from 10 West
which engi
*neersand postgraduate
students inMarine
and skills.
fourteen West European Countries and fifteen very successful Schools
have now been organised on a
wide
organised by one or two member Universities, with additional
lecturing
staff
Universities,
aware of the need
of providing an advanced tool of knowledge in he field of vibration,
noise and
specific objectives:
level of theoretical bases inorder to understand the physical phenomena involved;
- transmitting an update level of
knowledge
about
available or
Shipboard
 
"intensity".
and I
the
W = I *
S watts
over the
W = fIs dS
are the last ones to be
measured.
the wave propagation
wave and reach the
- for
infinite
plate:
kg/m'
C = ryJp, /e
y = ratio of specific
specific
heat
at
constant
pressure,
ca.
1.4
kg/mr
media correspond to sound
moss
omper
C
Fig. 1. 3
SYSTEM
given in
types of actions
as forces', since
along the internal
structures.Such forces are
are
generated
in the
main diesel
pertaining to
that, despite
various harmonic
components through
to achieve a balance,
to
aftmost ship hull
two
shortly afterwords
(Fig. 1.9).
Rough impact
may cause
referred
Furthermore waves of length
N-nodes
mode:
c)).
Occasionally
different-type
beam
modes
will
occur
showing
longitudinal,
horizontal
in
the modes found involve the superstructure in various
ways, basically for a combination
of the own
structure
effect of the hull
vibration will be of utmost importance because of the
resulting annoying effects on the crew.
The
main
components
need for further
composition
of
the
simple
several
modelling the structure involved through a
fine
according
to
stage of the
1.4.4 The
hull-beam parameters
Referring to
the
an N - degrees -
and
able
to
A
the
ship
about
which
equivalent
IMREOLO T" (S OtO a 3SnmiC. PT S S Uta
I
I
I
i
I
I
I
NZ
Fig.
1.13
/2/
1.23
if
the
frequency
range
is
specified,
waves
in
fluids:
a)
sound
pressure
level:
0. 07ca 1.0,,
sensation registered by ears and brain, several noise
rating reference curves have been proposed over the years.
These curves are simplified
in
fig.l.13.
As an example, ISO R 1996 Noise Rating Curves in octave
bands are
no
restrictions
somewhat). Most of the part of the energy that falls
in the audible range is therefore measured
with this
In most cases the sound levels measured in dB(C) and
dB(Lin) will have nearly, the same
value, so both of these
values have
sound
distribution in frequency of the energy,
while this is an
sensitive
the whole frequency range, but the signal is
passed
In fig
1.20 two
of fig. 1.13 and
Pequency Hm
level approximately
the ear is
obtained in the
These levels are then summed to express in a single
value the whole perception in the entire frequency field.
As stated
will
used.
1.30
a difference
thus:
Ltot =
(2)
+ Lx
*Example 1
measured at a point in
the
engine
to
three
i r*dted DO0
the total noise level has only
been
now
dB each,
measures.is only
obtained when
the three
completely abortive because
is negligible.
*Example 2
A noise
in an engine
strongest
source
94.5 dB,
an important factor in
dB reduction
in the
of about
engine
room
caused
only 5 dB.
not
source
of a function
the presence
of the
signal. [2]
transducer)
placed
in
FREE
VIBRATION;
FREE
or restraint. (1]
the
LOUDNESS
: The
intensive
attribute
of
also depends
upon the
MASS APPARENT : see APPARENT MASS
MASKING
sound.
force,
point
2.4
vibration
2.5
organ
handicap
B,
of
85
dB(A)
1999, the
dered
treshold is shifted
500, 1000 and
in dB(A)
exposure. A
equivalent with exposure
of 20 hours
permissible noise level of 85 dB
(A)
during
40
mostly are caused
or vibrating equipment. A
2.3.2 EVALUATION OF VIBRATION
are described in
evaluate whole-body
levels of
to the body.
directions,
which
are
is
the measured quantity
is used, where
the signal should
defi-
taken
to
80
on the requirements, and
the measurement data are
dB(A)
Facc-to-face No-nal voice Raised vice, Very loud or shouted Maximum voice level Very difficult to irnpos-
4unanmplified speech) at dis- level at
voice level at
dis. at distances
up to sibi,.
a dis-
tances up diitances tances up to 50cm 25 cm Unite, of I cm
to6,n upin 2n
- Usepnrss-t-ttalk Use special equipment
to difficult loudspeaker loudspeaker
in muff or Use insert
type orover-
in hel-
to
be
satisfactority
stisfactorily
intelligible
intelligible
dB
m
m
35
7,5
15
40
4.2
8,4
45
2.3
4.6
so
1.3
2,6
55
0.75
1,5
60
0.42
0.85
65
0.25
0.50
70
0.13
2.16
-0 50
Figure 2.2
ty with increasing age.
0
no
xpomsune
10~
vibrations depending
The approach to
electric
rotational mechanical energy into
energy.
means that
these
on the
human body.
For these
a significant "noise sourcen and has to be quantified as
regards
this
aspect.
the energy
is concentrated in a
Real
cases:
propeller
lines in the
rising
axial asymmetries in
3.2
propulsion engines: they are
radiate
We
strength
on
this
position".
The
transmission
aeroacoustical
propeller
ihull
plating
I
iba-mdto
accomodatlon
space.
3.4
the
receiving
position.
This
should
considering
strength,
can
result
of the
includes
finite
dimensions,
geometrical
interaction.
The
used in the
experimental tests, is
different
in
for exhaust systems
chose
be
reflected
back
are
decreases when the
than
fig 3.11
- Offices
60
of maritime countries were invited to 'establish provisions
for the reduction
exces-
sive
started work with
respect to noise
l im its
to
Administrations
on
that
time
annoying low
frequency sound
or obvious
tonal components
divided
the
noise
increase of
3 dB(A)
is permitted
implies
that
Comment:
and involved in
amount
of
fac-
acoustical
characteristics.
5.2
given of
sound levels-A
400
ships.
These
the
60
For ships in
the 5 000 to 50 000 dwt range with engine room
and
average level in
Above
engine room and
for
cabins
for this group of
that is installed in
5.5).
dwt
of
course
also
applies
on
the
various
high
-speed
the same
power, resulting
5.3.2
Workshops,
stores
Structure-borne
radiation is
frequency range (see Lesson
high
to
low
such as
on the
a
considerable
increase
in
sound
level.
Some
typical
vessels, without
and with
the small
pumps,
thrusters,
to plunger
frequencies with
.J.
8K
resiliently
mounted
and
..............
than 3.5 m
~ Without
floating
acconmodation
With/Without
floating
accommodation
With/Without
floating
acconmodation.
not
V
withstanding
ROOM,
deck level
structure
 
It is to be noted that, in the general case, the crank
arrangement
resultant
zero, a system
order
free
actions
plane through the
of the engine,
harmonics. In fact,
resultants other than
zero.
Let us analyse in detail the effects of the tolerances of the
balance weights of the
engines'to
This implies
1940, corresponding to
M = mass of the rotor in Kg
0) angular velocity of the rotor
Since, in the case of
crankshafts, the rotor has extensive
axial length, the balancing
and
of the balancing
160 kgm
disposition
from
6.33
crank
angle),
and rotational
been carried out
6
.41
moments
cylinder.
gear
wheels
shown can already
wheels and
3.3.4
GAS
Another
source
relatively low
induced
flow
noise.
3.3.5 VIBRATION
OF ENGINE
or
length of the
to the
of
the
individual
organs.
developed and still
lead sheets of
20-40 kg/m2, it
is
measuring
The determination
measuring
is a source of error in he subsequent calculation phase.
This method is useful
modifications
be
checked.
6.56
487
rpm
etc.
6.59
IN 6eA
17 1
pIT
W.OEP
CIKAC i ý,, oI,-I ýo o 0 1.,l P S
OR SWL-4.1 log
and
acoustic design
fan units in
3) the airborne noise in trunking and sound
traps
electric motor
connected directly
the blades
a
whole
a duct
show the best approach
We will
discuss the
from
insertion
loss
duct components.
of the
smooth
practical
example
is
presented
of
with the
back
in
CONVERSION OF SOUND POWER LEVEL TO SOUND PRESSURE LEVEL
It is necessary to convert the sound power level of a source
to
the duct, may
Care must be taken to ensure that duct
velocities are
MACHINERY ROOM VENTILATION
room and
measures.
openings
Constitute
areas such as bridge
spaces,
selected also
systems.
of
noise.
Woods,London
Fundamentals
of
Acoustics
duct or
minor dirnonsiou
of rectangular
72
PLENUM
SILENCER
TURN
ATTENUATION
INSULATED
TURNS
NOT
1-5
-5
END
REFLECTION
DIRECT PRESSURE 169 169 171 169 159 148 139 136
REVERBERANT PRESSURE
TOTAL
PRESSURE
REQUESTED VALUES
is calcula'3d
by multiplying
section
the
attenuation
of the incident
engine and the
decks and the bridge
performance via scale models
the acoustical
in a quick
have been presented in [3]1.
Basically
exhaust
system
for
made concerning the
the
the important
the
wave number (wO/c)
a
continuous
relatively
occur at low frequencies,
the exhaust
system is
inevitable, especially
the
bends.
Another
aspect
(1/2
increase
chapters
In figure 4 the measured sound
pressure levels from a diesel engine
with an unsilenced
chapter 2 it is assumed that,
because
between the two curves are
caused only
given in figure
are only generated
pipe.-
The relatively minima in the insertion loss occur at frequencies which may
be
calculated
c = velocity of
n =
silencer
is described as 'any section of a duct or pipe that has been
shaped
or
treated
with
same
time
allowing
'good'
the system performance
piping is
the
so-called
standing
waves
In
the
range
between
these
frequencies
the
characteristic
impedance
of
A successful and approved silencer is the 'double-expansion (equal) chamber silencer
with an
TLI of such
is that the length
to the chamber length. The
effects of the silencer length
and
the
Volume resonator silencers
These silencers offer
noise
ipecilum.
relatively narrow. The effect in that narrow
frequency range
could be
important to
in an exhaust system, each
tuned
silencers with respect
to the gas flow. There is no gasflow through the volume; the volume is coupled to the
main duct
Figure 12 shows
TU]. The resonant
acoustic
instance [3], [4]
and [5]. A major disadvantage of these silencers is the lack
of accurate
frequency) in the volume. This handicaps
optimum
type
of
silencers
chapter 10.
silencers. The
1 the length
the
sound
consequently the natural frequency. Figure 13 gives two examples of
1/4 X-silencers with the corresponding
TL'-loss
curves.
noise
reduction characteristics
for the higher frequencies. Sharp fluctuations in the TL, such as occuring
in reactive
silencers, are
not present.
figure 15.
The mathematical
model used for the calculation of the TL seems to be
optimistic
provides the user with the
information to estimate the noise reduction.
The advantage of the
combination with
the double
resonantor silencer
in figure
17. The
present design method for those silencers is partly theoretical but mostly
empirical,
design curves
which are
in general
ground.
9.Ie
which
of
of the subdivision
of the sources
given in paragraph
2, a preselection
Table
1:
engine
type
silencer
type
The usual
shaft speed
Bulky silencers should
systems of the second category - the medium speed engines - the
combination
source.
the application of an absorptive type
silencer gives
the best
22
exhaust systems
with target levels between NR-60 and NR-70 convented to a distance
of 10 meter from
several types of engines
is equal
can be used as guide-line. The
position
of
the
extent.
The
systemn
the length of
damping will also occur in the
piping between
engine and
minima occur can be
and silencer: f =(c/4 1) x n;
n =1, 3, 5. ..
silencer- f = (c/2 1) x n; n
= 1, 2, 3.
gas
resonator/absorption silenicer is shown in figure 25. Due
to
frequencies. The
mean flow in the main piping. In figure 26
an example of the influence
of the air flow
on the acoustic effect
of a volume
resonator is given.
The attenuation is remarkably reduced in the region of the resonance
point; the effects
velocity and
not considered further in this paper.
11 BACK PRESSURE
An exhaust silencer is introduced to attenuate the exhaust noise
without
causing
excessive
obstruction
one
of
causes of extra back pressure. This back pressure reduces the
maximum output of the
a piping of length
where f is the fanning friction factor for the pipe (-
0,0
Av
each
adds to
given for the
engine involved: the
remaining value is then available for the silencer. The back pressure accepted for most
diesel engines may vary between 1500 and
3000 N/m
will
be about three times the pressure drop of a
piece of pipe with the same as the silencer. For the reactive
silencer the back
large extent on the type and dimensions of the silencer.
9.13
Based
see
by the
lack of
this figure it is
The
improvement
exhaust system.
piping without a silencer.
of
transmission
loss.
(calculated
values).
9.34
resonator/absorption
transmission loss.
" two
9.39
500 (Hz)
9.46,
frequency
fm
(9).
9.51
band sound pressure levels of
modification b.
2.1
The
structure-borne
sound
path
The
structure-borne
sound
path.
illustrated in
both the sound
of these investigations
point
where
A almost
with one
It sounds
to
specialistic
here.
several
reasons,
is to minimize the number of bends in the
exhaust piping systems, especially for the larger engines. Bends directly at the
engine output must be considered with special care (see detail A in figure 6).
Thermal insulation
When rubber mountings are used, special attention should be paid to the thermal
aspects, in particular when these mountings are positioned close to the piping.
Temperatures of 70'C or higher in the rubber are inadmissible and in that case
special measures are
is recommended to maintain the temperature of
the rubber below
mountings.
In
figure
5
transfer to the mounting.
and
have to be insulated to prevent thermal short-circuiting. The
ventilating
in
maintaining
acceptable
temperatures
allow
considerably
mountings in the appropriate
directions
(preferably a factor 10 or more). This is required to guarantee that position and
vibrational
to the exhaust flange
this
at
that
smaller than for a
not
be
neglected.
The
behaviour.
guidance this
means that
- The mounting system
provided
with
suitable
have been
expansion
of
favourable
suspension
to
install
the
improved by arranging
Hz
Figure
2b:
Insertion
loss
due
to
resilient
mounting
axial thermal
tion
model
solved, good
agreement has,
and model
and
ship models to be used for
establishing the wake field, one
example
Recently a new tunnel of
similar
design
large tunnel in-
3. Facilities
depressurized towing
in
the
together
with
some
ties are discussed in /23 and 26/.
In spite of
ents
C
K =
(8)
pz
pI.
2
n
and
ency
scaling.
of scaling the wake distribution
is discussed, the
conclusions being somewhat
however, be emphasized
measured
of
more
at
frequencies
higher
than
those
corresponding
to
the
col-
lapse
blade frequency.
tunnel
walls.
I0.I3
are:
spectra
having
2) The model and full scale
measurements should be
wise a factor
 
.It should be mentioned that the formulas referred to above are
used
level
10.3.1
General
These
measures
world and already
blades
be
created
which
citation
TDW
tankers".
9th
lands Ship Model Basin,
ties". Conference
on Advances
Multiples of blade frequency
Ship No. 5
in Fig. 7.
propeller-hull on the
0 It
0 7F
having extreme skew
Pitch ratio
Pitch
ratio
Pns5R/PohR
frequency. From /74/.
Gadda)
the University
related
software
technical
aspects
and
(MAST, COMETT).
At present she is
1.
Because
these
notes
are
1 At fn,
T is reduced;
3. Peaks due to
wave effects are reduced;
5. At very high frequencies, damping may cause waves
to
damping.
(i)
of the
second part
one set of
of isolators
to support
the machine,
vibration in all of the six
degrees of
cn)nsiderably
The complete theoretical formulation
mobil'ties for response
14.
11.22
frequency ranges.
In-plane excitation
may be thought
but it
must t.,
forces and considering
if bending
control the
in
Chapter
which
can
upon
as
point, i.e.
there
and many
degrees of
freedom contributing to the total power transfer, this may' be reasonable. If broad bandwidths
are considered,
are short compared
low. In cases in
and particularly when
coupled to structures
then the motions
well correlated and relative
phase is important. It
of correlation between
connection points to the receiving structure. When predicting and measuring
responses, the degree of correlation is important
in
is termed
as the
IMPEDANCE Z,
M, i.e.
V -i
A general system
is shown above, with at points 1 2. The POINT MOBILITY is
given as the velocity at
a point per unit force
at
is
at positions 1
0
The TRANSFER MOBILITY is the velocity per unit force, when the force
and
velocity are
measured at two different points. The force in question is the only one responsible
for the
is
 1
F
2
F
1
system
M
12
= M)I
(ii)
that
groups of
elements series-wise
of electrical circuit
This can only occur
element, are massless, then
magnitude or phase)
elements.
Therefore for N elements connected end to end the mobility at
one end is
amass).
F11icm
11.37
as
1 and 2 is found by substituting for F
2
in
which
enables
on
or
even
velocities
+ ) dt
(1)
0
where
T
beams. The
excitation point
beam. When there
one
half
of
a flexural wavelength in the plate between beams the point accelerance tends to that
of an
from a machine
to mount
force transducers
between the
may be
input to a foundation
1
vibration transmission
in the x-direction,
the deflection, x is the distance
along the axis
The total power
 
andlongitudinal wave motion is sensed via the Poisson's ratio effect. Analyses have
been
type of
direction of
pipe.
To
summarise,
transducer configurations may be used to measure
transmitted
by:
<Px> =
2
<Psx> =
2
two-dimensional
at a distance
12.26
of A = 0.15k to O.2X. Mount the accelerometers first
and
in achieving the closest
method of
mounting the
the structure can be quite significant in this respect. The
signal
processing system should have a phase tolerance of± 0.3' or better.
6. Dynamic
range problems
components
in relation to practical testing and now that
digital
However,
occur in measurement
transducers,
measurement systems are
will
An
given in figure 17(b). A series of experiments
were carried out on a large steel plate of dimensions 2.5m x
1.3m and 5.8mm thickness. Dry
sand above and
boundaries.
lengths
of
cyanoacrylate adhesive but
that at the
the formn
four
acceleromercr
measurement
s
A n ~ ~ddtina accleomte w a placed.- at th ceteo h ra
o indct h
Aondaaddintionale ccelromegtermwsilaedatin
andw tohe rintnstyatend sisonfued duertoel ciclattiong fcpower.tions
thes igt hand
control
beams [30]. A long
which
pair of
in one half
modal responses.
for broadband
This is for
s the total floor
density of two
loss
factors. Proc. of a Conference on Recent Advances in Structural Dynamics,
(1980),
Southampton
University.
[23]
structures.
[29]
- -T
that the basic assumptions
at the Royal Institute
of
Measurement positions located
in shaded area.
and 17.
increase
16.2
a)
Source
will be very
mass attached
the
mass
mass and Ec
structures is to use
sandwich construction with acore
damping
by shear stresses in the viscoelastic
material, when elastic coating are bent. Strong damping can be obtained
for core of
low elasticity
modulus and high damping loss factor as polymers. The difficulty in using sandwich
construction
to
damped
basic
nature
but
material
frequency, and exhibits
13 I, here we just
give the formula
characteristics.
1 EH3 E
The
variable
X
where G2 = real part of complex shear modulus, p = wave
number of flexural vibrations.
FGR9.Loss
factor
;in the
finite case,
vibration modes
energy
transmission,
Energy
are avaliable, Energetic
in ref. 110 1I,
plate is
For a fixed value
bars
on
figure
the transmission
is frequency
data
from
internal
sources
to
radiation,
with
two
feet
fluid mass per
frequency (oc:
noise. Above the critical
frequency the bending wave
is supersonic and radiates a travelling pressure wave, in addition the radiation efficiency is very
strong just
above co
 
radiation factor is small, on the contrary at high frequency the waves
number of
piston is a good acoustic radiator.
The
governing
sound
vibrate
strongly and produce only small acoustic power if the displacement wave
number decomposition is mainly composed
of non radiating components.
increase the power radiated if the lower
vibration level is the result of a strong
decrease of non radiating
of radiating ones.
cawr*n s
I
transverse displacement can be expanded on the normal
modes:
a
and
b
that the modal amplitude anm
is given
by the
the generalized
natural angular
is greater
than the
mode wave
specific
impedance
(pc).
Radiation
reactances
weak below
in phase
of
modal
radiation
loss
117
I
ilimýyjsr) aJld
t sijlsw put
U QAU4
leqi uoiidwttssu
oy jo
*pow 23U S
olqui u Inzunwwns amr
owid oixuu jo .onnj uonprnpi
oqi A:uonbaxj jfl31)tfl rnp oopq flA2AM0H -airid o uiu Ui 'soAI1m
2uxpuoq 2U1[pAAE1I
saioutpuoi
osrn.jj
Joia0uj
zDu s )jvom Si oirjd 341 JO JoiDDJ uo 31puJulp
qiOm > m j Xnnuoa 041 UQ
(sv0
. z
3
nmin2uu
SIIIAUP
 
2
band is large.
stiffeners or
this is
only important at frequencies below and close to the ring frequency
WR
OR
(49)
R
p(l-V
2
boundary
conditions:
simple
supports,clamped
supports,
input
mobility
point (see reference
is
localized
the
the driving point of a
cylinder radiating in air.
unchanged with
shell quadratic
BKg
MEignim.2
Influence of amass located at the driving point, on the power radiated from a
cylinder
radiated power.
light fluid
frequency is
mass, by fluid
-ia ...,. .......n mdlb~.o i
Fl• S•4•t~dp•* -t g ,A ..*.i4•. • h flg*
Fin. 4 Radiation of a hell in ater with and without coating layer (After 22
Radiation from vibrating structures
encountered :
result in an increase
explainded
numbers, when
motion isecomposed n plane
wave spatial Fourier transform).
Two asic quantities are used to describe radiation : he power radiated and
the
the same
factor vary often in
opposite directions (increase of radiation factor results in a decrease of radiated power). The
radiation
very
small below as result of an acoustic short circuit. The curvature introduces a econd
maximum of radiation factor toward the ring frequency.
fTe basic phenomenon have been presented on se otructures like plates and cylinderS. For
real industrial structures there tendancies remain valid, but if etailed prediction of noise radiated
is wanted one
must use F.E.M
It is however, often not
necessary to include in he modelisation all structural details, and modal calculations
on plates and
are often sufficient.
Figure (25)compares a alculation and a measurement of the power radiated from a ylindrical
shell, one can see that
unpredicted
peaks
structural
defectts. To understand this let us remenber that well below the critical and ring frequency the
radiation efficiency isery small due to the cancellation effect of monopoles of same strength and
phase opposition. When a
structural defect is resent the array of equivalent monopoles is
perturbated,
and
sunpredicted peaks of power.
J.L. GUYADER
of a rectangular panel - J. Acoustic Soc. Am.51,
p.
to reverberant acoustic fields - J. Acoust.
Soc. Am.
1964.
acoustic radiation of elastically
1
with
general
boundary
p.93-102, 1977.
three layered elastic - viscoelastic
cylindrical shell -
60(6),
p.1256-126
4
, 1976.
p.
Al. 1988.
Vibration
on Noise and
Auxiliary Machinery pag.
LESSON :11.
Propagation
in
Structures
ACCOMMODATIONS
IN PRACTICE
Control
Onboard
Fast
Passenger
Vessels
to a room one introduces the difference of the pressure
level of the incident plane wave L(O) and the reverberant
pressure
one can remove
is
loss TL(8),
into source
mean intensity
<I> radiated
the panel from the pressure
level in the source room and the intensity level radiated in the receiving room.
R Lpl
room
with
strong
enough
absorption.
BASIC
MODELISATION.
THE
INFINITE
PLATE
Let
us
consider
510-2 ,b) 510-1
The Figure (8) presents a typical result obtained after numerical integration of (42).
The diffuse
of
the
also the lowest cofncidence frequency),
and above
it an
for
above i
frequency range
: 0 mm, Plate 2 : 20 mm, air gap: 10 cm
A typical result is shown
in Figure (10), it akes
several singularities
to appear:
medium separating
to the sum of those
of
the plate
of the
M, and
of
The
the
spring-mass
system:
=as
2
(51)
This
phenomenon is typical of double wall transmission. Around this frequency a
double
same
considerably
the
excitation is
of broad band type and the transmission loss is an average over frequency
; generally the double
the
the
breathing
*When people is dealing with
a diffuse field excitation an average over the angle of incidence 0
must be done as for single plates. The same mean effect is
observed. In particular the breathing
resonance phenomenon
is still responsible of a decrease of the transmission loss at the frequency.
n
0 ,
increase
better
varying cavity depth
A---A d=0.35;0--6 d=O.20; x---x d=0.10;A Ad=0.02
(After ref. [20])
several experimental results of
sound transmission through panels.
Figure
(13),
as one can see global tendencies are similar for predictions and measures, however
differencies
double panel,
In fact
the predicition of sound transmission through a real plate separating tworooms requires very
sophisticated theoretical method using modes of vibration of the finite
plates. Several papers have
study uses
[8]),
course
only
sufficiently large to
of papers are interested
the effect
are only modelled in the receiving room. The more complete
modelisation of direct sound transmission is presented in references [12] and [13],
it takes into
account finite plate
serve in the next section to demonstrate the effects of
source location, plate area, etc...
TRANSMISSION
THROUGH
description of this
problem is very
transmission loss. Concrete wall of 8 cm thickness.
II
plate and
through double plates
material and mechanical
presents the influence of mechanical
conection
the main defect, when using
double
panel
construction , they block the breathing phenomenon and make the double panel
to transmit sound
SANDWICH AND
MULTILAYERED PLATES
as it offers light plates of strong flexural
rigidity (honeycomb core), a strong loss factor (viscoelastic core). Several papers presents
theoretical and experimental
behaviour of asingle plate.
low as it depends
on the ratio of mass per unit area and bending stiffeness
of the
frequency
- For
viscoelastic
sandwich
the main influence of
critical frequency zone and
measured transmission
and isotropic plates.
sound is often,
Some empirical rules are sometimes used to estimate
the transmission
have been used successfully.
reference [18], for exemple, a second one was
presented
in
prediction seems
to be
reverberation time becomes
receiving
room
is
transmissions
increase
by
coupled
structures.
Application
to
flanking
transmission
vibratoire".
[22] CREMER,
ein fall".
by
at a
/19.3/) gives
c=0.75
c=1.55
at
larger distance from the source depends on the amount of
sound
this
total
sound
absorption
amount of sound
only
20 xt 5 = 1000 mn
 
19.6
with sound absorpti-
may function
room
position at
a large
mm)
glass or mineral
wool, in most
foams,
depending
"taylor made
a
wanted
the
19.8
of the machinery
and after installation of
the total insertion
loss is the
reinstallation
point is
enclosure.
the enclosure.
The natural
cooling is
all relevant
high
insertion
engine seating
avoided
by
system
for
of
A practical example of a complete design of a resilient
moun-
in
the airborne sound
of the machinery, e.g. in the room in which the
19.9
prima-
structure-borne sound e.g. by
linings, etc.
These measu-
the airborne
sound insu-
mounting systems are used
airborne
sound
may
system. In that
a balanced
at
level
the ECR is
ECR are:
- structure-borne sound
85-95
ble situations a very large
reduction in structure-borne sound
might be some 105 dB(A), with main contributions in
the mid-
mid- and high
and/or
floor)
that
insulating
materials.
- airborne
ECR:
sound
limits.
sound
sufficiently.
Generally
Figure
engines in two
room
auxiliary
level distribution
in an engine room on deck 1 (floor) in harbour (auxiliary
diesel
engines
density
Construction
mineral wo
eos lbaffyesr mpi se
a workshop, derived from,
efficiency
A floating floor consists of a resilient underlayer of glass
fibre
serves as
the floor
20.1). If a floating
positive influence, specific
system,
frequency should be chosen to
be as low as possible. This is
illustrated in Figure 20.2, which shows the differences
velocity
level
the toplayer
the range
of the floor must be resilient
(and
watertight).
Some measured
sound pressure
floor with the
The reduction
(in dB(A))
of
as
the propulsion
very lightly damped structural part vibrates over a long time
period after excitation (e.g. a church bell).
damping the
energy is
place when a sound
structure, the energy
- a
- a part is dissipated
a part
of the energy is dissipated at the edges, e.g. due to
friction.
following
effects:
20.9
2k 4k
frequency [Hz]
2
toplayer.
20.19
53 as 1. with floating floor
Figure
20.13
Airborne
cabin configurati-
E~q- frequency [Hz]
sound
excitation,
=
25 log f - 20 log f. - i0 log I a-2 [df]
27r8
board ships ,
Proceedings Internoise
35 nu chick toplayer of reinfor- toplayer of 5 . thick
steel
plat
ced
plastic
mortar
49
f - 2534
alcuatne d.dm asrred
35 eti ck with toplayer ofor the steel deck
sith steel-
20.38
in approval tests at
instantly
to
this lesson,
the subject; for further study
and
to consider the
various entries in
he reference list
are
In the following
paragraphs a concise
or physical phenome-
na at its location into an optical, mechanical, or most commonly into an electrical
signal
the
input
(either displacement,
velocity or acceleration), force, (sound) pressure etc. The electrical signal
might be
be either
analogue with discrete filter sections and rms circuits etc. or digital where, after
an analog/digital conversion
a Fast Fourier
simple
inclusive of shaping (into
complicated
flow methods, modal analysis, etc., see ref. 14/ /5/and /6/.
The capabilities of digital signal analysis and
manipulation nowadays
experiences very
various types of instruments
analyzed
result
instruments.
/M/.
be
the display iseffected.
Results are mostly
stored in digital
form on magnetic
use of a computer. The display is on a computer
screen
any
systems is available. The reader is advised to
become acquainted with
21.3
SOME TOPICS
This chapter describes some topics which are expected to be of
special interest
result.
21.3.1
been developped
which meet a series of requirements such as with respect to
sensitivity and frequency
2
etc.; fig.
difficulties.
The
position
acceleration m
of the accelerometer or even using a multiple of accelerometers
with the
simultaneously in a short
responses and
Also other
positions for
the sound
to
method usually gains
the method
reciprocal transfer
measurement, however, may even inhibit
larger
problems
by selecting an incorrect transfer relation,see
ref /6/, Appendix A,are frequent mistakes.
One
the
For further study
method
noise
sound path,
way
(with
the
engine
than
excitation
source.
This
and
5.
that path.
with
that
experiments can be executed for structu-
re-borne
answers.
essential
value
guidance notes or
requirements are given
adopted
standard
recommended to pay
paragraphs.
-calibration
for reliable measurements;
must be recorded for
way as
of
averaging
averaging is required,
level
meter.
from the
meter applying
of the
cabin
and
intensively
M = seismic
mass, P =
of
advanced
21.16
e.g. cargo,
passengers, cruise/leisure,
On
technically
research, fishing
some military vessels the noise radiated into the water may
be
of
and
simple
as
problems and
design
parameters.
2.0
SAFETY
certain
safety
regulations,
specified
by:
- International
integrity in
waterline.
locate critical pipes
and hence
commercially
and
having poor
smaller
vessels. The machinery
too large the
see
control
earlier
can
vibration
problems
required
design
the customers
additive
effect on
customers choice.
4.0 OTHER
noise and
vibration control
space, weight,
bow
thrusters
superstructure arrangement
national rules
for measurement
requirements
cabins,
optimal
extremely complex
a ship
changes
spirals
all will
single step and than to
return the
acceptance at sea of the same
vessel;
beginning
experience
discussed in the
and of the
any
explanation.
worth
personnel
involved
a qualified way and
back
Control Staff
should attend
when necessary to
plan and integrate,
ships?
are
mentioned reason the
in
dB
conclusive way.
23.3.2. Assessment
requirements onboard does not exist. However the
most
widely
used
parameters,
the attention particularly
will often determine
to noise
In the
same way
sound insulation
nowadays
the
threshold
wall
partition.
23.7.5.
New
and
radiation
of
technical
knowledge,
to:
- adoption
-
integrated
damping
Catamaran
Figure
1,
Different
types
of
high
speed
vessels
M.-3
Sources
building the
vessel. The
criteria discussed
conditions,
noise levels
the crew areas. The criteria
have been set to
in the passenger
and
criteria
vibration and shock - Guidelines for the overall
evaluation
levels.
According
mm/s (peak) in
to represent an average
shipboard vibration environment, see
figure 4. For vibration
adverse comments are probable.
DnV's experience from shipboard
rise to complaints after
Is;.-~* -W. ~ nn
24.6
program NV 590.
A Dir.
Mid. - 68-70
shaft
misalignment,
perform
140 positions. The measurements were carried out at two
conditions, 120 revs/mmn
the
basis
were to establish the
comparison with the expected vibration levels. In addition,
pressure impulses above the SB propeller were measured in two
points
(110
between the propeller excitation
controllable
regard to
vibration level at 120
order to arrive at the expected vibration level at the
contractual speed
highly
skewed
the two propellers was therefore
performed
lifting surface approach. The results from this
analysis were:
two
forces for th e
pitch propeller are 75 A% of the f ixed pitch
propeller at approximately 53 MW. The resulting scaling
factor
x
0.75=
2.7.
In addition to the propeller change, a vane wheel was to be
added
aft of each propeller in order to improve the propulsion
efficiency. The effect of the vane wheel was not
taken
into
account
since
to
on the
aft ship are naturally exposed
to
propeller
vibration.
As an example
of the Vibration
th e
1.
24.22
resonances were found
24 30
21 17
The measurement
carried out
vanes and not caused
the
aft
cavitation on th e
propeller and the phenomenon
taking place behind the
extensive full scale measuring
stable
sheet
cavitation
which
was
test carried
out in
propeller at 55 MW is illustrated in Fig. 5 As
expected, the
blade passing
value along
order)
value
9in) aft of the propeller plane. Another unexpected
finding was
the frequency
pressure
it
most
pronounced in the frequency band 24 Hz to 48 Hz. Sound in this
frequency
the
occur was the fluctuating nature
of the pressure signal. The result presented in Fig. 5 is the
average value over approximately 1 minute, while the peak
value is approximately twice the mean value.
Vibration level was recorded at the
pressure
transducers
in
the
As-fitted
propeller
New
design
D =
5.8
m
D
Integrated
component
at 70 MW for
the new design is lower than this component for the as-fitted
design at 55 MW.
vibration
will
only 90% of
'
The visually observed tip vortices at the model test revealed
that the new propeller
service with the new propeller design. The results
are listed
Average Pressure impulses kPa
DESIGN
0 2 3 4
ILFRAME 15516
to shock phenomena, and will also address the associated
problem of
damage assessment,
obtained
from
should be
of a
specific consideration
that since by damage
to
from shock
2. FUNDAMENTAL
CONSIDERATIONS
Mechanical
shock
may
be characterised by a variety of physical quantities such as, for
example, velocity
shock, blast
define rigorously
periodic
phenomena, that is, sudden disturbances which do not repeat in a finite time interval.
Shock is
occurrence
response of a system
model
with currently
pulses
of freedom
representation is very useful because of the
relatively simple dynamic
response prediction and also because the
single degree of freedom analogy may be used to predict the
response
by
and elasticity have an infinite number of
degrees of
the dynamic response may be obtained
by, considering motion in a
few lower modes only. The
effects of shock
if system
studies may therefore be seen to
fall into the familiar pattern of dynamic analysis. The
problem is illustrated in Figure 1.
Three
quantities are involved: the excitation, the response and the system transfer function
which relates the
excitation.
are
to estimate the third.
Two
types of analyses are possible, time domain and frequency domain. Figure 1 ndicates
time
functions are given
analysis must be carried
are three quantities:
the
very complicated, analysis in the time available may be precluded.
In both
instances, experimental studies are necessary. It is often also necessary to carry out
shock
to some of the
system
response
analysis.
shock or transient is
complex spectrum according to
complex spectrum
spectra
of frequency.
by a continuous
spectrum of frequency
in the frequency domain may be examined in a manner
similar to that
shown below
P(t) = P0
I -TIe-
t r
P
,=P0
IP(o)I =
a
a
deflection of the
constant.
This
Figure
f(r)At,
and if h(t) is the impulse response of a system
(remember, response to
to the
That is the product of the magnitude of the impulse
multiplied
can
impulses
such
that
t
A-t -- gives
or Duhamel integral and is expressed
in
convolution
analytical
expressions
may be
equation and evaluation of the integral.
An application of the convolution integral to the prediction of the
response of an undamped
pulse is shown here
and IF(f)l is the Fourier spectrum
of the shock.
by similar means, that is to mount the
body to be
resilient supports.
Thus the essential features of the isolator are its resilience and
energy dissipating mechanisms. The
latter may be achieved
in
order to simplify analysis, that the springs and dampers are separate elements, that the springs
are massless, viscous
damping only is present and that the elements exhibit linear
characteristics. These are idealisations of course, but
although the presence of nonlinear
elements and
Two classes
(a) shock motion of the support or foundation
where an
by the supported equipment
from
on the foundation.
of isolator elements, which may be of acommercially available type
or a special
in which they
system which
produces coupled
in analytical work, to
shown below in
in the form
of shock spectra
to synthesise shocks
since
the
be allowed for in
systems.
of view
of the
the
above.
Whilst
the
which
plifications, mainly
of the interaction
The recents decades
have however witnessed
two major breakthroughs,
been the
efficiency
these methods