Paul A. Lepper

Temporary shift in masked hearing thresholds in a harbor porpoise (Phocoena phocoena) after exposure to seismic airgun stimuli

An auditory study was conducted to derive data on temporary threshold shift (TTS) induced by single impulses. This information should serve as basis for the definition of noise exposure criteria for harbor porpoises. The measurements of TTS were conducted on a harbor porpoise by measuring the auditory evoked potentials in response to amplitude-modulated sounds. After obtaining baseline hearing data the animal was exposed to single airgun stimuli at increasing received levels. Immediately after each exposure the animals hearing threshold was tested for significant changes. The received levels of the airgun impulses were increased until TTS was reached. At 4 kHz the predefined TTS criterion was exceeded at a received sound pressure level of 199.7 dB(pk-pk) re 1 microPa and a sound exposure level (SEL) of 164.3 dB re 1 microPa(2) s. The animal consistently showed aversive behavioral reactions at received sound pressure levels above 174 dB(pk-pk) re 1 microPa or a SEL of 145 dB re 1 microPa(2) s. Elevated levels of baseline hearing sensitivity indicate potentially masked acoustic thresholds. Therefore, the resulting TTS levels should be considered masked temporary threshold shift (MTTS) levels. The MTTS levels are lower than for any other cetacean species tested so far.

Insect attraction to wind turbines: does colour play a role?

The phenomenon of wildlife mortality at wind turbine installations has been generating increasing concern, both for the continued development of the wind industry and for local ecology. While an increase in aerial insectivore activity in the vicinity resulting from insect attraction to turbines remains a strong possibility, little research exists on the possible causes for such events. In this paper, the relative attraction of a selection of specific turbine colours and other hues is assessed in order to determine if turbine paint colour could be influencing insect numbers at these installations. The common turbine colours ‘pure white’ (RAL 9010) and ‘light grey’ (RAL 7035) were among those found to attract significantly more insects than other colours tested, suggesting colour may well have a role to play in potential mitigation.

Perception of Low-Frequency Acoustic Signals by a Harbour Porpoise (Phocoena phocoena) in the Presence of Simulated Offshore Wind Turbine Noise

Using auditory evoked potential (AEP) methods, a study was conducted on a harbour porpoise (Phocoena phocoena) at the Dolfinarium Harderwijk in The Netherlands. The study measured the audible range of wind turbine sounds and their potential masking effects on the acoustic perception of the animal. AEPs were evoked with two types of acoustic stimuli: (1) click-type signals and (2) amplitude-modulated signals. The masking noise resembling the underwater sound emissions of an operational wind turbine was simulated. At first, the animal’s hearing threshold was measured at frequencies between 0.7 and 16 kHz. Subsequently, these measurements were repeated at frequencies between 0.7 and 2.8 kHz in the presence of two different levels of masking noise. The resulting data show a masking effect of the simulated wind turbine sound at 128 dB re 1 µPa at 0.7, 1.0, and 2.0 kHz. This masking effect varied between 4.8 and 7.3 dB at those frequencies. No significant masking was measured at a masking level of 115 dB re 1 µPa. The available data indicate that the potential masking effect would be limited to short ranges in the open sea, but limitations exist to this conclusion and all estimates are based on existing turbine types, not taking into account future developments of larger and potentially noisier turbine types.

Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

A technique for the calibration of hydrophones using an optical method is presented. In the method, a measurement is made of the acoustic particle velocity in the field of a transducer by use of a thin plastic pellicle that is used to reflect the optical beam of a laser vibrometer, the pellicle being acoustically transparent at the frequency of interest. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. A commercially available laser vibrometer is used to undertake the calibrations, and results are presented over a frequency range from 10 to 600 kHz. A comparison is made with the method of three-transducer spherical-wave reciprocity, with agreement of better than 0.5 dB over the majority of the frequency range. The pellicle used is in the form of a narrow strip of thin Mylar©, and a discussion is given of the effect of the properties of the pellicle on the measurement results. The initial results presented here show that th...

What is the source level of pile driving noise in water

To meet the growing demand for carbon-free energy sources, the European Union (EU) has ambitious plans to increase its capacity for generation of offshore wind power. The United Kingdom and The Netherlands, for example, plan to increase their offshore power-generating capacity to 33 and 6 GW, respectively, by the year 2020. Assuming that this power is generated entirely by wind and that a single wind turbine can generate up to 10 MW, at least 3,900 offshore turbines would be required by these two states alone to achieve this goal. A popular turbine construction method known as “pile driving” involves the use of hammering a steel cylinder (a “monopile”) into the seabed. A concern has arisen for the possible effect on mammals (Southall et al. 2007) and fish (Popper and Hastings 2009) of the sound produced by the succession of hammer impacts required to sink the pile to its required depth (tens of meters).

Rethinking Underwater Sound-Recording Methods to Work at Tidal-Stream and Wave-Energy Sites

Commercial-scale devices to extract energy from tidal streams and waves may be new, but an associated industry is developing fast. In most countries, device introduction will require investigation and some level of proof that they do not unduly harm local wildlife. Of the impacts that they might have, the emission of acoustic energy (noise) into the marine environment is important. In operation, it is possible, though unlikely, that they will emit sufficient noise to cause auditory damage to sensitive species, but some level of area avoidance/attraction and masking is likely. Nevertheless, all such devices will require perceivable acoustic signatures for animals to detect and avoid colliding with them. To understand these issues, information on operational device acoustic characteristics is required along with information on existing background noise levels at sites suitable for extraction of marine energy. However, the energetic features of these locations with intense lateral, vertical or oscillatory motion mean that conventional methods of underwater sound recording are unsuitable. Here new methods for sound measurement specifically tailored to tidal-stream and wave-energy sites are introduced. The methods are illustrated following performance tests and real measurements at the European Marine Energy Centre tidal test site in Orkney, UK.

Wind turbines and bat mortality: Doppler shift profiles and ultrasonic bat-like pulse reflection from moving turbine blades

Bat mortality resulting from actual or near-collision with operational wind turbine rotors is a phenomenon that is widespread but not well understood. Because bats rely on information contained in high-frequency echoes to determine the nature and movement of a target, it is important to consider how ultrasonic pulses similar to those used by bats for echolocation may be interacting with operational turbine rotor blades. By assessing the characteristics of reflected ultrasonic echoes, moving turbine blades operating under low wind speed conditions (<6 m s(-1)) were found to produce distinct Doppler shift profiles at different angles to the rotor. Frequency shifts of up to ±700-800 Hz were produced, which may not be perceptible by some bat species. Monte Carlo simulation of bat-like sampling by echolocation revealed that over 50 rotor echoes could be required by species such as Pipistrellus pipistrellus for accurate interpretation of blade movement, which may not be achieved in the bats approach time-window. In summary, it was found that echoes returned from moving blades had features which could render them attractive to bats or which might make it difficult for the bat to accurately detect and locate blades in sufficient time to avoid a collision.

The application of scanning vibrometer in mapping ultrasound fields

Optical interferometry has been used for calibrating hydrophones at frequencies greater than 500 kHz for many years with very small uncertainties. Optical method have been found to be an effective non-perturbing method for detecting ultrasound fields with very fine spatial and temporal resolution; ultrasound fields can be mapped using a membrane and a scanning vibrometer, and two-dimensional fields can be imaged when the optical beam from a Laser Doppler Vibrometer (LDV) is aligned normal to the axis of the sound beam from a arbitrarily shaped projector. This paper presents a theoretical analyses for using a LDV system to detect ultrasound fields and further presents the result of some initial optical scan measurements performed using an LDV system. Results are presented of the particle velocity distributions produced by a focused ultrasound projector measured using a scanning LDV on an acoustically compliant, optically reflective membrane using a scanning LDV. Further measurement results are also presented of two-dimensional field maps obtained by positioning the laser beam perpendicular to the acoustic axis of a projector and detecting the line-integral acousto-optic effect. Visual maps of the acoustic waves radiating from the projector and the reflected and scattered propagating field by objects in water were obtained using this method and are presented in this paper.

Monitoring the temporal and spatial characteristics of the noise radiated from marine piling

Noise is often an unintended by‐product of offshore activities, and a significant source of impulsive underwater noise is marine piling, where a pile is driven into the sea‐bed using a hydraulic hammer. This paper describes work undertaken to monitor the underwater radiated noise during offshore marine piling. The measurements made include full characterisation of the temporal variation of the resulting sound field, including frequency content of the pulses and the increasing amplitude typically observed during the soft start period. Results are also shown for the spatial variation of the field, including the effect on propagation of significant bathymetry changes (for example, due to the presence of a sand bank). The energy within each acoustic pulse is an appropriate measure of the sound field, having the advantage that it is relatively straightforward to add the contributions from each pulse in order to derive an overall sound exposure level (SEL) in a manner analogous to methods familiar from air acou...

Underwater noise generated from marine piling

Marine piling impact piling is a source of high-amplitude impulsive sound that can travel a considerable distance in the water column and has the potential for impact on marine mammals and fish. It involves steel piles being driven into the seabed using powerful hydraulic hammers, and is a commonly used construction method for fixing structures to the sea-bed in the offshore industry, and for the installation of offshore wind turbines in shallow coastal waters such as those around the UK. This paper describes methodologies developed for measurement of marine piling including estimation of the energy source level. Measurement results are presented of measurements made during the construction of an offshore windfarm, involving piles of typically 5 m in diameter driven by hammers with typical strike energies of around 1000 kJ. Acoustic data were recorded as a function of range from the source using hydrophones deployed form a vessel, allowing the transmission loss to be confirmed empirically. The use of fixe...