Monitoring guidance
Methods for the assessment of noise (and other forms of energy) have been discussed by the Technical Group on Underwater Noise and other forms of Energy (TG Noise). Methodologies are still needed for impulsive noise and ambient noise.

TG Noise notes that there are no international standards for the measurement, modelling or storage of data related to underwater ambient noise, and recommends that such international standards be developed, including the measurement of radiated sound from sources such as airgun arrays and underwater explosions (standards for the measurement of sound radiated from ships and pile driving equipment are already being developed by ISO).

The group provided a monitoring guidance document to support the implementation of Descriptor 11: Monitoring Guidance for Underwater Noise in European Seas:
Part I is the executive summary for policy and decision makers responsible for the adoption and implementation of the MSFD at national level. It provides key conclusions and recommendations to support the practical guidance for Member States and enable the assessment of the current levels of underwater noise.
Part II is the main report of the Monitoring Guidance for Underwater Noise in European Seas. It provides specifications for the monitoring of underwater noise, with dedicated sections on impulsive noise (Criterion 11.1 of the Commission Decision) and ambient noise (Criterion 11.2 of the Commission Decision) designed for those responsible for monitoring/modelling and registering underwater noise levels.
Part III, the background information and annexes, gives additional information, examples and references that support the Monitoring Guidance specifications.
Underwater noise, as per that considered under the MSFD, can be classified as:
Impulsive noise
The MSFD Technical Group on Noise recommends that indicator 11.1.1 be monitored by setting up a register of the occurrence of impulsive sounds as a first step in establishing their current levels and trends.

Airguns, pile-driving equipment, explosives, and sonar working at relevant frequencies and some acoustic deterrent devices are the most important sound-sources that should be considered for inclusion in the register of impulsive noise. Additional sources that could also be of concern include boomers, sparkers and scientific echo sounders. TG Noise recommends thresholds for inclusion in the register, which ensure that all sources that have the potential for significant population -level effect will be included in the register. However, the use of these (relatively low) thresholds will result in the registration of sources with a relatively low potential for significant impact . TG Noise concluded that there is a need for more detail in the register than just the day and location; of this additional information, the source level is the most important.

The information required to derive pulse-block days (the number of days on which a certain threshold (pulse) is exceeded in an area (block)), are:
  • Position (geographic position (latitude/longitude), licensing block/area)
  • Date
  • Source properties: Essential (minimum)
  • Source level or proxy;
Additional data will be beneficial for improved assessment - where available the following may also be recorded:
  • Source spectra;
  • Duty cycle;
  • Duration of transmissions (and actual time/time period);
  • Directivity;
  • Source depth;
  • Platform speed
It is possible that many operators (e.g. navies using sonar devices) may have concerns about releasing sensitive information. Where detailed information of source properties is requested it is proposed
Ambient noise

TG Noise concludes that the combined use of measurements and models (and possibly sound maps) is the best way for Member States to ascertain levels and trends of ambient noise in the relevant frequency bands. Member States should be careful to balance modelling with appropriate measurements.

The use of modelling for indicators and noise statistics, and possibly the creation of noise maps, ensures that trend estimation is more reliable and cost-effective, for the following reasons:

i. The use of models reduces the time required to establish a trend, with a fixed number of measurement stations (the expected trend in shipping noise, based on observations in deep water, is of the order of 0.1 dB/year; and therefore it takes many years, possibly decades, to reveal such small trends without the help of spatial averaging);

ii. The use of models reduces the number of stations required to establish a trend over a fixed amount of time (similar reasoning), therefore reducing the cost of monitoring;

iii. Modelling helps with the choice of monitoring positions and equipment (selecting locations where the shipping noise is dominant as opposed to explosions or seismic surveys being dominant). The use of models provides Member States with an overview of actual noise levels and their distribution across the sea area, thereby enabling identification of a departure from GES. In addition, there are advantages to using modelling that could contribute to a greater understanding of the potential impacts of noise. Mapping has been used in implementing the MSFD for quite some time, so lessons can be learned from previous experience. Relevant EU experience and regulation is summarised in part II, which provides some useful background information regarding the MSFD.