attenuation and can be detected hundreds of miles away from their sources.
Mammals The production and perception of infrasound has been observed in multiple mammals, including whale, elephant, giraffe, hippopotamus, and rhinoceros. For most of these animals, observations are preliminary and their sensitivity to infrasound has not been quantified. If an animal produces a low frequency sound, and uses it in communication, it suggests the animal might also be sensitive to infrasound.
Elephants Elephants are the terrestrial animal in which the production of infrasonic calls was first noted by M. Krishnan, later discovered by Katy Payne. The use of low frequency sounds to communicate over long distances may explain certain elephant behaviors that have previously puzzled observers. Elephant groups that are separated by several kilometers have been observed to travel in parallel or to change the direction simultaneously and move directly towards each other in order to meet. The time of estrus for females is asynchronous, lasts only for a few days, and occurs only every several years. Nevertheless, males, which usually wander apart from female groups, rapidly gather from many directions to compete for a receptive female. Since infrasound can travel for very long distances, it has been suggested that calls in the infrasonic range might be important for long distance communication for such coordinated behaviors among separated elephants.
Infrasound production and perception Recordings and playback experiments support that elephants use the infrasonic components of their calls for communication. Infrasonic vocalizations have been recorded from captive elephants in many different situations. The structure of the calls varies greatly but most of them range in frequency from 14 to 24 Hz, with durations of 10–15 seconds. When the nearest elephant is 5 m from the microphone, the recorded sound pressure levels can be 85 to 90 dB SPL. Some of these calls are completely inaudible to humans, while others have audible components that are probably due to higher frequency harmonics of below 20 Hz fundamentals. Sometimes, vocalizations cause perceptible rumbles that are accompanied by a fluttering of the skin on the calling elephant's forehead where the nasal passage enters the skull. This fluttering can also occur without causing any perceptible sound, suggesting the production of a purely infrasonic call. The mechanism of infrasonic call production in elephants has not been determined. Playback experiments using prerecorded elephant vocalizations show that elephants can perceive infrasound and how they respond to these stimuli. In playback experiments, certain behaviors that occur commonly after vocalizations are scored before and after a call is played. These behaviors include lifting and stiffening of ears, vocalization, walking or running towards the concealed speaker, clustering in a tight group, and remaining motionless ("freezing"), with occasional scanning movements of the head. The occurrence of such behaviors consistently increases after the playing of a call, whether it is a full-bandwidth playback or a playback in which most of the energy above 25 Hz was filtered out. This filtering shows that the behaviorally significant information of the call is contained in the infrasonic range, and it also simulates the effect of frequency-dependent attenuation over distance as it might occur in the wild. Behavioral responses do not increase for pure tone stimuli that are similar to recorded infrasonic calls in frequency and intensity. This shows that the responses are specifically to signals that were meaningful to the elephants. The use of prerecorded playbacks and behavioral scoring also shows that the infrasonic elephant calls are behaviorally significant over long distances. The degree of response behaviors performed by an elephant group, such as lifting of ears, walking towards the speakers, “freezing”, or scanning movements, was compared visually before and after the presentation of a stimulus, scoring a trial as a positive response if the amount of behaviors is greater after the stimulus. In one particular experiment performed on elephants living in the wild, the presentation of playbacks for 20–40 seconds from loudspeakers at distances of 1.2 km and 2 km caused a significant increase in response behaviors. Since the playbacks were done at half the amplitude at which they were recorded, it is estimated that these calls would be perceptible by elephants at distances of at least 4 km Even this may be an underestimate because animals do not respond every time they perceive a conspecific call, and they are probably less likely to respond to calls from further distances even if they do perceive them. There are some confounding factors that might influence the results of this kind of experiment. Firstly, the animals might actually be more sensitive than the experiments would indicate owing to habituation of the animals to the playback stimuli after several trial repetitions. To avoid this, researchers present several different types of playbacks in random order. Another problem that might arise in interpreting field experiments done on groups of animals is that animals may be responding to signals from other elephants in the group rather than the playback stimulus. However, an assumption is made that at least one animal in the group did perceive and respond directly to the stimulus.
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