Page 311 - Introduction to Naval Architecture

296 VIBRATION, NOISE AND SHOCK

Vibration testing of equipment
Most equipments are fitted in a range of ships and in different positions
in a ship. Thus their design cannot be tailored to too specific a
vibration specification. Instead they are designed to standard criteria
and then samples are tested to confirm that the requirements have
been met. These tests include endurance testing for several hours in
the vibration environment. Table 11.3 gives test conditions for naval
equipments to be fitted to a number of warship types.
In Table 11.3 the masthead region is that part of the ship above the
main hull and superstructure. The main hull includes the upper deck,
internal compartments and the hull.

NOISE

2
16
The internationally agreed unit for sound intensity is 10~ watts/cm . At
1000 Hz this is close to the threshold of hearing. Noise levels are
expressed in decibels, dB. If two noise sources have intensities of W! and
w 2, the number of decibels denoting their ratio is:

In saying that a noise source had a certain dB value, w 2 would be taken
16 2
at the reference level of 10" watts/cm .
Instruments recording noise levels in air record sound pressure so
that:

2
2
In this expression the pressure is measured in dynes/cm (0.1 N/m )
5
and p2 would correspond to the threshold of hearing fa - 2 X 10"
2
2
N/m , A sound pressure level of 1 dyne/cm is equivalent to a noise
level of 20 log (1/0.0002) dB = 74 dB.
In the open, sound intensity falls off inversely as the square of the
distance from the source. At half the distance the intensity will be
quadrupled and the difference in dB level will be 10 log 4, which is
effectively 6dB. Doubling the distance will reduce the dB level by 6.
The combination of two equal noise sources results in an increase of
3dB. Sound levels are subjective and for the noise level to seem to a
human to have doubled requires a dB increase of 10.
This subjectivity arises because a typical noise contains many
components of different frequency and these will affect the human ear