Page 314 - Introduction to Naval Architecture

VIBRATION, NOISE AND SHOCK 299

useful concept is that of radiation efficiency which relates the sound
power radiated to the mean square vibration velocity of the surface. It
is frequency dependent.
It is not possible to go into the theory of noise generation and
transmission in a book such as this but the reader should be aware of
17
the general factors involved .
Reducing noise levels
Generally anything that helps reduce vibration will also reduce emitted
noise. Machinery can be isolated, the isolating system preventing
excessive vibration of the machine and transmission of large forces to the
seating. The system must attenuate high frequency vibration and protect
against shock. That is it must take account of vibration, noise and shock.
Because of the different frequencies at which these occur the problem
can be very difficult. For instance a mount designed to deal with shock
waves may actually accentuate the forces transmitted in low frequency
hull whipping. Dual systems may be needed to deal with this problem,
Air borne noise can be prevented from spreading by putting noisy items
into sound booths or by putting sound absorption material on the
compartment boundaries. Care must be taken to ensure such treatments
are comprehensive. To leave part of a bulkhead unclad can negate to a
large degree the advantage of cladding. Flow noise from pipe systems can
be reduced by reducing fluid speeds within them, by avoiding sudden
changes of direction or cross section and by fitting resilient mounts.
Inclusion of a mounting plate of significant mass in conjunction with the
resilient mount can improve performance significantly.
Where noise mounts are fitted to noisy machinery care is needed to
see that they are not 'short circuited' by connecting pipes and cables.
The similarities for vibration and shock isolation will be apparent.
In recent years active noise cancellation techniques have been
13
developing . The principle used is the same as that for active vibration
control. The system generates a noise of equivalent frequency content
and volume but in anti-phase to the noise to be cancelled. Thus to
cancel the noise of a funnel exhaust a loudspeaker could be placed at
the exhaust outlet. For structure borne noise from a machine force
generators could be used at the mounting. Systems have been made to
work efficiently but it is not always easy to get the necessary masses, and
other equipment, into the space available.

SHOCK

All ships are liable to collisions and in wartime they are liable to enemy
attack. The most serious threat to a ship's survival is probably an