(Phys.org) -- Mammals produce sounds in two ways, either via the flow of air over vocal folds (vocal chords), as in humans and many other mammals, or via active muscular contractions as in the cats purr. Until now no one has been sure how elephants produce their lowest tones, but a new study has solved the mystery.
African elephants produce "infrasounds," which are low-frequency (<20 Hz) vocalizations capable of travelling up to 10 kilometers. Their frequency is usually too low for them to be audible to the human ear.
A new study by an international team of voice scientists and biologists has cleared up the mystery of how these infrasounds are produced. The researchers studied the excised larynx of an elephant that had died of natural causes and tried to use it to create infrasounds in the laboratory.
The study, led by voice scientist Christian Herbst of the University of Vienna, Austria, aimed to settle the long-standing question of whether elephants make infrasounds in the same way that humans and many other mammals produce sounds, by air flowing across the vocal folds to create vibrations, or through active contractions of the muscles.
Scientists can study the process in humans by inserting cameras into the larynx and observing what happens when different sounds are made, but this method is not possible in living animals such as elephants.
The research team excised the larynx within a few hours of the elephants death in a Berlin zoo, and they froze it and transported it to the University of Viennas Department of Cognitive Biology laboratory. They tested the larynx by adjusting the vocal folds to a position used for vocalizations (called a phonatory position) and blowing streams of humidified warm air through it to simulate the action of the elephants lungs. The larynx produced infrasounds virtually indistinguishable from those produced by living elephants.
The results demonstrated that muscular activity is not required to produce infrasounds, and that they are produced by the flow of air, a type of vocalization known as myoelastic-aerodynamic or flow-driven mode. If elephants produced infrasounds through active muscular contractions, in the same way as a cats purr is produced, the larynx would not have produced the sounds with the brain absent. The research does not, however, prove that elephants never use active muscular contractions to produce any sounds.
Myoelastic aerodynamic vocalization is the method used in humans (producing frequencies of about 50 to 7,000 Hz) and many other mammals, including echo-locating bats (100,000 Hz or higher). The researchers also found other features of the elephant larynx that matched nonlinear phenomena in other species, such as screaming in humans. In nonlinear vocalizations the vibration is chaotic rather than periodic.
The paper was published on 3rd August in Science.
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More information: How Low Can You Go? Physical Production Mechanism of Elephant Infrasonic Vocalizations, Science, 3 August 2012: Vol. 337 no. 6094 pp. 595-599. DOI: 10.1126/science.1219712
Elephants can communicate using sounds below the range of human hearing (infrasounds below 20 hertz). It is commonly speculated that these vocalizations are produced in the larynx, either by neurally controlled muscle twitching (as in cat purring) or by flow-induced self-sustained vibrations of the vocal folds (as in human speech and song). We used direct high-speed video observations of an excised elephant larynx to demonstrate flow-induced self-sustained vocal fold vibration in the absence of any neural signals, thus excluding the need for any purring mechanism. The observed physical principles of voice production apply to a wide variety of mammals, extending across a remarkably large range of fundamental frequencies and body sizes, spanning more than five orders of magnitude.