62
C. Heeps . V. May
of close-up recognition, but many organisms respond to variations in pressure and
marine mammals communicate and locate themselves sonically. Underwater communication and detection for military purposes depends upon the application of sound as
does seabed mapping and investigation. However, the public understanding of the nature and use of sound in the sea is extremely limited. Although many people have heard
the sounds of dolphins, whales and other marine mammals and know that they use
sound to navigate, hunt for food and communicate, very few have seen the seabed portrayed on sonographs. An informal survey of scientists, engineers as well as the wider
public showed that whilst many are aware that the speed of sound in air is about 340 m S-I
(95% of respondents), the same percentage did not know if sound travels faster in air or
water. Even fewer knew that the average speed of sound in water is about 1540 m S-I. In
a scientific group, including individuals with degrees in physics, knowledge of the speed
of sound in water was either absent or faulty. In order to overcome this lack of know ledge, a number of related projects have been developed which rely primarily on most
visitors' fascination with dolphins.
A hydrophone was sited in 12 m water 400 m offshore in Durlston Bay in 1993. Underwater sounds in the Bay are cabled to the centre where they can be recorded for
future analysis and transmitted as part of an interpretative display on underwater acoustics. Filters are used to enhance dolphin sounds for human hearing. Marine sounds
within frequencies 10 to 20 kHz are recorded, but filtering can detect sources within an
envelope of 200 kHz. The hydrophone picks up not only the echo location of the dolphins as they map out their environment, search for food and communicate with each
other, but also a wide range of ambient sounds, including those made by other marine
animals, passing ships, seismic exploration, power boats, personal watercraft, pile driving, even land-based quarrying, thereby giving a true indication of noise levels in coastal
waters. There appear to be two distinct sounds associated with cetacean activity. One is
a comparatively slow click rate, which usually coincides with sightings of one or more
bottlenose dolphins (Tursiops truncatus) just offshore. The second sound, a much faster
click rate with high whistles, is present most of the time. These may be produced by
common dolphins (Delphinus delphis) or harbour porpoise (Phocoena phocoena). There
are many other sounds, including a continuous crackling which may be snapping
shrimps or mussels opening and closing their shells. Unidentified explosions may come
from local quarries and naval sonars have been picked up from frigates operating over
20 km away. High frequency fast repetition clicks have been recorded for about 80% of
the time since July 1994. High frequencies propagate less well in water so the frequent recording of these clicks implies that the sound source was generally within 500 m of the
hydro-phone. A dolphin sightings scheme, using volunteer observers, has highlighted the
importance of the waters off Durlston for feeding and migrating bottlenose dolphins.
Even today, many first-time visitors to the centre do not believe that dolphins can be
seen off the Dorset coast until they encounter the displays within the centre. Using this
new knowledge, visitors are then encouraged to consider noise pollution in the sea as
an environmental issue, the potential disturbance on wildlife from some human coastal
activities and how the information collected can actually aid managers in their management tasks. In addition to the dolphin sightings scheme, a comprehensive photoidentification survey is providing valuable information with respect to dolphin
populations in this section of the English Channel. Collectively these projects are known
as Coastwatch which "aims to raise awareness of the coastal waters off Durlston Head".
C. Heeps . V. May
of close-up recognition, but many organisms respond to variations in pressure and
marine mammals communicate and locate themselves sonically. Underwater communication and detection for military purposes depends upon the application of sound as
does seabed mapping and investigation. However, the public understanding of the nature and use of sound in the sea is extremely limited. Although many people have heard
the sounds of dolphins, whales and other marine mammals and know that they use
sound to navigate, hunt for food and communicate, very few have seen the seabed portrayed on sonographs. An informal survey of scientists, engineers as well as the wider
public showed that whilst many are aware that the speed of sound in air is about 340 m S-I
(95% of respondents), the same percentage did not know if sound travels faster in air or
water. Even fewer knew that the average speed of sound in water is about 1540 m S-I. In
a scientific group, including individuals with degrees in physics, knowledge of the speed
of sound in water was either absent or faulty. In order to overcome this lack of know ledge, a number of related projects have been developed which rely primarily on most
visitors' fascination with dolphins.
A hydrophone was sited in 12 m water 400 m offshore in Durlston Bay in 1993. Underwater sounds in the Bay are cabled to the centre where they can be recorded for
future analysis and transmitted as part of an interpretative display on underwater acoustics. Filters are used to enhance dolphin sounds for human hearing. Marine sounds
within frequencies 10 to 20 kHz are recorded, but filtering can detect sources within an
envelope of 200 kHz. The hydrophone picks up not only the echo location of the dolphins as they map out their environment, search for food and communicate with each
other, but also a wide range of ambient sounds, including those made by other marine
animals, passing ships, seismic exploration, power boats, personal watercraft, pile driving, even land-based quarrying, thereby giving a true indication of noise levels in coastal
waters. There appear to be two distinct sounds associated with cetacean activity. One is
a comparatively slow click rate, which usually coincides with sightings of one or more
bottlenose dolphins (Tursiops truncatus) just offshore. The second sound, a much faster
click rate with high whistles, is present most of the time. These may be produced by
common dolphins (Delphinus delphis) or harbour porpoise (Phocoena phocoena). There
are many other sounds, including a continuous crackling which may be snapping
shrimps or mussels opening and closing their shells. Unidentified explosions may come
from local quarries and naval sonars have been picked up from frigates operating over
20 km away. High frequency fast repetition clicks have been recorded for about 80% of
the time since July 1994. High frequencies propagate less well in water so the frequent recording of these clicks implies that the sound source was generally within 500 m of the
hydro-phone. A dolphin sightings scheme, using volunteer observers, has highlighted the
importance of the waters off Durlston for feeding and migrating bottlenose dolphins.
Even today, many first-time visitors to the centre do not believe that dolphins can be
seen off the Dorset coast until they encounter the displays within the centre. Using this
new knowledge, visitors are then encouraged to consider noise pollution in the sea as
an environmental issue, the potential disturbance on wildlife from some human coastal
activities and how the information collected can actually aid managers in their management tasks. In addition to the dolphin sightings scheme, a comprehensive photoidentification survey is providing valuable information with respect to dolphin
populations in this section of the English Channel. Collectively these projects are known
as Coastwatch which "aims to raise awareness of the coastal waters off Durlston Head".