IACS recommendation: Measuring underwater ship noise

The International Association of Classification Societies (IACS) issued Recommendation No. 181 (2024) to provide guidance on measuring underwater radiated noise (URN) from ships. The recommendations aim to standardize methodologies, data processing, and reporting practices across IACS members.

The recommendations align with ISO standards, including ISO 17208-1 (deep water measurements), ISO 17208-2 (source level determination), ISO/DIS 17208-3 (shallow water measurements), and ISO 18405 (terminology).

For deep water measurements (depth greater than 150m or 1.5x ship length), compliance with ISO 17208-1 and 17208-2 is required. For shallow water measurements (depth less than 150m or 1.5x ship length), considerations include defining the ship's acoustic length (typically in the rear third of the vessel), identifying the reference point (near the engine room and propulsors), and selecting sites to minimize background noise and frequency cutoffs.

Hydrophones used should have flat sensitivity across 10 Hz–20 kHz (±2 dB, max ±3 dB). Instruments must be calibrated before and after deployment, and three hydrophones are recommended for improved accuracy and redundancy.

Tests must maintain constant power and machinery settings, with a recording duration of at least 10 seconds at a distance of 100 meters or more. Measurements should represent typical ship operating conditions.

The recommendations specify two metrics for reporting: Radiated Noise Level (RNL), for ship comparisons, periodic assessments, and ecological impact evaluations; and Source Level (SL), for environmental studies and sound mapping. Reports should include details about the ship (hull condition, last dry dock date), environmental conditions (current speeds, CTD readings), and operational specifics (speed, heading). Special considerations, such as shallow water frequency cutoffs and unusual conditions, must also be documented.