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Michael A. Ainslie
Postal:
TNO, PO Box 96864 2509 JG The Hague The Netherlands
Email:
michael.ainslie@tno.nl
A Century of Sonar:
Planetary Oceanography, Underwater Noise Monitoring, and the Terminology of Underwater Sound
No self-respecting science – or scientist – would tolerate a factor of four uncertainty in the interpretation of a reported measurement or model prediction, arising from poorly defined terminology alone, so why do we?
Introduction
The current terminology of underwater sound, as documented, for example, by (Urick, 1983), was developed during and after the Second World War (ASA, 1951; Urick, 1967), and has evolved little since then (Jensen et al., 2011). When exam- ined against a modern requirement, with particular attention to the needs of plan- etary oceanography and underwater noise, this 60-year old terminology is found wanting.
The Sonar Equations and “Noise Level”
The development of passive and active sonar during the first half of the 20th cen- tury, motivated by the loss of RMS Titanic in April 1912 and by two world wars (Wood, 1965; Hackmann, 1984), was directed almost exclusively towards the de- tection and localization of objects in seawater. In order to understand the perfor- mance of underwater detection systems, a theoretical framework (known today as the ‘sonar equations’ (Urick, 1983)) was developed for quantifying that perfor- mance in terms of the signal-to-noise ratio (Horton, 1959).
The traditional meaning of the term “noise level,” in the sonar equations, is the level of the masking background against which a signal is to be detected. In the 21st century, underwater sound is increasingly seen as a potential pollutant, for which “noise level” is then taken to mean the amount of that pollutant. For the bulk of this article, I focus on the sonar equation, and return in the final section to the possible impact of noise on aquatic life.
The sonar equations are in widespread use on Earth (Urick, 1983), and are start- ing to find application in space (Arvelo and Lorenz, 2013). Whether in search of signs of extra-terrestrial life in subsurface water oceans (Hussmann et al., 2006) or of vast hydrocarbon resources on distant moons (Stofan et al., 2007), these new uses encounter harsh conditions that are very different to those on Earth. These extreme conditions serve to expose a fundamental ambiguity in the way the indi- vidual terms in the sonar equations are expressed as levels. Although the ambi- guity is minor for the range of conditions usually encountered in water on Earth, it becomes important in some situations, and is seen on closer inspection to be a symptom of a deeper malaise, namely a dearth of widely accepted definitions for even the most basic terminology used in undersea acoustics. My main purpose is
12 | Acoustics Today | Winter 2015, volume 11, issue 1 ©2015 Acoustical Society of America. All rights reserved.