The show examines the research methods for both naturalistic observation as well as analysis of the various frequencies of sounds. As it turns out, elephants can create sounds that are below the level of human hearing which is about 20 Hertz. The researcher has a wonderful explanation the process of how this was discovered.
The context of this for me is a demonstration of how there are visual and auditory frequencies that we cannot detect. More on this topic later.
Here are some other sites that examine both the elephant communication system and infrasound.
The Elephant Listening Project
History/Overview of Infrasound on Wikipedia
posted by Chuck Schallhorn