Low level, low-frequency sounds: Measurement and analysis challenges

Low-frequency sounds from both natural and civilization sources can travel for long distances (tens or hundreds of kilometers), producing local regions of enhanced as well as reduced intensity depending upon range and time. Thus the identification of sources can be difficult, since merely mapping sound level in the direction of increasing intensity may not help to identify distant sources. A brief review of some examples of low-frequency background sounds provides the basis for a discussion of potential methods for identifying sound sources in a complex propagation environment. At low frequencies (