Page 22 - Electronic Navigation Cyber Book

the physical dimensions of the steel plate need to be small and the plating
will have to vary in thickness from ship to ship because of different
frequencies used.
Thus for pierced hull installation the problem of using a frequency suitable
for reasonable penetration no longer applies and higher frequencies can
be used.

With a very low frequency, the size of the oscillator becomes
inconveniently large; secondly, there is a lack of selectivity from water
and other noises within the audible range and finally, less directivity.
The higher frequencies give more improved selectivity from the noise
and better directivity is possible, but there is less penetration.

Echo Sounding - Full cycle of operation

The full cycle of operations for one sounding is as follows:

The recording stylus starts each cycle as it moves to past the zero. It
triggers an electronic generator, which produces a known number of
electrical oscillations, which are applied to the transmitting transducer
(Tr/Tx). The Tr/Tx creates the sound pulse, which is injected into the sea,
travels to the bottom, is reflected and returns as an echo to the receiving
transducer, where it is converted back into an electrical pulse. This is
amplified and applied to the stylus, which has moved across the
recording paper, to indicate the depth against a suitable scale. The stylus
moves across the paper at a constant speed which is decided by the
designer after he has decided the following:
Maximum depth to be displayed,

The width of the paper and the SW velocity to be used.

The pulse length to be used for transmission is governed by a number of
factors. The minimum theoretical depth that can be measured is equal to
half a pulse length. Since sound travels at approx. 1500 meters per
second, a pulse length of 1 millisecond (ms) will mean that the theoretical
minimum depth, which could be measured, would be 1.5 meters. In
practice, it would be about this value.

The difference between the theoretical and the practical values is
because the transducer being a resonant device does not stop oscillating
immediately the electrical pulse ceases. It shows a tendency to "ring"
when energized and this is usual for the time taken for 10 to 12 cycles. If

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