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variation in the received sound levels measured along different radials (Wilkes and Gavrilov, 2017). Received levels recorded on fixed-range and towed measurement systems were substantially different (~10 dB) between piles inclined in opposite directions (Vigness-Raposa et al., 2017; Martin and Barclay, 2019). These differences were observed independent of the strike energy used for individual hammer strikes (Amaral et al., 2020). The pile orientation affected the incident angle of the radi- ated pressure wave front on the seabed, which resulted in the directivity of the radiated sound varying based on the azimuth. The steeper the incident angle of the radiated wave front on the seafloor, the more energy was absorbed in the sediment. The azimuthal dependence to the radiated sound field and resulting sound levels are important factors to consider when determining the potential marine mammal and fish impact zones around pile-driving activities for inclined piles.
Vibratory Pile Driving
Vibratory pile driving is another method used to drive piles into the seafloor and could be used prior to impact pile driv- ing to ensure that the pile is stable in the seabed (JASCO and LGL, 2019) or for the installation of sheet piles to construct temporary cofferdams (Tetra Tech, 2012). In this process, the pile is vibrated at a certain frequency, typically between 20 and 40 Hz, to drive it into the sediment rather than ham- mering the top of the pile (Matuschek and Betke, 2009). The vibratory process produces lower level continuous sounds (see tinyurl.com/st4h9tq) compared with the high-ampli- tude impulsive noise produced during impact pile driving.
The high-amplitude pressure waves generated in the water column during impact piling are not present with vibratory piling, and the highest sound pressures are expected near the seafloor as a result of the propagating low-frequency interface waves (Tsouvalas and Metrikine, 2016). The radi- ated spectrum will be strongly influenced by the vibration frequency, will have peaks at the operating frequency and its subsequent harmonics, and will vary as the operating frequency is adjusted according to changing operational conditions such as sediment type (Dahl et al., 2015a). To assess the impact of nonimpulsive sound on marine life, the SEL metric is used (Southall et al., 2019).
Additional Construction-Related Sounds
The construction of an offshore wind project generates sound during other activities apart from pile driving, including during the laying of electric cables on the
seabed and from the operation of the vessels used during construction. The primary source of noise during the cable laying process is from vessel operations and the potential use of dynamic positioning thrusters to hold vessels in position. An environmental assessment per- formed for the Vineyard Wind project off the coast of Massachusetts concluded that the sounds generated from these activities were generally consistent with those from routine vessel traffic expected in the area, and, therefore, they were not anticipated to be a significant contributor to the overall acoustic footprint of the project (JASCO and LGL, 2019).
Operational Sounds of Wind Turbines
The construction of a wind farm takes place over a period of months, whereas the typical wind farm life span is between 20 and 25 years. Once completed, the turbines will operate nearly continuously, except for occasional shutdowns for maintenance or severe weather. Therefore, the contribution of sound to the marine environment will be more consistent and of longer duration during the oper- ational phase than during any other phase of the life of the wind farm (Nedwell and Howell, 2004). The underwater noise levels emitted during the operation of the turbines are low and not expected to cause physiological injury to marine life but could cause behavioral reactions if the animals are in the immediate vicinity of the wind turbine (Tougaard et al., 2009; Sigray and Andersson, 2011).
In some shallow-water environments, sound due to ship- ping traffic or storms could dominate the low-frequency ambient-sound field over the sound emitted from the wind turbines. Therefore, evaluating the relative sound levels from the wind turbine compared with those from other sources is important when considering the potential impacts to marine life. Measurements made at 3 different wind turbines in Denmark and Sweden at ranges between 14 and 40 meters from the turbine foun- dations found that the sound generated due to turbine operation was only detectable over underwater ambient noise at frequencies below 500 Hz (Tougaard et al., 2009).
The main sources of sound generated during the opera- tion of wind turbines are aerodynamic and mechanical. The mechanical noise is from the nacelle, which is situ-
ated at the top of the wind turbine tower and houses the gear box and generator (Figure 1). As the wind turbine blades rotate, vibrations are generated that travel down
Summer 2020 • Acoustics Today 17