A recent paper was published by marine research firm JASCO on localizing walruses using a single hydrophone. The technique they have refined is detailed in the article and involves assessing “multipath signals” – echoes from the sea surface and seafloor – along with the direct signal to track the movements of walruses in the Arctic. This is remarkable because locating a sound source usually involves two or more receivers.
For example, all terrestrial vertebrates have two ears which allow us to locate sound sources. We do this using a number of different cues. One of the obvious cues is amplitude: if the sound is louder in one ear over the other we assume that the sound source is closer to the louder ear. Another more subtle cue is ‘time of arrival’ which allows us to locate a sound source by the difference between the times it takes for the sound to arrive at one ear and the other.
These two cues serve to give us a general bead on the horizontal source of a sound – whether it is to the left or right, in front of us or somewhere in between. This method is adequate for birds, frogs, and lizards in the context of their survival needs.
But we mammals have more complex localizing needs from sound so we have developed outer ears that provide us with more subtle cues – allowing us to determine things such as how close or far, and the height or azimuth of a sound.
Mammals with movable ears such as cats and horses can use their outer ears like periscopes to hone in on a source of sound. Primates such as us humans have fixed ears, so we derive location cues from secondary reflections off the pinnae of our ear.
It happens that all the whorls on our outer ears collect and reflect sound into our ear canal with a tiny time delay after the primary sound hits our ear drums. This is a bit like the “multipath” cues that JASCO teased out in their work (but with much less math).
These three cues – amplitude, time of arrival, and delay of the secondary reflection allows us to pinpoint the source of a sound with uncanny accuracy.
Of course marine mammals (with the exception of sea lions, polar bears, and otters) do not have outer ears. We know that they have equal, or even more complex localizing needs from sound, especially the Arctic animals that spend a good amount of time in the deep and dark waters of the Arctic winter.
It has been less than 100 years that humans have started to use sonar in marine environments. The folks at JASCO are rolling back sonar frontiers, showing us that complex data can be derived from single receivers.
Marine mammals such as whales and seals have been adapting to their acoustic environment for 20-30 million years. It stands to reason that these animals have evolved some pretty complex adaptations to sound perception.
Hopefully we can come to understand and learn from some of these adaptations before we cloud out their arctic environment with sounds to which they have not adapted.