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7.2.4 Directional Index (DI)
The Directional Index gives a reduction in noise level, governed by the
properties of the transmit/receive transducer.
An omni-directional transducer will theoretically pick-up noise from all
directions. A directional transducer will hear noise from only one direction.
Thus a method of reducing the apparent background noise, is to utilise a
highly directional transducer.
The ISA500 utilises a 6º conical acoustic beam. Meaning that any potential
interference effects which exist outside of this beam, will not have a negative
impact on the operation of the unit.
7.2.5 Detection Threshold (DT)
The Detection Threshold is a property of the acoustic system. It is defined as
the minimal signal to noise ratio required in order to detect the acoustic
signal.
The threshold can be lowered by minimising the device self-generated noise,
utilising advanced acoustic signalling, and by having a highly capable
matching filter, or a highly sensitive transducer on the receive side to detect
the signal.
The ISA500 utilises a proprietary acoustic correlator to detect the returning
acoustic signal, low noise digital electronics and a highly sensitive composite
transducer to enable it to detect extremely small acoustic signals.
An appreciation of the Sonar equation will provide an understanding of the
fundamental operation of the ISA500. It may also help during installation and also
when fault finding as it provides an indication as to influential factors.
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