The way elephants make the low-frequency rumbles they use to communicate over long distances has been revealed by scientists in Austria. Humans and other mammals use sound extensively; whales and elephants produce frequencies as low as 9Hz, and some species of bat are capable of ultrasound frequencies exceeding 110,000Hz.
In all of these cases, these vocalisations are produced by passing air over the vocal folds, making them vibrate. But there are two ways to produce sounds like this. One is known as myoelastic aerodynamic noise production and is the means by which we speak and sing. To make these sounds we close the vocal cords or "glottis" and apply push air from the lungs against them. At a certain pressure the vocal cords part and a rush of air ensues. This causes the pressure to drop and the vocal cords then come together again, re-starting the process and setting up vibrations that resonate in the mouth and throat.
Purring, on the other hand, which cats typically do, involves rhythmic opening and closing of the vocal cords under nervous control at the rate of about 30Hz. This creates surges of air through the alternately open and closed glottis and sounds of the same frequency.
But do elephants do this to produce the human-inaudible infrasonic noises they are known to emit? To find out, Christian Herbst and his colleagues at the University of Vienna took advantage of the natural death of an elephant at a Berlin zoo to study the the animal's larynx. The team excised the entire elephant voicebox and connected it to an artificial lung to push in air. They closed the vocal cords by bringing together the supporting cartilage structures called the arytenoids, as would occur in life, and placed sensors to record sound production and the contact between the vocal cords.
Isolating the vocal cords from their air supply like this means that purring is impossible, so any ensuing sound must be being produced by myoelastic-aerodynamic means. As they turned up the pressure to about 4 times would it takes to vibrate a human voicebox, the elephant larynx started making sounds of about 16Hz, close to the 20Hz frequency of infrasounds recorded from living elephants in the wild. High-speed photography confirmed that the vocal cords were repeatedly opening as pressure built up and then closing again at air passed through, setting up vibrations in the tissue.
Concluding their paper in Science this week, the team point out that this study is the first to directly observe the sound production mechanism of elephant infrasound vocalisations. "We have shown that low-frequency phonation can be created by flow-induced self-sustaining oscillations of the vocal folds, in accordance with the myoelastic-aerodynamic theory of voice production." But the team also caution that they cannot rule out the possibility that elephants can also purr. But there proabably aren't many brave enough to volunteer to implant the electrodes that would be required to find out...