Page 313 - Introduction to Naval Architecture

298 VIBRATION, NOISE AND SHOCK

Table 11.4

location Permitted noise level (dBA)

Engine room 110
Workshops 85
Bridge 65
Mess room 65
Recreation space 65
Cabins 60

briefly in Chapter 12. Apart from noise making it hard to hear and be
heard, crew performance can fall off because prolonged exposure to
noise can cause fatigue and disorientation. It can annoy and disturb
sleep. High levels (about 130 to 140dB) will cause pain in the ear and
higher levels can cause physical harm to a person's hearing ability. Thus
noise effects can range from mere annoyance to physical injury. The
IMO lay down acceptable noise levels in ships according to a
compartment's use, Table 11.4.

Noise calculations
There are a number of acoustical calculations a designer can apply to
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ship noise estimation and for the design of noise control systems .
Both finite element and statistical energy analysis methods are used.
Since the level of structure borne noise from a machine depends upon
the forces in the machine and the structure on which it is mounted the
concept of structural mobility is introduced. This is the ratio of velocity to
force at the excitation point. The structural mobility, velocity and force
will all vary with frequency. For a machine mounted rigidly on a plate,
the structural mobility depends upon the mass per unit area and
thickness of the plate and upon the velocity of longitudinal waves in the
plate and wide variation can be expected throughout the frequency
range. This factor can be used to deduce the flow of power into the
structure. This will be proportional to the mean square vibration
velocities of structural elements to which the subsequent sound
radiation is proportional. The level of power flow can be minimized by
avoiding resonances. In theory this can be done either by decreasing
gready the structural mobility, that is making the seating very stiff, or
increasing it gready which can be achieved by fitting a flexible mount.
In practice it is impossible to make a seating stiff enough to avoid
resonance over the whole frequency range and a flexible mount is the
better solution. When a flexible mount is used the structural mobility
approach can be used to measure its isolation effectiveness. Another