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The 21st International Congress on Sound and Vibration
13-17 July, 2014, Beijing/China
HOW TO LOWER THE NOISE LEVEL IN THE OWNER’S
CABIN OF A YACHT THROUGH THE IMPROVEMENT OF
BULKHEAD AND FLOOR
Edoardo Alessio Piana, Anna Marchesini
Università degli Studi di Brescia, via Branze 38 – 25123 Brescia (Italy)
e-mail: [email protected]
High quality yachts are not always developed to give good vibro-acoustic performances. A
research project has been carried out in cooperation with a yacht manufacturer with the main
goal of improving the transmission loss of a bulkhead dividing the crew cabin from the owner’s cabin. First of all the comfort class of the yacht was investigated. The overall noise and
vibration levels proved to be below the expectations. The bulkheads dividing the crew cabin
from the engine room and the owner’s cabin have been tested. The transmission loss resulted
to be very poor. To completely characterize the behavior of the bulkhead, an intensity measurement session has been performed in order to obtain a mapping. The report shows the influence of the joists inside the sandwich panel and the presence of two critical areas: one corresponding to the passage of the air conditioning ducts and another at the interface with the
floor. Consequently different measurements have been performed to evaluate the structureborne noise raising from the floor. This brought out the need of studying the connection between the main fiberglass structure and the floor layer in order to increase the transmission
loss. At the moment such a connection is realized using a stiff glue. Focusing on the adhesive, it is important that it fulfils various requirements, first of all a good resistance to aggressive environments such as marine water, and a good tensile strength to resist to the movements of pitch and roll of the ship. Different materials have been tested in laboratory for the
determination of the dynamic stiffness and in order to suggest the best solution.
1.
Introduction
The main concern of the company making the yacht was the privacy issue rising from a particular model, where the crew cabin is adjoining to the owner’s cabin. First of all it is important to
choose some criteria to decide the transmission loss required for that particular partition. Following
the criteria given by the Rules for the classification of Ships1, the Comfort Class category for the
room was decided. It was possible to verify that the Sound Insulation Index calculated as the sum of
the relevant noise criteria and the single rating number for the transmission loss between two cabins
should be, at least, 88 dB. Moreover, the same parameter should be equal to 100 dB, if calculated
between a cabin and the engine room.
The Comfort Class considers three main groups (Table 1), depending on the quality to be
achieved. Class 1 represents the highest comfort level and class 3 represents an acceptable situation.
The maximum allowed noise level for transit condition in passenger cabins varies from 55 to
44 dB(A). Considering that yachts are luxury products, it would be preferable if the requirements
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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
for top grade cabins, also intended for overnight cruising, were satisfied. In that case, for harbour
condition, the maximum allowed level drops off to 35 dB(A). Therefore the single rating number
for the sound insulation of the bulkhead should be at least 33 dB. In most cases a value above 45 dB
is suitable.
Table 1: Sound Insulation Indexes according to DNV Comfort class
Lp + R’W (dB)
Positions
Cabin to cabin (crew)
88
Cabin to cabin (passenger)
90
Cabin to corridor
87
Cabin to stairways
100
Cabin to engine rooms
100
Cabin to public spaces
100
Machinery / technical spaces to passenger corridor
100
As to the vibration levels, the Rules for Classification state also that, for the highest comfort
class, it is allowed a floor acceleration up to 0.5 mm s-1 in harbour condition and 1.0 mm s-1 during
navigation. The accelerations should be evaluated in the frequency range from 1 Hz to 100 Hz.
2.
Preliminary measurements setup
As preliminary test, a recording of the noise level in the owner’s cabin during navigation has
been performed. The SPL time history shows sound pressure levels varying from a minimum value
1 - Misura
1IntvT.H. (Filevalue
N. 3) - OVERALL
slightly below 70 dB(A) to
a maximum
of 86- AdB(A) for the maximum speed (Fig. 1).
2 - Misura 1IntvT.H. (File N. 3) - OVERALL - A - Running Leq
90
13:02:32
1= 84.1 dBA
2= 80.2 dBA
dB(A)
80
70
60
12:57
h.m.
12:59
13:01
13:03
13:05
13:07
Figure 1. Sound pressure level in the owner’s cabin during navigation.
Therefore all the sound pressure levels measured during this test prove to be too high if compared to the comfort class limits and, referring to transit condition, the vessel does not fulfil any of
the above requirements. It was also possible to determine that the most annoying noise was coming
from the engines and not from the water impinging the hull.
In order to characterize the main sources on board, a measurement session focusing on the
engine room and the devices placed in it has been carried out in harbour condition. For this aim it
was used a multichannel analyser with different configurations. According to Fig. 2, three different
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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
microphones were placed on board: one in the engine room, one in the owner's cabin and the last
one in the VIP cabin, placed in the forefront of the yacht. In a second measurement session the microphones have been replaced by two accelerometers: a tri-axial transducer was placed in the engine
room and a mono-axial one in the owner's cabin. During this session, it was decided to carry out
two different measurement types: a first one placing the mono-axial accelerometer on the bed structure, a second one placing the accelerometer on the floor of the cabin.
Figure 2. Microphones and accelerometer positions.
In idle conditions and using only one of the two engines, the average sound pressure level inside the engine room is about 100 dB. In such condition, the relevant sound pressure levels in the
other rooms are: 85 dB(A) in owner’s cabin and 77 dB(A) in VIP’s cabin.
As a reference, the overall vibration level measured in the owner’s cabin is 0.622 mm s-1. In
Fig. 3 a typical velocity spectrum is displayed.
Figure 3. Comparison between velocity levels obtained placing an accelerometer on the engine and on the
floor of the owner’s cabin.
During this preliminary tests the yacht manufacturer decided that the noise coming from the electric
power generator and the HVAC system could be considered of secondary importance, so the attention was focused on the sound insulation of the bulkhead.
3.
Transmission loss of the bulkhead
From the preliminary measurements it was discovered that the main problems on the vessel
were caused by the poor insulation between the engine room and the owner's cabin and between the
owner’s cabin and the crew cabin. As a principle, the performance of both the bulkheads had to be
investigated in order to determine their transmission loss and improve it. Notwithstanding this, the
manufacturer asked to improve only the performances of the bulkhead dividing the crew cabin from
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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
the owner’s cabin. Fig. 4 shows the transmission loss for the bulkhead dividing the owner’s cabin
and the crew cabin. The single rating number in this case was R’w=29 dB.
Some different specimens of various panels already used by the manufacturer on other yachts
have been tested in a laboratory to decide if there was one suitable to be used instead of the one
normally mounted.
The core of the specimens for all the bulkheads was made of rigid foam, so it was possible to
test them according to Nilsson’s method2, since the outer laminates moved in phase.
In that case it is possible to determine the transmission coefficient τ(φ) at the angle of incidence φ according to Eq. (1).
2
2
2
2












f

f




4
4
 ( )  1 
cos    (sin  )    
 cos    (sin  )  1 
  2 c
 f c 
 
 fc 
 2 c


1
(1)
As a consequence, the transmission coefficient for diffuse incidence τd .can be computed according to Eq. (2).
 /2
 d  2   ( ) cos  sin  d
(2)
0
Finally, the transmission loss can be computed as:
R  10 log d
(3)
Figure 4. Transmission loss of the bulkhead originally mounted between crew cabin and owner’s cabin.
It was found that some panels had a single rating number of 28 dB and a critical frequency of
590 Hz. The first choice to improve the acoustic performances was to change the PVC foam core
with mineral wool. In that case the main concern of the manufacturer was to have some resistance
to the compression stresses given by the main deck. In order to fulfil this requirement, the panels
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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
were bonded every 640 mm by means of two steel “Z” studs fastened to the Okumè plywood by
screws. Instead of the PVC foam, a low density mineral wool has been placed between the panels.
In this case the specimen has been analysed when mounted on board following the recommendation of the ISO 140 and ISO 717 standards (Fig. 5) giving a single rating number of 38 dB
and a critical frequency at 590 Hz.
The bulkhead built on the yacht is 2.85 m wide and 1.95 m high. Some previous measurements confirmed that the junctions are of great importance. With some new improvement the single
rating number achieved for the transmission is 41 dB and fulfils the minimum requirement (Fig. 5).
As a final improvement of the partition, the thickness of the two external layers was changed
to make the panel asymmetric (from 20+20 mm to 15+25 mm), without growing the overall mass.
Even if the transmission single rating number did not increase that much, some resonances disappeared.
Afterwards intensity measurements have been carried out in accordance with the method provided by ISO 15186-2.
In the harbour, an omnidirectional sound source was placed in the crew cabin and the intensity measurements were carried out in the owner’s room.
Figure 5. Transmission loss plot of the original bulkhead (on the left), transmission loss of the new bulkhead
(mineral wool core, Z stud junctions - Δ), transmission loss of the new bulkhead (different mass for unit area
of the laminates - □).
In order to compare the transmission loss obtained from intensity measurements with the results from ISO 140-4, the standard suggests a procedure to determine the apparent sound reduction
index taking into account the contributions from the direct and all flanking paths. The transmission
loss can be determined through the formula:
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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014


 S 
0.1L 
RI'   L p1  6  10 log   10 log  S Mj 10 Inj   K C
 S 0 
 j


(4)
where Lp1 is the average sound pressure level in the source room, S is the area of the partition
under test, SM is the total area of the measurement surfaces and
is the average normal sound
intensity level. The index j includes all the receiving room surfaces.
KC represents an adaptation term depending on the area of all the boundary surfaces Sb2 in the
receiving room, V2 is the volume of the receiving room and λ is the wavelength at the frequency of
interest:
 S 
K C  10 log1  b 2 
8V2 

(5)
To evaluate the quality of the measurement environment, for reflective test specimens, the criterion to be satisfied is given by the following equation:
FpIn   pI0  7
(6)
Where FpIn is the difference between the sound pressure level and the normal sound intensity
level on the measurement surface and δpI0 represents the pressure-residual intensity index.
The results for the tests carried out in the owner’s cabin applying this technique are displayed
in Table 2:
Table 2. Qualification of the measurement surface.
1-third octave
[Hz]
FpIn
100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150
[dB]
6.1
4.7
5.4
5.7
6.4
7.1
6.6
5.1
5.6
5.8
6.5
6.1
7.6
8.0
6.4
8.3
δpI0 [dB]
17.5
18
18
19
19
20
20
21
22
23
22
23
23
20
25
24
Difference [dB]
11.4 13.3 12.6 13.3 12.6 12.9 13.4 15.9 16.4 17.2 15.5 16.9 15.4 12.0 18.6 15.7
On the other hand the check for the influence of flanking transmissions was not verified for
all the 1/3 octave band and all the flanking surfaces. The results from this kind of test are shown in
Table 3 and put into evidence that for the 250 Hz one-third octave band the criterion is not satisfied
for all the surfaces. Considering the different surfaces, it is clear that the floor and the ceiling are
radiating noise and give a strong contribution to flanking transmission.
Table 3. Check for flanking transmission (X represents criterion not satisfied).
1-third octave [Hz] 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150
Left surface


X

X









X
X
Right surface




X






X




Floor


X

X
X
X




X




Ceiling
X

X

X
X










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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
Despite this fact, the apparent sound reduction achieved by ISO 15186-2 is comparable to the
values obtained with ISO 140-4 (Fig. 6). A major advantage provided by the intensity method,
however, is to obtain a mapping of the sound intensity emitted by the surface under test.
Figure 6. Comparison between transmission loss obtained using ISO140 (triangles) and ISO 15186 (squares)
(on the left) and coloured map obtained from intensity measures (on the right).
4.
Floating floors in the owner’s cabin
The improvement of the bulkhead transmission loss has put into evidence some noise contributions that previously could be neglected: air conditioning, non uniformities caused by ducts and
structure-borne noise coming from the floor. As to the floors, some impact measurements were performed placing a tapping machine on the main deck: the SPL in the owner’s cabin was 81 dB and in
the crew’s cabin was 76 dB. These results are mainly due to the connections between the hull and
the floors, made using stiff glue. From laboratory measurements, the dynamic stiffness of such glue
is 94 MNm-3. Obviously this value is too high and one of the effects of such stiff junction is to increase the noise propagation from the engine room to the cabins. One option to uncouple the floors
from the hull is to use mineral wool. Unfortunately, in this case, the minimum thickness required
for the wool to be effective is too high if compared with the requirements of the shipyard: the useful
gap to place the resilient element of the floating floor is just 5 mm.
In order to find a softer glue, three different products were tested, according to UNI EN ISO 29052
Standard. The results are displayed in Table 3: a better solution to reduce structural-borne noise
could be found by using silicon instead of polyurethane.
Table 3. Comparison between three different glue.
Density Elongation Dynamic Stiffness
(kg m-3)
(%)
(MN m-3)
Test number
Category
1
polyurethane
1295
> 300
94
2
polyurethane
1265
600
55
3
silicon
895
-
5
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21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
5.
Conclusions
The main aim of this work is to find out how to improve the sound insulation of the bulkhead
placed between the owner’s cabin and the crew cabin. Replacing the rigid foam used as core with a
mineral wool has proved to be very effective. Moreover by using metal profiles instead of wooden
studs and using two external laminates with different thickness proved to be even more effective.
During the tests carried out to determine the improvements, some intensity measurements
were performed. This kind of measurements are quite complicated and slow, but they allow to
achieve much detailed results, such as intensity maps that put into evidence leakages and flanking
transmissions, such as the one found for the floor.
As to the glue used as resilient material for the floating floor, it was proven that it is too stiff
to uncouple efficiently the deck from the hull. Some soft silicon can give much better results.
REFERENCES
1
2
3
DNV, Rules for Classification of Ships – Part 5 Chapter 12 – COMFORT CLASS, (January
2009)
Nilsson, E. and Nilsson, A. Prediction and measurement of some dynamic properties of
sandwich structures with honeycomb and foam cores, Journal of Sound and Vibration, 251
(3), 409–430, (2002).
Bies, D. A. and Hansen, C. H., Engineering noise control – Theory and practice, Spon Press,
376–381, (2009).
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