Page 312 - Introduction to Naval Architecture

VIBRATION, NOISE AND SHOCK 297

differently. To define a noise fully the strength of each component and
its frequency must be specified. This is done by presenting a spectral
plot of the noise. This approach is needed for instance in considering
the importance of radiated noise in terms of its likely detection by
enemy sensors or weapons. For human reactions to noise an alternative
is to express noise levels in dB(A). The A weighted decibel is a measure
of the total sound pressure modified by weighting factors which vary
with frequency. The end result reflects more closely a human's
subjective appreciation of noise. Humans are more sensitive to high
(1000 Hz and over) than low (250 Hz and less) freqencies and this Is
reflected in the weighting factors.
Primary sources of noise are the same as those which generated
vibration, that is machinery, propulsors, pumps and fans. Secondary
sources are fluids in systems, electrical transformers and the sea and
waves interacting with the ship. Noise from a source may be transmitted
through the air surrounding the source or through the structure to
which it is attached. The structure on which a machine is mounted can
have a marked influence on the amounts of noise transmitted. The
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actions are complex . Not only is it difficult to predict the transrnision
losses in typical structures but airborne noise may excite structure on
which it impacts and directly excited structure will radiate noise to the
air. For machinery, combustion forces, impact forces and rapidly
changing pressures generate structural wave motions in the machine
which radiate to the air or travel through the mounting system into the
ship's structure. For a propulsor much of the noise will be transmitted
into the water. That represented by pressure fluctuations on the
adjacent hull will cause the structure to vibrate transmitting noise both
into the ship and back into the water. Other transmission paths will be
through the shaft and its bearings. At low powers noise will arise from
the hydrodynamic forces generated by the propulsor working in a non-
uniform wake. At higher powers, or when manoeuvring, cavitation can
occur and then the noise increases dramatically. For pumps and fans
the impeller produces noise which can travel through the fluid along
the pipe or trunk or be radiated from the conduit.
A designer will be concerned to limit noise because:

(1) Internal noise levels can affect the performance of the crew and
the comfort of passengers.
(2) Noise transmitted into the water can betray the presence of the
ship. It can trigger off enemy mines or provide a signal on which
weapons can home. It can reduce the effectiveness of the ship's
own sensors.
It is the former effects which are of primary concern here. The
importance of the latter for the signature of warships is discussed