Among elephants, a typical male rumble fluctuates around an average minimum of 12 Hz (more than 3 octaves below a man's voice), a female's rumble around 13 Hz and a calf's around 22 Hz.(5) In normal human speech the vibration rate may vary over a 2:1 ratio, in other words over one octave, while the range of a singers voice is over two octaves.(6)

Elephants are capable of producing different sounds ranging over more than 10 octaves, from 5 Hz to over 9,000 Hz(7). Indeed, within a single call an elephant may slide over a range of 6 octaves!(8) Imagine a musical composition of some operatic elephants!

Elephants can produce very gentle, soft sounds as well as extremely powerful sounds. Some of the calls produced by elephants may be as powerful as 112 decibels (dB) recorded at 1 meter from the source(9). Decibels are measured on a logarithmic scale and to give you some idea of how loud some elephant sounds are we have presented a table with some examples of typical sound levels you might encounter(10).

Certain calls by elephants are associated with particular postures of the head and ears and it is our belief that by holding its head in a certain posture and by flapping its ears in a particular rhythm and angle an elephant is able to affect the musculature around the larynx thus modifying a particular call to achieve the desired sound (for example the ear wave and the musth rumble)(12).

Quite different results may be achieved with the same basic rumble at source (duration and frequency) depending upon whether the elephant holds its mouth wide open or closed, its head held high or low, the ears steady, flapping slowly or rapidly, or perhaps raised and folded. And depending upon the positioning of the trunk and the speed and duration of air moving through it, elephants are able to produce a wonderful mixture of higher frequency sounds.

First of these is the elephant's trunk which in an adult male may add as much as 3 meters on to the length of the resonating chamber.

Second, the structures of the hyoid apparatus (a series of bones at the base of the tongue) and the musculature that support the tongue and the larynx in elephants are different from other mammals. The hyoid apparatus of elephants has five rather than nine bones, and these are attached to the skull by muscles, tendons and ligaments, rather than by bones as in most other mammals. This rather loose arrangement allows for a greater movement and flexibility of the larynx and is, therefore, thought to facilitate the production and resonance of low frequency sounds(14).

Third, in most mammals the hyoid apparatus provides support for the tongue and for the larynx but the looser arrangement in elephants also houses a pharyngeal pouch,(15) a structure unique to elephants located at the base of the tongue, which in addition to providing elephants with an emergency source of water, appears to function in the production of low frequency calls. In humans, and by inference also in elephants, the muscles of the larynx help to contract and relax the vocal cords. The greater the flexibility of the larynx, the greater the ability of these muscles to stretch and relax, which in turn affects the contraction and relaxation of the vocal cords and consequently the pitch or frequency of the sound that is produced. So, the modification in elephants of the hyoid apparatus to house the pharyngeal pouch also permits an enlargement of the resonating chamber by lowering the loosely attached larynx. Consequently, elephants are able to produce very low frequency sounds.

In an area between the southern Selous Game Reserve and the Mozambique border in Tanzania, Donald Mpanduji(17) watched as a mother elephant twice withdrew water from her pharyngeal pouch and then used it to spray over her infant. The mother elephant must have recognized that her infant was suffering from the heat and rather than use the water herself she showered the cooling water over her baby, indicating, Joyce believes, an ability to empathize.

Sound traveling through air attenuates by the reverse square law at 6 decibels (dB) for every doubling of the distance from the source. Thus, for example, a sound measuring 100 dB at one meter from the source will be reduced to 94 dB at 2 meters, 88 at 4 meters, 82 dB at 8 meters, and so on. Sound also attenuates through "excess attenuation" as it travels through the environment. The degree of excess attenuation is affected by the frequency of a sound and the type of habitat it is passing through. But very low frequency sound, such as the very low frequencies produced by rumbling elephants, suffer from little if any excess attenuation(20). In grassy savannas and woodlands elephants communicating over distances of more than 100 m should be able to perceive low frequency calls better than higher frequency calls. Elephant groups are frequently over 100m in diameter and sub-groups of related elephants are often separated by several kilometers. Powerful very low frequency sounds are the means by which these individuals stay in touch with one another.

Something interesting happens to the transmission of sound at different times of the day. Out on the savanna, it has been shown that environmental conditions follow a pretty regular diurnal cycle. Around evening a strong temperature inversion usually forms and doesn't dissipate until dawn. The greatest calling areas are achieved during the formation and dissolution of these nightly inversions, especially with cloudless and relatively undisturbed weather. Under such conditions it is possible for an elephant to have a calling range of 300 square km (an area almost the size of the entire Amboseli National Park)! In other words an elephant may be able to detect the calls of another elephant almost 10 km away. During the day, without the help of an inversion and with factors such as heavy sun and wind often coming into play, calling area size is drastically reduced, ranging from a couple dozen to 150 square km(24).

Not only are the elephants' very low frequency rumbles well suited for long distance communication, but being sounds with a rich harmonic structure they also allow a listening elephants to "calculate" the distance of the calling elephant. This is because at close range the full harmonic structure will be intact while with increasing distance the upper frequencies will become relatively weaker eventually leaving only the lowest frequencies to persist(25).

Mammals with small heads and narrow spaced ears are better able to hear high frequency sounds than mammals with large heads and wide-set ears. Large mammals are generally specialised in lower frequency hearing because larger skulls can encompass longer ear canals (meatuses), wider tympanic membranes (the membrane that closes the middle ear off from the exterior) and spacious middle ears. How do these three factors favour higher sensitivity at low frequencies?

In normal air-conducted hearing sound waves set the tympanic membrane and the middle ear bones (or ossicles) in vibration, thus producing movements on the oval window and changing pressure gradient in the cochlear fluid.

One difficulty with low frequency sound is the signal to noise ratio. In the lower frequencies there tends to be a higher level of background noise, and so animals that specialize in low frequency hearing must have a way of distinguishing signal from noise. The amount of sound energy collected by the tympanic membrane increases with increasing membrane area, thus enhancing the signal to noise ratio at the level of the inner ear. So, the larger the tympanic membrane the better an animal is able hear at low frequencies. The tiny middle ear bones, or ossicles (the malleus, incus and stapes), have to be able to withstand the greater forces produced by the vibrations of a larger tympanic membrane, and so animals with large tympanic membranes also have massive (relatively!) middle ear ossicles(27). An incus of an adult female African elephant (actually collected by Joyce from the skull of the mother of Eudora, an elephant named Emily who died in September, 1989 when she was 39 years old) weighed 237 mg.(28) The malleus and stapes of this elephant were estimated to be 278 mg and 22.6 mg respectively and the tympanic membrane area 855 mm2.

Large tympanic membranes do, however, present a problem: Mammalian tympanic membranes are extremely thin and the risk of scratching and damaging them may have prevented the tympanic membranes of most large mammals from evolving too large. The enormous skull of the elephant, however, has allowed the evolution of an outer ear canal of about 20 cm in length, providing adequate protection for its very large tympanic membrane. Since the large elephant middle ear bones do not impede the transmission of low frequencies and the large tympanic membrane allows high signal to noise ratios, the elephant middle ear reflects a special adaptation to low frequency hearing(29).

Finally, one more structure of the elephant's ear, the cochlea, may facilitate low frequency hearing(30). Together with their relatives the Sirenians (the dugongs and manatees), elephants are unique among modern mammals in having reverted to a reptilian-like cochlear structure that may facilitate greater sensitivity to lower frequencies(31). Since the cochlear structure of reptiles facilitates a keen sensitivity to vibrations it has been suggested that the similar structure in elephants may allow them to detect vibrational signals, too(32).

So with all of these special adaptations, just how low can elephants hear? The only study of elephant hearing sensitivity was carried out on an Asian elephant(33). Unfortunately, the study was completed a couple of years before it was known that elephants produce very low frequency sounds(34). As a result extremely low frequency sounds were not tested. But we do know from this study that elephants have very good hearing into the infrasonic (below human hearing) range. This particular elephant, a juvenile Asian female, was able to hear down to 16 Hz at 65 dB. Since 65 dB can be described as a moderate to noisy sound, presumably elephants can hear significantly lower than this. Joyce has recordings of elephant calls as low as 8 Hz and other people have reported calls as low as 5 Hz, so we think we can safely presume that elephants have a way of detecting these extremely low frequencies otherwise why would they produce them? Recent studies have shown that elephant rumbles are also transmitted through the ground, via seismic communication(35). If someday we show that elephants are unable to hear down as low 5 Hz then perhaps we will discover that they are instead picking up these low sounds through their sensitive feet.

At the other end of the scale, elephants are unable to hear above 12 kHz making them the animal with the lowest high frequency hearing limit of any mammal tested(36).

One juvenile Asian elephant whose hearing was tested was able to localise clicks and noise bursts to within 1 degree. She was less good at distinguishing tones, but was better able to distinguish lower frequency tones than higher frequency tones; below approximately 300 Hz she was able to localize tone within 10 degrees with 75% accuracy, 20 degrees with about 80% accuracy and 30 degrees with 90% accuracy.(38)

Elephants may be able to detect these seismic vibrations, or rayleigh waves, through two possible means, bone conduction and the use of massive ossicles of their middle ears(40) or possibly by mechanoreceptors in the toes or feet that are sensitive to vibrations.(41) The tip of an elephant's trunk has layers of cells called Pacinian corpuscles that are extremely sensitive to vibrations(42) and it has been suggested that perhaps these cells may also occur in the fleshy pads of an elephant's feet.(43) Movements or vibrations deform the layers of Pacinian corpuscles, sending a nerve signal to the brain. Although these corpuscles are found in other mammals, too, they are particularly densely packed in the tip of an elephant's trunk.

You can find an updated article about seismic signals from elephants here.