The well-known Dolfinarium in Harderwijk (The Netherlands) is equipped with a
rehabilitation pool for dolphins and porpoises, which are mediacally treated
after being rescued if stranded on a beach.
During this treatment period
the marine biologist of the Dolfinarium, R. Kastelein Ph.D., who is
concidered worldwide to be a leading edge investigator in this field, takes
the opportunity to set up a training program for some of the porpoises after
their complete recovery.
The objective of his investigation is to find ways
to prevent these sea mammals being captured in the nets of the fishermen.
Once in contact with the nets the natural reaction of the dolphins and
porpoises is a number of rollovers, which is lethal for them: the porpoise
will be entangled in the nets and drown.
Knowing that dolphins and porpoises use their acoustic sensors to determine
objects, the idea is to develop a small salt-water proof stand-alone pinger,
which generates marine mammal repulsive underwater sounds. These pingers
would have to be fixed onto the nets.
To be able to quantify the term 'repulsive sounds' including their sensing
directivity, tests are needed with these marine mammals.
For this purpose the Dolfinarium in Harderwijk
asked LabForce1 to manufacture 16 hydrophone transducers (a piezo electric cristal, which converts electric
energy and the other way around). After manifacturing, testing and
delivery, these transducers are currently being used in the Neeltje Jans in
Zeeland where the Dolfinarium has a test site (a floating pen).
Each hydrophone 
is fixed in a 14-m-diameter circle at 22.5 degrees distance
from each other and the porpoise is trained to position himself in the
centre of the circle.
After a few seconds one of the 16 hydrophones at
randomly is emitting an acoustic signal.
If the porpoise swims directly to
the emitter, the activity is recorded as a 'hit'. If he indicates the wrong
transmitter, it is recorded as a 'miss'.
From these recordings the directivity of the porpoise's receptor might be
judged. By varying the emitted signal (amplitude, frequency) and studying the
animal's performance per parameter, the best signal to be used in future
commercial pingers can be derived.
