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Table 3: Who polices the police? ANSI and ISO define “sound pressure” to mean p(t), the Table 3: Who polices the police? ANSI and ISO define “sound pressure” to mean p(t), the
difference between instantaneous pressure and static pressure, whereas IEC defines the
difference between instantaneous pressure and static pressure, whereas IEC defines the
same term to mean ρ , the RMS value of p(t). Similar differences arise for the definition same term to meRManS pRMS, the RMS value of p(t). Similar differences arise for the
of “sound pressure level.” definition of “sound pressure level.”
Organization Sound Sound Pressure Reference Pressure Level (SPL)
American National Standards Institute (ANSI)
p(t) 10 log10[pRMS2/p02] dB (ANSI, 2013)
International Organization for Standardization (ISO)
p(t) 10 log10[p(t)2/p02] dB (ISO, 2007)
International Electrotechnical Commission (IEC)
pRMS 20 log10 [pRMS /p0] dB (IEC, 1994)
of this ambiguity are small, and we don’t notice them. For some applications, however (e.g., calibration of transducers in fresh water (Horton, 1959), even small ambiguities lead to significant errors, while for others we depart sufficiently from the standard seawater conditions that the effects are no longer small. Even if we never put a hydrophone in Gany- mede’s oceans, the question “what is the reference intensity?” will still arise for model predictions reported in decibels.
The “obvious” way to remove the ambiguity is to follow a na- tional or (better) international standard instead of the con- vention. After all, the whole purpose of standardized termi- nology is to facilitate unambiguous communication, and if there existed a single unambiguous standard terminology, the adoption of that standard would indeed be the solution. Unfortunately, while there is international agreement on ref- erence values for sound pressure (1 μPa) and sound intensity (1 pW/m2), different national and international standards bodies have chosen different definitions even for basic ter- minology such as “sound pressure” and “sound pressure lev- el,” pointed out below and summarized by Table 3, so there remains some harmonization work to be done.
Why it Matters, Here on Earth
The examples considered so far involve exotic conditions on distant moons, but there is no need to look so far for ex- amples of the need for national and international harmoni- zation. Sound (or “noise”) in water is increasingly seen as a potential pollutant, and offshore contractors are required by regulators to assess or mitigate the risk of exposing marine animals to noise (Lucke et al., 2014). For example, the Na- tional Marine Fisheries Service (NMFS) in the US (NMFS, 2013), and the Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU) in Germany (BMU, 2013) specify thresholds of levels (e.g., of sound pressure or of sound exposure) that are either not to be exceeded at all or only by permit. While it is up to each national author-
ity to define the terms used in its national regulations or guidelines, underwater sound, like salmon and dolphins, shows scant regard for national boundaries, creating regula- tory confusion if the nationally adopted terminologies differ from one another, and highlighting the need for internation- al harmonization. In Europe, the Marine Strategy Frame- work Directive (EC, 2008) requires EU Member States to co-operate at a regional level to achieve good environmen- tal status. Specifically, EU Member States are required to monitor trends in underwater noise levels (EC, 2010), and associated monitoring programs can only yield comparable results if the Member States are measuring the same quan- tity (Dekeling et al., 2014).
Recent advances in scientific knowledge about risks of un- derwater sound have been reviewed by (Southall et al., 2007) (for marine mammals) and (Popper et al., 2014) (for fishes and sea turtles). These expert reviews provide stakeholders with sorely needed risk criteria, and insights into the latest scientific findings. In the absence of widely accepted inter- national standard terminology, the onus is on the authors of such reviews related to underwater sound to provide a com- plete list of acoustical terms used, and their definitions. To the extent that neither review defines all the terms used, the onus is then placed on the reader (who is unlikely to have the same in-depth expertise as the authors) to refer to the original research literature to find the missing definitions, increasing both the reader’s effort and risk of misunder- standing.
The International System of Quantities
We are witnessing the birth of a new science, planetary oceanography, and a new societal concern, underwater noise pollution. Before reaching maturity, both will need a more precise terminology than is presently available.
Help is on its way in the form of a long-standing collabora- tion between ISO and IEC that has borne as fruit the 14-
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