3 Pile source level determination
3.1 Summary of general concepts
The sound generated and radiated by a pile as it is driven into the ground is complex, due to the many components which make up the generation and radiation mechanisms. Larger pile sizes can require a higher energy in order to drive them into the seabed, and different seabed and underlying substrate types can require use of different installation techniques including varying the hammer energies and the number of hammer strikes. In addition, the seabed characteristics can affect how sound propagates from the pile through the sub-surface geology, thus fundamentally affecting the acoustic field around the activity. The type of hammer method used (i.e. the force-impulse characteristics) can also affect the sound characteristics.
Underwater sound source level is usually quantified using a dB scale with values generally referenced to 1 μPa pressure amplitude as if measured at a distance of 1 m from a hypothetical, infinitesimally small source (often referred to as the Source Level). This quantity is often referred to as an equivalent monopole source level. In practice, it is not usually possible to measure at 1 m from a large structure, which in reality is more akin to a distributed sound source, but the metric allows comparison and reporting of different source levels on a like-for-like basis. In reality, for a large sound source such as a monopile, this conceptual point at 1 m from the (theoretical, infinitesimally small) acoustic centre does not exist. Furthermore, the energy is distributed across the source and does not all emanate from this imagined acoustic centre point. Therefore, the stated Sound Pressure Level (SPL) at 1 m does not occur at any point in space for these large sources. In the acoustic near field (i.e. close to the source), SPL will be significantly lower than the value predicted by the Source Level.
A useful measure of sound used in underwater acoustics is the SEL. This descriptor is used as a measure of the total sound energy of an event or a number of events (e.g. over the course of a day) and is normalised to one second. This allows the total acoustic energy contained in events lasting a different amount of time to be compared on a like for like basis. The SEL is defined as:
where 
is the integration time of the sound “event”, 
is the squared sound pressure at a time 
and 
is the reference time-integrated squared sound pressure of 1 µPa2s. For impulsive sounds it has become customary to utilise the T90 time period for calculating and reporting root mean squared (rms) sound pressure levels. This is the interval over which the cumulative energy curve rises from 5% to 95% of the total energy and therefore contains 90% of the sound energy.
3.2 Proposed pile source modelling methodology
The estimation of source levels for sound propagation modelling of piling is an important aspect of the noise modelling methodology. Ideally, use can be made of noise data measurement for similar piles, installed using a similar hammer in similar conditions. However, since noise modelling for proposed wind farms often proposes the use of piles and hammers for which there is no currently available measured data, it is often necessary to utilise an alternative method to estimate the source level inputs to the model. One such method used in some previous noise modelling assessments is the use of energy conversion factors, which involves estimating the proportion of the hammer energy which is transmitted into the water column as sound. However, the subject of sound generation due to impact piling is an active area of research and the evidence base is constantly being updated by new measurements, research and published papers. It is therefore important to ensure that the methodology used for determining the source levels of piling take into account the most recent research.
It is proposed to utilise scaling of measured data during pile driving for similar operations to Ossian in order to determine source levels. The subject of sound generation due to impact piling is an active area of research and the evidence base is constantly being updated by new measurements, research and published papers. A recent peer-reviewed paper (von Pein et al., 2022) presents a methodology for the dependencies of the SEL on strike energy, diameter, ram weight, and water depth that can be used for scaling measured or computed SELs from one project to another. The method has been shown to be usable within practical ranges of accuracy, especially if the measurement uncertainties are taken into account. The paper suggests that scaling should be performed over either a small number of very similar piling situations or over a larger data set with according averaging. This is a recently published method for deriving the sound source level which provides a more scientifically robust method compared to using an energy conversion factor (the conversion factor method simply assumes that a percentage of the hammer energy is converted into sound irrespective of parameters such as pile size, water depth and hammer specifications). Since the von Pein et al. (2022) methodology takes into account several site-specific and pile-specific factors, in addition to hammer energy, and because it is based on a scientifically rigorous and peer reviewed study, it is therefore considered to be a significant improvement on the use of simple conversion factors alone.
Using the equation below (von Pein et al. 2022), a broadband source level value is calculates for the noise emitted during impact pile driving operation in each operation window.
In this equation, E is the hammer energy employed in Joules, d is the pile diameter, mr is the ram mass in kg, h is the water depth in m, 
is the reflection coefficient and 
is the propagation angle (approximately 17° for a Mach wave generated by impact piling). The equation allows measured pile noise data from one site (denoted by subscript 0) to be scaled to another site (denoted by subscript 1).
This methodology therefore takes into account the following factors:
- Pile diameter
- Pile length.
- Pile penetration.
- Water depth.
- Rated maximum hammer energy of the proposed hammer.
- Hammer energy being used.
- Ram mass for the hammer.
- Acoustical parameters of the soil and water.
The peak SPL can be calculated from SEL values via the empirical fitting between pile driving SEL and peak SPL data, given in Lippert et al. (2015), as:
SPLpk = 
.
Root mean square (rms) sound pressure levels were calculated assuming a typical T90 pulse duration (i.e. the period that contains 90% of the total cumulative sound energy) of 100 ms. It should be noted that in reality the rms T90 period will increase significantly with distance which means that any ranges based on rms sound pressure levels at ranges of more than a few kilometres are likely to be significant over estimates and should therefore be treated as highly conservative.
The piling scenarios for Ossian have not yet been finalised, but it is envisaged that these will include the following phases:
- Initiation (including slow-start);
- Soft start;
- Ramp up; and
- Full power piling.
Mitigation methods such as use of Acoustic Deterrent Devices (ADDs) and engineering means of reducing sound emissions will be investigated as part of the sound modelling exercise if required.