Free-falling crushing as an underreported marine construction noise source: field characterization and implications for marine mammal exposure assessment

Free-falling crushing (FFC), which breaks reefs through the gravity-driven impact of a massive steel crusher, is increasingly used as a non-explosive alternative to underwater blasting in environmentally sensitive waters. However, its underwater noise emissions remain poorly characterized, limiting its incorporation into marine noise pollution assessment. In this study, FFC underwater noise was measured during subsea reef-removal operations in the coastal waters of Zhangzhou, China, using calibrated hydrophones deployed at multiple ranges and receiver depths. Analysis of 64 impacts produced by a 35-ton crusher showed that FFC generated short-duration, impact-related underwater noise with substantial event-to-event variability in impulsiveness, received levels, and time-frequency characteristics. At 60 m from the crushing point, the kurtosis-to-time ratio (β/Δt) ranged from 0.5 to 334 s⁻¹, while single-event SEL ranged from 145.5 to 174.3 dB re 1 μPa2·s. The observed variability occurred under broadly stable environmental conditions, suggesting that source-related differences during impact-induced reef fracturing may have contributed to this variability. These findings indicate that FFC should not be regarded as acoustically negligible simply because it avoids explosive detonation, and they provide baseline acoustic information for incorporating this underreported construction noise source into marine noise exposure assessment.