Over the years marine biologists and oceanographers have specialized in using passive acoustic methods for studying marine life and the sound that they create. Hydrophones can be designed and built to match very specific needs. These are extremely powerful tools for surveying marine populations and for investigating their vocal behavior in an environment much more transparent to sound than to light, and has a minimum disturbance to the animals.
A simple stereo hydrophone array is towed behind the boat on 200m of cable at all times and monitored regularly. This means that even when making oceanic crossings data on the distribution and abundance of cetaceans is being collected. The stereo towed hydrophone simply consists of two hydrophone elements spaced 3m apart inside a housing tube, providing a streamlined casing for on the move recordings. With this equipment, accurate surveys of cetacean populations can be made on a very low budget
How do the two "microphones" in the towed hydrophone allow the listener
to gauge whether the vocalizing whale is ahead or behind the vessel?
The clicks of sperm whales (Physeter catadon) travel for long distances and propagate more or less spherically - that is they are not very directional in the frequencies which travel well, meaning the whales don't have to be pointing at you for you to be able to hear them. Thus the sound waves will arrive at either the rear or forward element of the hydrophones first depending on whether the whale is ahead of or behind the boat. The two elements in the towed hydrophone give a stereo effect when listened to on headphones, with one ear listening to the forward element and other to the rear element. The listener can, with experience, judge whether the sounds are arriving in the right or left ear first and thus tell whether the whale is ahead or behind, and maneuver the boat accordingly.
Why are they towed so far from the boat?
A towing cable length of at least 100m is necessary to separate the sensitive hydrophone from the often-noisy boat. Boat noise consists either of engine noise when motoring or the sound of waves hitting the hull when sailing in even moderate conditions. By towing the hydrophone at this distance, the range at which cetaceans can be heard over such boat noise is greatly increased.
Using a directional hydrophone on the rear of the vessel can give precise bearings to whales for closer observation. By rotating the hydrophone until the whale’s loudest sound is heard, get a directional fix. This task is easily accomplished even if the whales are over a mile away.
Using hydrophonic equipment, sperm whale groups have been tracked for up to three days at a time, allowing scientists to build up a much better picture of the behavior of this animal. In addition marine biologists have investigated, through the building of specialized hydrophones, whale vocalizations beyond the normal range of human hearing. For example, Raymond Browne, an electronics Ph.D. student based in Birmingham, U.K. is looking for low frequency sounds emitted by fin whales (Balaenoptera physalus), while Olly Chappell, a consultant electronic engineer who works for IFAW has designed equipment which can detect the ultrasonic (very high frequency) emissions of harbor porpoises.
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