Caitlin O'Connell-Rodwell is Co-founder/CEO of Utopia Scientific and an instructor in the Department of Otolaryngology at Stanford University in California. Research in the O'Connell-Rodwell laboratory focuses on communication through vibrations in large mammals.
In a previous blog, I discussed Dr. O'Connell-Rodwell's research as presented in a feature podcast from The American Physiological Society. Her research on elephant communication sparked quite a bit of conversation. Therefore, I decided to invite Dr. O'Connell-Rodwell to be interviewed, and she agreed. Here is an exerpt from our exchange. Her website is Utopia Scientific.
Dr. Dolittle: In the podcast you mention that elephants travel and have to communicate over great distances. About how far do these calls reach?
Dr. O'Connell-Rodwell: In the air, elephant low frequency vocalizations called rumbles, emitted in the range of 20 Hz propagate around 2-4 km on average in the air, and even up to 10 km under special atmospheric conditions (similar to the SOFAR channel with whales) where cold air creates a two-dimensional channel such that sounds spreads cylindrically rather than spherically, meaning that there would be half the energy lost for every doubling of distance (3dB versus the normal 6dB), hence how the sounds can travel up to 10 km. In the ground, there is no theoretical outer limit to how far vibrations can propagate. In the air, Snell's Law dictates that sounds refract back into the atmosphere after 10 km. Vibrations in the ground do not have this outer limit, thus vibrations have the potential of propagating much further than airborne sounds. It has been difficult for us to measure the absolute outer limit of ground-borne vibrations due to the many human-generate sounds that exist in the ground ranging from radio waves to generators to 60 Hz electrical signals. However, from the data we have been able to collect, vibrations can travel at least as far as airborne signals and most likely much farther. Either way, the fact that two channels exist to receive a vocalization (both the air and ground) means that there are two opportunities to receive a signal, and given that the propagation rate is different for both the air and the ground, elephants could conceivably detect the difference in arrival time between the two signals and estimate the distance of the vocalizing elephant (similar to how we count the time delay between lightning and thunder to estimate the distance of an approaching storm). There are other advantages involving the ease of localizing a shorter wavelength signal when vibrations traveling as Rayleigh waves at the surface of the earth tend to have a slower velocity than those in the air (this is true at my field site in Namibia), making the wavelengths shorter and easier to localize.
For further videos of responses to acoustic and seismic playback experiments, visit Utopia Scientific.