The definitions of “level,” “sound pressure,” and “sound pressure level” in the International System of Quantities, and their implications for international standardization in underwater acoustics

Abstract

The International System of Quantities (ISQ), incorporating definitions of physical quantities and their units, was completed in 2009 following an extensive collaboration between two major international standards organizations, the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). The ISQ encompasses all SI units as well as selected units outside the SI such as the byte (including both decimal and binary multiples), bel, neper, and decibel. The ISQ, which includes definitions of the terms “level,” “sound pressure,” and “sound pressure level,” is presently being used to underpin an underwater acoustics terminology standard under development by ISO. For this purpose, pertinent ISQ definitions are analyzed and compared with alternative standard definitions, and with conventional use of the same terms. The benefits of combining IEC and ISO definitions into a single standard, solving some long-standing problems, are described. The comparison also reveals some teething problems, such as internal inconsistencies within the ISQ, and discrepancies with everyday use of some of the terms, demonstrating the need for continued collaboration between the major standards bodies. As of 2014, the ISQ is undergoing a major revision, leading to a unique opportunity to resolve these discrepancies.