Elephants can communicate using sounds below the range of human hearing (infrasounds below 20 hertz). These vocalizations have been presumed to be 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).
In an experiment, direct high-speed video observations of an elephant larynx demonstrated 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.
This according to “How low can you go? Physical production mechanism of elephant infrasonic vocalizations” by Christian T. Herbst, et al. (Science CCCXXXVII/6094 [3 August 2012] pp. 595–599). Below, a podcast interview with Dr. Herbst provides examples and further details (including the fact that the elephant had died of natural causes).
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