Standardized underwater measurements of a ferry’s source levels in the St. Lawrence’s shallow coastal environment

• Source levels increased by 2–4 dB kn − 1 with strongest trends above 10 kHz. • Acceleration events raised low-frequency acoustical source power by 1–2 orders of magnitude. • Diesel-electric ferry noise levels were lower than pure diesel propulsion, especially below 100 Hz. Accurate characterization of ship source levels is essential to assess the acoustic footprint of maritime traffic in biologically sensitive regions. This study presents standardized underwater radiated noise (URN) measurements for the N.M. Saaremaa I , a diesel-electric ferry scheduled to operate in the St. Lawrence Estuary, a critical habitat for endangered belugas. Using two seabed-mounted digital hydrophones and a shallow-water measurement protocol adapted from recent standards, we derived monopole source levels (mSLs) across 37 decidecade frequency bands under various operating conditions, including constant-speed transits, acceleration runs, and idle states. Our results show a consistent increase in mSLs with vessel speed, reaching slopes of 2–4 dB kn − 1 , with strongest correlations above 500 Hz. Acceleration events produced band emissions exceeding 180 dB re 1 µPa 2 · m 2 at 25 Hz, while mid-to-high frequency levels ( ≳ 1 kHz) were often lower than during steady-speed operation. These findings highlight the dual nature of acceleration noise, potentially degrading acoustic habitat for baleen whales while increasing collision risk for odontocetes near terminals. This work provides one of the first in-situ acoustic profiles of a ferry ahead of service in a high-use marine mammal corridor and offers empirical input for noise modelling, regulatory assessments, and mitigation planning in shallow coastal environments.