Standard definitions

A consistent vocabulary and standard system of units is essential for work in any scientific discipline. Such a system facilitates efficient and unambiguous communication among members of the community. The underwater acoustics community has struggled with a common vocabulary and standard system of units for quite some time. This situation derives, in part, from the fact that many of the participants in this community have been trained in other disciplines and later migrated into this field.

A number of investigators have introduced suggestions for a standard system of units to satisfy the requirements that are unique to the underwater acoustics community. Recently, for example, Carey (1995) clarified the use of SI metric units for measurements and calculations used in underwater acoustics and bioacoustics, while Hall (1995) re-examined the dimensions of units for source strength, transmission loss, target strength, surface-scattering strength and volume-scattering strength. In addition, technical dictionaries (e.g. Morfey, 2000) provide useful guidance on proper terminology and usage.

Work in modeling and simulation also requires a consistent vocabulary. In this regard, the Institute of Electrical and Electronics Engineers (1989) published a glossary of definitions for modeling and simulation terms. The US Defense Modeling and Simulation Office (DMSO) assembled the DOD Modeling and Simulation Glossary, which prescribes a uniform M&S terminology, particularly for use throughout the Department of Defense. In addition to the main glossary of terms, this highly useful manual includes a list of M&S-related abbreviations, acronyms and initials commonly used within the Department of Defense (refer to Appendix C for the DMSO website).

Recent communications in the forum of the Journal of the Acoustical Society of America (Clay, 1999; Hickling, 1999; Chapman, 2000) highlight confusion over use of the decibel (dB). In Chapter 3, the use of the decibel in underwater acoustics will be explained in greater detail. Confusion over the use of decibel has created significant technical and legal problems for those acoustical oceanographers and sonar technologists who must interact with their counterparts in aeroacoustics and bioacoustics. In air acoustics, a reference sound pressure of 20 ^Pa is used while the current choice in underwater acoustics is 1 ^Pa. Part of the confusion arises from the use of decibels to represent ratios of dimensionless quantities as well as ratios of absolute quantities having physical dimensions. When reporting ratios of dimensionless quantities (such as reflection losses), decibels can be used without further qualification. However, when decibels are used to represent ratios of absolute quantities having physical dimensions (such as radiated noise levels), the reference quantities (pressure and distance) must be clearly stated. Since acoustical measurements are first made in SI units (sound pressure in pascals and radiated source power in watts) and then converted to decibels, some researchers have argued that confusion caused by the decibel could be removed by reporting acoustical measurements in SI units.

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