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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:-
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Imaged-Based Acoustic Signal De-noising This new method based on a
combination of signal and image processing techniques, aims to remove acoustic
noise from FM tones, enhance the spectrogrammes and use spectrogramme
segmentation to perform reliable tracing of the fundamental frequency
variation of a dolphin or dugong whistle (or other FM tones). This
package of tools is
necessary for accurate classification of whistles produced by marine
mammals. Click here to learn
more about spectrogrammes. |
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This technique is jointly developed by MMRL and the Acoustic Research
Laboratory (ARL). SSA is a Singular Value Decomposition (SVD) based
procedure that decomposes a time series into a number of times series
components. The sum of all these time series components is equal to the
original time series. However, the time series components can be grouped
together on the prior to summation on the basis of some criteria to
group like time series components together. Currently grouping is
performed by using statistical properties such as kurtosis and is summed
separately to separate tonals, transients and spectrally smooth noise.
However, the number of components from the decomposition has to be
sufficiently large for adequate grouping, and, as the length of the time
series and number of components increase, this translates to higher
requirements for computation time and memory. The technique has proved
useful when extracting dolphin echolocation clicks from tonal noise such
as radio interference, and boat noise. |
