environmental acoustics

Wolves Canis lupus use long distance vocalisations (howling) for territory maintenance and group consolidation (Harrington, Mech 1979; Nikolskij, Frommolt 1989). The acoustic feature of long distance signals should be designed for best transmission in the habitat (Morton 1975). Additionally, animals should specialise in calling during hours when transmission is best. Nocturnal temperature inversions should increase communication range (Larom et al. 1997). Measurements on the acoustic features of chorus howling were obtained from three captive grey wolf packs. The choruses consist of two components: a part with nearly no frequency modulations (A-part) and a part with deep frequency modulations (B-part). Sound energy in the A-part is concentrated around 400 Hz, whereas in the B-part maximum amplitudes are in the region between 800 until 1200 Hz. In general the B-part has higher amplitudes. It is proposed that the B-part would be more effective in sound propagation in respect to environmental noise and excess attenuation. The diurnal distribution of howling was compared for wolf packs in different habitats and in captivity. Wild wolves howl primarily during night-time whereas captive animals vocalise almost during daytime. These findings indicate that the diurnal changes in vocal activity are primarily caused by ecological factors other than the sound transmission conditions.

Bird songs, which are transmitted through a natural habitat, will always be degraded in different ways. Used as a holistic term, song degradation will usually include three aspects: attenuation and addition of background noise resulting in a reduced signal-to-noise ratio (SNR), blurring, and elongation with echoes. Degradation may therefore influence a number of different communication network activities. It may, for instance, constrain information transfer, especially over long ranges, and at the same time provide ranging cues. Some of the factors causing or influencing song degradation, e.g. temperature gradients, wind speed, relative humidity and background noise, are likely to vary over the day. The options for communication activities that depend on sound degradation could therefore be predicted to show a similar variation over the day. Normally, they are believed to be best at dawn. We investigated this by transmitting typical sound elements from the species characteristic terminal motif part of the blackcap song in a typical blackcap breeding habitat, a deciduous forest. The experiment was made in Denmark late in April at the blackcap's return from the wintering area and before foliation was complete. The sounds were transmitted at three different times of the day: dawn, morning, and afternoon. All of the above mentioned aspects of song degradation were quantified. The experiments showed a significant decrease in excess attenuation and increase in SNR over the day, where as the measures representing the blurring (blur ratio) and the elongation with tails (e.g. the tail-to-signal ratio) showed no diurnal variation. The results have a number of important implications for different communication network activities. These will be discussed, but it should be clear that the traditional view of sound propagation and communication being most effective early in the day does not always hold true.

In gill-net fisheries around the world, very large numbers of small cetaceans are killed each year as bycatch (IWC report 1994). If commercial fishing is to continue in areas where the incidental catch of cetaceans is significant then technical improvements to the method of fishing are required to mitigate the impact. Modifying fishing gear by the addition of acoustic alarms which signal the position of the net by the transmission of low level sounds was pioneered by Lien and his colleagues in Newfoundland and this technique successfully reduced baleen whale interactions with set fishing nets and traps. These simple devices, which were constructed by the fishermen, were also tested in a bottom set gillnet fishery in the Bay of Fundy where harbour porpoise mortality is high. Encouraging results from this Canadian research resulted in an improved device being developed in the USA for use in a subsequent Gulf of Maine study. The low frequency signals developed as baleen whale deterrents were also tested on captive harbour porpoises at Harderwijk in Holland where it became very evident that the frequencies used were inappropriate. The technology used in these early devices is electro- acoustically inefficient and the operating (battery) costs rather high. In Europe studies of a variety of potential acoustic deterrent devices were carried out by Loughborough University which included tests with a harbour porpoise of a wide variety of signal frequencies and waveforms. These sounds were synthesised digitally and the behaviour of a 9ee swimming animal, contained within a large floating net enclosure, were observed. The new generation of micro-controller based beacon-mode alarms developed at Loughborough University is discussed here. These devices synthesise the sounds shown to be most aversive to a porpoise and implement new features intended to minimise habituation rates and maximise battery life. A preliminary field test with wild harbour porpoises in Scotland during September 1996 showed that these devices induce a dramatic avoidance behaviour, displacing the animals in a short test to a range greater than 640 m. The design and engineering of this new technology device is discussed in the context of preparing them for a large-scale commercial fishery trial in Denmark.

Large numbers of small cetaceans are caught each year as incidental catch in gill-net fisheries around the world. The 1994 EC ASCOBANS agreement stresses the importance of reducing marine mammal bycatch, in particular of the harbour porpoise Phocoena phocoena. With the uncertainty of the severity of the problem in other fisheries it has become necessary to research into the scale of the problem in the different fishery types. Two 3-year EC (AIR DG XIV) projects have been initiated to analyse the scale of the problem in the pelagic trawls and to advise on possible methods of reducing cetacean bycatch in this fishery, one with the remit to analyse the scale of the problem in commercial fisheries (BIOECO), the other to analyse the reasons for cetacean bycatch and suggest methods of reducing it (CETASEL). As part of CETASEL, trials have been taking place both at sea with pelagic trawls and in dolphinaria in Europe. A cetacean rehabilitation centre in Neeltje Jans, Holland, has been used to examine the behaviour of a single wild harbour porpoise to different forms of acoustic disturbance produced by electronic means. The results of such tests provide valuable information as to the animal's tolerance to sound pressure levels at various frequencies and to different signatures. This information can be used further to design effective deterrents which only produce the signals which are known to deter the relevant species. The porpoise was housed in a floating net pen in minimum (tidal) of 4 metres of water providing a shallow, controlled environment in which the animal can be monitored as it re-acclimatises to the open sea. Signals introduced to the animal via four transducers produced the required aversive effect, however patterns of behaviour emerged which could not easily be explained. The porpoise did not move to the furthest point from the source, instead preferring the furthest point in line with the transducers. Mathematical modelling of the propagation of the signal in the water, showed a possible reason for the behaviour. The complicated signal pressure level pattern around the projectors showed the near field effects of the sparse array (a feature confirmed by measurements in the field), and highlighted the end-fire of the array as the single point in the enclosure where a large stable area was present. If the spacings of the projectors in the array were non integer, which was highly likely, a form of null was formed along the line of the transducers, the exact preferred position of the porpoise. This paper gives the details of the tests and results, and shows the situations modelled in post processing.

Shipping has been responsible recently for many collisions with cetaceans in the Canary Islands. A series of experiments, including the playback of artificial sounds of different frequencies, was conducted to test a system designed to keep sperm whales apart from the ferries routes. The results showed that the whales did not react to low frequency playbacks which suggests sperm whales from an area which has heavy vessel traffic have a high tolerance for noise. After the collision of a ferry with two sperm whales, ears were extracted from the two individuals, in order to assess the health of their inner ears. CT scans showed that there were no fractures or other overt evidence of impact, or ship strike related injuries; however, ears from both animals had reduced auditory nerve volumes. 0ne animal also had patches of dense tissue in the inner ear. These findings are consistent with auditory nerve degeneration and fibrous growth in response to inner ear damage. In combination with the results from the playback experiment, these results suggest that low frequency sounds from shipping may be affecting hearing and increasing collision rates. These findings are however, preliminary, and histologic analyses are underway to determine whether the primary cause of the ear changes seen with CT are disease or noise induced.

The U.S. Navy's SURTASS LFA is an active sonar system under development since the late 1980's, for employment from a small number of auxiliary ships assigned to conduct undersea surveillance to detect, classify and track potential threat submarines. SURTASS LFA system development is important because traditional passive systems are facing increasing challenges in their detection capabilities due to improved submarine quieting technologies. SURTASS LFA hardware consists of an array of acoustic transmitting components suspended an average of 100 meters beneath the host ship, which travels at a maximum of 3-4 knots This vertical array of transducers transmits waveforms (100-500 Hz) which are much longer than other active sonar systems. Echoes reflected off objects such as submarines are then detected on passive towed horizontal line arrays (HLA), and processed and evaluated to identify and classify them. The U.S. Navy will soon be at the stage of transitioning the system from a test and evaluation status to the fleet for worldwide employment to enhance undersea warfare capabilities. At present, the U.S. Navy has only one system. If proposals are carried through to completion, a total of four systems will be procured, two each for the Atlantic and Pacific fleets. High sound levels can potentially be harmful, either by affecting hearing or by causing other physiological effects; however, the combination of high transmission losses in the marine environment and the implementation of proven mitigation measures (visual and acoustic monitoring, source ramp- up, sound pressure level monitoring, shut-down criteria), reduce the potential for harmful sound levels from the SIJRTASS LFA system reaching marine animals and divers to a negligible level. During the development of the SURTASS LFA system, the U.S. Navy prepares various environmental analyses prior to sea tests, which are coordinated with the appropriate agencies responsible for wildlife protection. In July, 1996, the U.S. Navy announced a proposal for operational employment of the system and the initiation of a worldwide environmental impact statement (EIS) to evaluate the potential environmental effects of such deployment. The EIS is one element of a synergistic plan composed of three primary thrusts; 1) a comprehensive scientific research program (SRP) under the aegis of some of the world's most prominent marine bioacousticians, using the SURTASS LFA system as a scientific tool to collect much-needed data on the potential effects of low frequency sound on marine mammals; 2) an intensive diver risk assessment involving in-water low frequency sound measurements with human subjects under the direction of the U.S. Navy Submarine Medical Research Laboratory; and 3) the EIS, which will be available to the public in draft form in 1998.

The role of underwater sound as a potential stressor in the marine environment is now widely recognised and the designers of sonars find themselves increasingly constrained by environmental legislation which requires them to consider the possible harmful effects of high power sound transmissions on marine life (e.g. fish and marine mammals) and on human beings. This paper describes a formal process of environmental impact assessment being developed in support of the procurement of future sonars in the UK. The basis of this process in environmental legislation is briefly reviewed but the main purpose of the paper will be to consider the complex scientific and technical issues surrounding environmental impact assessment. In particular, Environmental Assessment (EA) for sonar systems requires a process of cause and affect modelling to be undertaken. Sonars may produce both energy and substance pollution (e.g. explosives may release toxic compounds). The 'precautionary principle' which is enshrined in environmental legislation puts the onus on the polluter to prove that his particular form of pollution does not have a harmful effect on the environment. But how are environmental impact criteria defined? Toxic effects are relatively easy to test for and to quantify. Sound energy on the other hand is rather more difficult. Consideration of the hearing sensitivity of fish, for example, leads to the notion of safe exposure level and probability of avoidance. But how representative are the experiments on which these criteria are based (e.g. the impact of seismic airguns on fish catch rates)? How can we assess the reliability of the scientific evidence given the uncertainties elsewhere, e.g. poor or inadequate knowledge of sound propagation characteristics (including non-linear effects associated with impulsive sound sources), uncertainty in environmental conditions, natural variability and the cumulative effects of repeated exposure to sound energy transmissions? There have been few coincident measurements of sound intensity in the ocean at the ranges at which a particular species exhibits avoidance behaviour and many studies make simplifying assumptions regarding acoustic propagation, e.g. spherical spreading out to unrealistically long ranges, These and other topics will be reviewed with example calculations 'to illustrate particular aspects of the EA process which is being developed by the authors.

Ferry companies are increasingly utilising high-speed wave-piercing catamarans to provide fast alternatives to conventional services. The number of such ferries operating in the UK has doubled in the last 5 years, but the environmental impacts of this trend, including possible cetacean disturbance arising from noise pollution, have received little attention. In March 1997 a new high-speed ferry service began operating from Poole in Dorset, England, passing through the Durlston Marine Research Area, the site of a long-term bottlenose dolphin monitoring project. Recordings of the ferry were obtained, from portable and seabed mounted fixed hydrophones, in order to assess the potential for disturbance of the study animals. The most significant sound outputs are two sharp peaks around 500Hz. Apart from these, machinery noise also produces a continuous spectrum across the range 100Hz to above 5kHz. The other major noise source is from displaced water, contributing to noise levels in the higher part of the spectrum, particularly above 10kHz. For bottlenose dolphins, the ferry would appear unlikely to cause disturbance on acoustic grounds. In keeping with this, comparison of bottlenose dolphin sightings data before and since the commencement of the ferry service found no discernible change in the timing or frequency of dolphin activity in the study area. However, this was very much a preliminary short-term study and further data are required before firm conclusions can be made.

Most species of marine mammals seem highly reliant on and sensitive to underwater sounds. Sounds important to marine mammals may include calls from conspecifics, odontocete echolocation sounds, predator and prey sounds, and environmental sounds (e.g. surf or ice noise). Some man-made noises are known or suspected to have negative effects on marine mammals, including noise-induced masking, disturbance, hearing impairment, and possibly stress. However, marine mammals are adapted to a variable and often naturally noisy environment. Also, even when levels of man-made noise are well above natural ambient levels, negative effects on marine mammals are not always obvious. Data available up to early 1995 were summarised in the book "Marine Mammals and Noise'' (Richardson et al. 1995, Academic Press). Since then, advances have occurred in some but not all areas of particular concern:

(1) When can marine mammals hear man-made noise? Additional data are becoming available for some small- and moderate-sized odontocetes, pinnipeds, and manatees. There is still an urgent need for direct audiometric data from baleen and sperm whales.

(2) Does man-made noise mask important natural sounds? Data are available on masking in a few species of captive odontocetes and pinnipeds. However, we need data on masking processes and significance when free-ranging marine mammals are exposed to typical man-made sounds, including variable, non-tonal, and directional sounds.

(3) When does man-made noise disturb mammals, and when is disturbance strong enough to constitute harassment? Disturbance effects are graduated, not ''all or none''. Sometimes no disturbance is apparent even at short ranges with high received levels (RQ. At other times there is strong disturbance even at long ranges with low RLs. Strong and/or prolonged disturbance may have negative biological elects even if there is no physical damage. However, infrequent brief disturbances may have no biological significance, and if so should not be considered "harassment''. Additional controlled studies, both field and captive, are needed.

(4) What are the thresholds for noise-induced auditory impairment and non- auditory effects, and what types of man-made sounds could elicit them under field conditions? The first data on Temporary Threshold Shift (TTS) in marine mammals have been released. recently. TTS work with additional species and exposure conditions is needed. However, TTS results have limitations in establishing damage risk criteria (DRC), and relationships between TTS and harassment are uncertain.

(5) Noise-induced stress in marine mammals is almost entirely unstudied.

Mitigation measures sometimes used to reduce noise effects include seasonal and geographic restrictions, ramping up, and real-time monitoring plus localised mitigation. We need more data on the effectiveness of ramping up, visual and/or acoustic monitoring, and localised measures such as minimum approach distances, minimum altitudes, and shutdown radii. Progress is being made toward understanding noise effects on marine mammals, in focusing on the most serious issues, and in devising mitigation approaches. However, the issues are complex and the needed studies are often difficult. Some major emitters of underwater sound remain reluctant . to become involved in the process. It will take time, money and cooperation to conduct the needed studies, to determine which situations need mitigation, and to devise, test and implement effective yet practical mitigation measure.

There is a growing concern in the literature about the effects of low frequency sounds (LFS) on marine mammals. A primary way to assess these effects on marine mammals involves the study of disturbance reactions. Detailed research of disturbance reactions of submerged marine mammals requires 3- dimensional localization and tracking of the animals. Animals such as sperm whales Physeter macrocephalus are localized passively with the use of travel time differences (TTD) of their vocalizations received by multiple hydrophones at known positions. Classically, straight-line paths of sound propagation between source and receiver are used to calculate source position. A more accurate calculation of source position involves naturally occurring non- constant sound speeds. This gives rise to arced paths of sound propagation between source and receiver. An algorithm is used to recursively pinpoint source position in a medium with a non-constant sound speed. 5 hydrophone array configurations are tested, each with 30 randomly generated source positions. Average errors of the 150 source position calculations (x, y, z) are (±1.58m, ±1.70m, ±10.44m) for the straight line, and ±0.76m, ±0.87m, ±1.10m) for the algorithm. On average, the algorithm improves the source depth calculation by an order of magnitude.