Sources of underwater sound and their characterisation (abstract)

After centuries of speculation, punctuated by occasional theoretical or experimental advances, the first intensive research into underwater sound took place 100 years ago, applied initially to provide advance warning of icebergs after the loss of RMS Titanic in 1912, and later to counter the U-boat threat during World War I. The need to understand the performance of these early underwater detection systems, now known as sonar, led to the development of a mathematical framework known as the ‘sonar equations’, within which sound sources are characterised by their ‘source level’, which together with ‘propagation loss’ and related terms enable sonar engineers to predict the detection ranges of their systems. Today a new concern has arisen, for the well-being of the animals with which we share the sea. Because of the history of sonar and sonar engineering, the concept of ‘source level’ is widely used to characterise anthropogenic sound sources like airguns, explosions, ships and pile drivers, but is it necessary or even useful for sources other than sonar transmitters? The concept and applicability of source level is reviewed for the most important sources of anthropogenic sound and compared with alternative
source properties such as radiated power, radiated energy and total free-field energy. In particular, which properties of an underwater sound source are best correlated with its potential for harm? Finally, international efforts towards the harmonisation of the terminology for measurement, reporting and impact assessment of underwater sound are summarised, with particular attention to initiatives of the International Organization for Standardization (ISO).