On definitions of signal duration, evaluated on close-range airgun signals.

In impact assessments for underwater noise, the duration of a transient signal is often expressed by the 90%-energy signal duration τ90 %. Consequently, the rms sound pressure is computed over this duration. Using a large set of measurements on marine-seismic airgun signals, it is shown that τ90 % is often very close to the interval between the primary and secondary pulse (the bubble period) or a small integer multiple thereof. In this situation τ90 % is a measure of the duration of the relative silence between primary and secondary peaks, which is not the intended measure. Rarely, τ90 % quantifies the duration of the main peak, leading to a much lower value of τ90 %. Since the number of peaks included in τ90 % is sensitive to the nature of the signal, relatively small differences in the signal lead to large differences in τ90 %, causing instability in any metric based on τ90 %, e.g., the rms sound pressure. Alternative metrics are proposed that do not exhibit these weaknesses. The consequences for the interpretation of sound pressure level of a transient signal, and the benefits of using a more stable metric than τ90 % are demonstrated.

Characterization of the acoustic output of single marine-seismic airguns and clusters: The Svein Vaage dataset.

The acoustical output of marine-seismic airguns is determined from recordings of the sound pressure made on hydrophones suspended below a floating barge from which the airguns are also deployed. The signals from multiple types of airguns are considered and each type is operated over a range of deployment depths and chamber pressures. The acoustical output is characterized in terms of a "source waveform" with dimensions of the pressure-times-distance and in an infinite idealized medium, could be divided by the source-receiver distance to give the sound pressure at that receiver. In more realistic environments, the source waveform may be used to predict the pressure at any arbitrary receiver position simply by the application of a time-domain transfer function describing the propagation between the source and receiver. The sources are further characterized by metrics such as the peak source waveform and energy source level. These metrics are calculated in several frequency bands so that the resulting metrics can be used to characterize the acoustical output of the airguns in terms of their utility for seismic image-processing or possible effects on marine life. These characterizations provide reference data for the calibration of models that predict the airguns' acoustical output. They are validated via comparisons of the acoustic pressure measured on far-field hydrophones and predicted using the source waveforms. Comparisons are also made between empirically derived expressions relating the acoustic metrics to the chamber volume, chamber pressure, and deployment depth and similar expressions from the literature.

Application of damped cylindrical spreading to assess range to injury threshold for fishes from impact pile driving.

Environmental risk assessment for impact pile driving requires characterization of the radiated sound field. Damped cylindrical spreading (DCS) describes propagation of the acoustic Mach cone generated by striking a pile and predicts sound exposure level (LE) versus range. For known water depth and sediment properties, DCS permits extrapolation from a measurement at a known range. Impact assessment criteria typically involve zero-to-peak sound pressure level (Lp,pk), root-mean-square sound pressure level (Lp,rms), and cumulative sound exposure level (LE,cum). To facilitate predictions using DCS, Lp,pk and Lp,rms were estimated from LE using empirical regressions. Using a wind farm construction scenario in the North Sea, DCS was applied to estimate ranges to recommended thresholds in fishes. For 3500 hammer strikes, the estimated LE,cum impact ranges for mortal and recoverable injury were up to 1.8 and 3.1 km, respectively. Applying a 10 dB noise abatement measure, these distances reduced to 0.29 km for mortal injury and 0.65 km for recoverable injury. An underlying detail that produces unstable results is the averaging time for calculating Lp,rms, which by convention is equal to the 90%-energy signal duration. A stable alternative is proposed for this quantity based on the effective signal duration.

Application of kurtosis to underwater sound.

Regulations for underwater anthropogenic noise are typically formulated in terms of peak sound pressure, root-mean-square sound pressure, and (weighted or unweighted) sound exposure. Sound effect studies on humans and other terrestrial mammals suggest that in addition to these metrics, the impulsiveness of sound (often quantified by its kurtosis ß) is also related to the risk of hearing impairment. Kurtosis is often used to distinguish between ambient noise and transients, such as echolocation clicks and dolphin whistles. A lack of standardization of the integration interval leads to ambiguous kurtosis values, especially for transient signals. In the current research, kurtosis is applied to transient signals typical for high-power underwater noise. For integration time (t2-t1), the quantity (t2-t1)/ß is shown to be a robust measure of signal duration, closely related to the effective signal duration, τeff for sounds from airguns, pile driving, and explosions. This research provides practical formulas for kurtosis of impulsive sounds and compares kurtosis between measurements of transient sounds from different sources.