This news article is part of Issue 2 of our annual newsletter, which chronicles our research in 2022. To view more articles describing our research progress from our second year, read the newsletter in full here.
by Michael Ainslie, JASCO Applied Sciences
U.S. President William Howard Taft once said, “Don’t write so that you can be understood, write so that you can’t be misunderstood.” A prerequisite for exchanging scientific ideas without misunderstanding is a shared, common interpretation of the terms used in the discourse. And not only science can fall victim to misaligned terminology; dire consequences can occur in practical life.
In 1628 a Swedish warship sank on her maiden voyage — having capsized due to instability — because two teams involved in the construction defined the foot (the unit of length) in two different ways. Today we have the International System of Units (SI) and the International System of Quantities (ISQ) to safeguard us against similar disasters, but we still find ways of losing a spacecraft by choosing not to follow international standard units (Mars Climate Orbiter), or having to redesign a bridge already under construction whose spans would otherwise not have met due to the lack of an international standard for sea level (Laufenburg Hochrheinbrücke).
Left: Actual and planned trajectories of the Mars Climate Orbiter in 1999; the orbiter was 25 km too close to survive. (source) Centre: The lower gun deck of the warship Vasa. (source) Right: The modern Hochrheinbrücke in Laufenburg belies the mishap in its design.
Acousticians took heed of Taft’s advice, and the first international standard for underwater acoustical terminology (ISO 18405:2017 Underwater acoustics – Terminology) was published in in 2017, followed three years later by ISO 80000-8:2020 Quantities and units – Acoustics, which consolidated 1 mPa as the international standard reference value of underwater sound pressure in the ISQ. ISO 80000 (ISQ) and ISO 18405 define basic terms like ‘sound’, ‘sound pressure’, ‘sound pressure level’, ‘sound exposure level’, ‘decidecade’, ‘source level’ and ‘soundscape’ (Ainslie et al., 2021). And yet there remains a lack of key terminology needed for characterisation of vessel sounds and impact assessment and mitigation of underwater radiated noise from ships.
Building on the firm foundation provided by the ISQ and ISO 18405, a SATURN work package team led by JASCO and supported by BV, DNV, IBL, PLOCAN, TNO, and UPC is developing standard terminology for the project. Examples of the wide scope of this effort can be drawn from general underwater acoustics (‘acoustic near field’), underwater bioacoustics (‘acoustic habituation’), vessel acoustics (‘radiated noise level’, ‘structure-borne sound’) and sound mapping (‘temporal observation window’). And spirited academic debate about the meaning of ‘source level’ is nowadays replaced by snappy discussion on Twitter.
The first draft, issued in August 2021, was circulated internally. The resulting feedback led to an improved second draft published twelve months later, which has become the SATURN project workhorse and was adopted by the EU technical group on underwater noise (TG Noise) for its guidance on threshold values announced in November 2022. The final project standard is scheduled for publication in October 2024 and will include, among further expansions, new terminology for particle motion.
While there is much good news to report, there are setbacks too. The ISQ definition of the decibel (dB) was withdrawn in 2019 amid lack of agreement, leaving undefined the most widely used unit in acoustics. JASCO is supporting the International Electrotechnical Commission (IEC) to fill this important gap with a new standard IEC 80000-15 Quantities and units – Logarithmic and related quantities, presently under development. Meanwhile SATURN is left with the choice to wait for IEC to issue its new standard, or develop its own definition of the dB. The times of capsizing warships and missed bridge crossings may hopefully be behind us, but there is still drama in standardisation.