Vocalisations of marine mammals, fish and other
organisms are of interest to many scientists who study animal behavior. However, it
can be difficult to make high quality recordings of underwater sound due to unwanted
environmental noise. This can be a problem not only for the scientists but also for
the animals that have to live in increasingly noisy environments and whose health and
well-being might be compromised by not being able to hear useful sound signals.
Contrary to popular opinion, the sea in general is a surprisingly noisy place and
coastal zones even more so. Natural noise sources include wave and wind action, bubbles,
rainfall and even snowflakes landing on water; noise produced by marine mammals, fish, and
particularly Snapping shrimp (Alpheus heterochaelis); and man-made noise such as
shipping and small boat noise, oil drilling, mining and general off-shore geological
explorations, and sonar systems.
While some of these sources produce sound in limited frequency bands (specific pitches)
and are therefore relatively easy to filter out of scientific recordings of marine mammal
vocalisations, some sources cover a very wide range of frequencies and are much more
difficult to remove. This is a similar problem to trying to listen to someone talking at
a very noisy traffic junction or during a rock concert: you can shout to a certain extent
but sometimes the problem is impossible to overcome. Snapping shrimp are a particularly
important source of this kind of broadband environmental noise. They occur in large colonies
in shallow tropical waters, particularly around rocks and man-made structures and the noise
made by these small crustaceans each snapping a single claw many times dominates the sound
field in tropical coastal regions. The result sounds like an egg frying in oil.
Click here to a short audio clip of snapping shrimps.
Unwanted noise is not limited to the study of marine mammal vocalisations and is a common
problem in more general acoustic signal processing. MMRL has developed several techniques
for removing acoustic noise and separating out different signal components from a composite
signal comprising of ‘transient sounds’ such as dolphin echolocation clicks, and ‘tonals’
such as dolphin whistles and Humpback whale song.
Despite a lot of research over many years, there is still no easy way to remove broadband
transient noise from broadband transient acoustic signals because they both share superficially
common signal features. MMRL is working with the NUS Department of Electrical and Computer
Engineering and TMSI’s Acoustic Research Laboratory to address this problem which is
fundamentally mathematically based. Some of the methods we are exploring are as follows:-