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Assessment and discussions of impact of underwater noise on aquatic organisms are greatly hampered by the general lack of standards in analysis methods and measuring units. This report summarises points of importance, whether one is measuring sound or evaluating the work of others.
Instruments must be appropriate for the sounds one wants to record and with sensitivity that maximizes the signal to noise ratio of the recordings. All parts of the recording chain must have sufficient bandwidth and be properly calibrated to ensure comparability with other measurements.
Sound, which is variable in time, in particular ambient noise, must be characterised by appropriate statistical measures, such as medians and percentiles or other measures of variation. Sound propagation around a sound source should likewise be characterised by multiple measurements in different directions and distances.
Additional information relevant for interpretation of measurements must be given. This includes not only the equipment used but also environmental parameters, measurement positions and measurement protocol.
Agreed and de facto standards, if existing, should be followed in analysis and when reporting results. Particular concern connects to the use of the dB-scale, where not even peer-review can be taken as a guarantee that usage is correct or at least consistent. Different studies can in many cases not be compared at all and other only with sufficient caution.
In cases where standard analysis methods, such as 1/3-octave spectra or rms-averages, are not considered fully appropriate for a specific application, the results from new analysis methods should always be accompanied with results from standard methods, to allow comparability with other studies.
Methods for analysing impact must be adapted to the specific objectives. A distinction should be made between making a full characterization of a sound source from a physical/acoustical point of view and a bioacoustics assessment related to a particular species or species group. In the first instance as many features of the sound are to be included but in the latter the objective must be to describe the sound, to the best of abilities, with respect to how it is perceived and may affect the focal species.
Measurements should cover either the entire frequency range of the sound under analysis or the audible range for the focal animal. Likewise the sensitivity of instruments should be at least as large as the auditory system of the focal animal or, if this is not realised, to include this constraint in the discussion of results. Particular attention should be whether the self-noise of recording instruments is the limiting factor for conclusions on impact.
