scope:

This part of ANSI/ASA S12.64 describes the general measurement systems, procedures, and methodologies used for the measurement of underwater sound pressure levels from ships at a prescribed operating condition. It contains methodology for the reporting of one-third octave band sound pressure levels. The resulting quantities are the sound pressure levels normalized to a distance of 1 m. Since the underwater sound pressure levels are affected by the presence of the free surface (and sometimes the bottom), such quantities are considered "affected source levels," herein referred to as source levels.

The underwater sound pressure level measurements are performed in the geometric far field and then adjusted to the 1 m normalized distance for use in comparison with appropriate underwater noise criteria. However, this standard does not specify or provide guidance on underwater noise criteria or address the potential effects of noise on marine organisms.

This standard is applicable to any and all underway surface vessels, either manned or unmanned. The methods have no inherent limitation on minimum or maximum vessel size. This standard is not applicable to submerged vessels or to aircraft. This standard is limited to vessels transiting at speeds no greater than 50 knots (25.70 m/s). The measurement methods mitigate the variability caused by Lloyd's Mirror surface image coherence effects (see 3.16), but do not exclude a possible influence of bottom reflections. No specific computational adjustments for either of these effects are part of this standard. A specific ocean location is not required to use this standard, but the requirements for an ocean test site are provided.

The intended uses of the methods described in this standard are: to show compliance with contract requirements, to enable periodic signature assessments, and for research and development. The intended users include: government agencies, research vessel operators, and commercial vessel owners that need to operate in acoustically sensitive waters.

This standard offers three grades of measurement, each with a stated applicability, test methodology, uncertainty, system repeatability, and complexity. A summary of the attributes of each "Grade" (denoted A, B, and C) is given in Table 1.