J, Sound Vib. (1969) 10(2), 187-197 THE USE OF DAMPING MATERIALS FOR NOISE REDUCTION ON A PASSENGER SHIP A. E. TURNER Imperial Chemical Industries Ltd., Petrochemical and Polymer Laboratory, Runcorn, Cheshire, England (Received 12 November 1968) A survey of noise and vibration levels in a car-carrying passenger ship indicates that an important contrib’ution to noise in some of the passenger cabins is due to reverberation of large areas of metal forming the walls and floor in an adjacent store area. Trials are described in which approximately 800 ft2 of a viscoelastic damping material is applied to these surfaces. A small but significant reduction in cabin noise level is obtained. It is argued that large scale use of damping material at the time of building could result in appreciably quieter ships. 1. INTRODUCTION The two ships m.v. Ulster Queen and m.v. Ulster Prince were brought into service as car ferries on the Liverpool to Belfast overnight run during 1967. They are of all-welded steel construction and of gross weight 4478 tons. They are powered by diesel engines giving a maximum speed of 173 knots. The UZster Prince is shown in Plate 1. The second-class passenger berths on these ships are situated on the lower deck where unfortunately they have been found rather too noisy for comfort. To the surprise of the shipowners the cabins most affected were those situated towards the stern and away from the engine. The layout of the Ulster Prince is shown in Figure 1 and those cabins giving rise to the most annoyance have been marked on the figure. At the invitation of the owners an initial survey of noise levels in one of the ships was undertaken. This survey showed that the sound pressure level in the noisiest cabins was between 68 and 70 dBA with the ship at full speed ahead in calm weather. A reading on the linear scale of the sound level meter used indicated 102 to 104 dB. Investigation of the surrounding area suggested that the largest contribution to the noise level in the cabins arose from the vibration of walls and floors of the store area immediately behind the cabins. These walls and floors were constructed entirely from mild steel. They reverberated con- siderably with the vessel underway, so much so that it was difficult to write legibly in this area. In view of these discoveries it was decided to attempt to reduce the vibrational amplitudes of the floor and walls of this store area by applying a suitable damping material. If the vibration of these areas did give rise to the largest contribution of noise in the cabins then damping would effect an improvement. The extent of the improvement would depend on the relative contributions of other noise sources. Previous attempts at noise reduction on ships by damping treatments have been confined to a two-dimensional model. A paper by van To1 at the Leuven Symposium on “The Damping of the Vibration of Plates by Means of a Layer” described some rather unsuccessful attempts in this direction [l J. This paper was based only on a model of a thin vertical section through a ship and was therefore somewhat unrepresentative. It is known that noise in ships is largely structure-borne rather than airborne. This test on an actual ship was intended to give a more 187
Transcript
J, Sound Vib. (1969) 10 (2), 187-197
THE USE OF DAMPING MATERIALS FOR NOISE REDUCTION ON A PASSENGER SHIP
A. E. TURNER
Imperial Chemical Industries Ltd., Petrochemical and Polymer Laboratory, Runcorn, Cheshire, England
(Received 12 November 1968)
A survey of noise and vibration levels in a car-carrying passenger ship indicates that an important contrib’ution to noise in some of the passenger cabins is due to reverberation of large areas of metal forming the walls and floor in an adjacent store area. Trials are described in which approximately 800 ft2 of a viscoelastic damping material is applied to these surfaces. A small but significant reduction in cabin noise level is obtained. It is argued that large scale use of damping material at the time of building could result in appreciably quieter ships.
1. INTRODUCTION
The two ships m.v. Ulster Queen and m.v. Ulster Prince were brought into service as car ferries on the Liverpool to Belfast overnight run during 1967. They are of all-welded steel construction and of gross weight 4478 tons. They are powered by diesel engines giving a maximum speed of 173 knots. The UZster Prince is shown in Plate 1.
The second-class passenger berths on these ships are situated on the lower deck where unfortunately they have been found rather too noisy for comfort. To the surprise of the shipowners the cabins most affected were those situated towards the stern and away from the engine. The layout of the Ulster Prince is shown in Figure 1 and those cabins giving rise to the most annoyance have been marked on the figure.
At the invitation of the owners an initial survey of noise levels in one of the ships was undertaken. This survey showed that the sound pressure level in the noisiest cabins was between 68 and 70 dBA with the ship at full speed ahead in calm weather. A reading on the linear scale of the sound level meter used indicated 102 to 104 dB. Investigation of the surrounding area suggested that the largest contribution to the noise level in the cabins arose from the vibration of walls and floors of the store area immediately behind the cabins. These walls and floors were constructed entirely from mild steel. They reverberated con- siderably with the vessel underway, so much so that it was difficult to write legibly in this area.
In view of these discoveries it was decided to attempt to reduce the vibrational amplitudes of the floor and walls of this store area by applying a suitable damping material. If the vibration of these areas did give rise to the largest contribution of noise in the cabins then damping would effect an improvement. The extent of the improvement would depend on the relative contributions of other noise sources.
Previous attempts at noise reduction on ships by damping treatments have been confined to a two-dimensional model. A paper by van To1 at the Leuven Symposium on “The Damping of the Vibration of Plates by Means of a Layer” described some rather unsuccessful attempts in this direction [l J. This paper was based only on a model of a thin vertical section through a ship and was therefore somewhat unrepresentative. It is known that noise in ships is largely structure-borne rather than airborne. This test on an actual ship was intended to give a more
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(facing page 188)
SHIP NOISE REDUCTION BY DAMPING LAYER 189
representative answer to the question as to whether damping materials could be effective against noise on ships in addition to curing the individual problem.
2. DETAILED NOISE AND VIBRATION MEASUREMENTS
A more detailed survey of noise and vibration levels was now undertaken on both vessels. Noise levels were measured in one of the noisiest cabins (berths 436 to 439), in the electrician’s store and in the steering gear area. Vibration levels were measured at several points in the electrician’s store. The measurements were taken using a Brtiel and Kjaer precision sound level meter. For the vibration measurements a B. & K. accelerometer and integrator unit were also used. The vibrational amplitudes given by converting the metre readings in dB to cm are much smaller than actually experienced. This is probably due to the presence of very low-frequency vibrations outside the range of the equipment, i.e. below 20 Hz. All vibration level results have therefore been left in decibels as given by the sound level meter. In order to obtain as much information as possible, tape recordings were made of the output from the sound level meter and these were later fed into a narrow band analyser.
Results showed that the two vessels were almost identical. Results given below in Tables 1
TABLE 1
Sound pressure levels prior to treatment
Frequency (Hz)
Sound pressure level (dB) ,
Berths Electrician’s Steering 436 to 439 store gear
31.5 gw 103 - 63 89 109 -
125 82 102 - 250 70 91 - 500 62 88 -
1000 56 82 - 2ooo 51 82 - 4ooo 44 71 -
Overall A 67 93 95 Overall B 82 102 104 Overall C 94 109 110 Overall linear 100 111 112-114
t See text.
and 2 refer largely to the Ulster Queen but results for the Ulster Prince may be assumed identical. The positions of vibration level measurements are shown in Figure 2.
The S.P.L.‘s in the area of the steering gear were found on subsequent occasions to have little or no bearing on the noise level in the store area when the access doors into the stores were closed. It was also found that higher intensity noises generated adjacent to the steering gear when the ship was in dock were barely audible in the cabins. The noise level in the cabin must therefore arise from structure-borne noise either through the boundaries of the stores or via the hull of the ship. The very large vibrational amplitudes of floor and partitioning walls in this area indicated that these were the predominant paths and that the resulting noise levels in the cabins could be reduced by damping these surfaces. There would, of course, be con-
190 A.E.TURNER
TABLET
Vibration levels indicated by Briiel and Kjozr sound level meter prior to treatment?
Position Vibration level (dB)
A 55 B 40 c 34 D 50 E 36
t Positions as shown on Figure 2.
tributions to the sound level transmitted from the engine room and from the propellors via the hull of the ship. The relative contributions of each would only become evident by actually trying the damping treatment. From the fact that the cabins further forward were quieter it was known that noise from the engine room was less important.
Passenger cabins
C (Mid height)
Steering gear
Bo’suni’ rope store
Figure 2. Measuring positions for vibration levels in electrician’s store.
It was thus clear that extra insulation in the form of sound barriers would not be effective. To isolate the cabins by rebuilding them on vibration isolators was considered too expensive. A damping treatment in the store area appeared to be the only possibility. The question of what contribution arose via paths through the hull would have to remain open.
The results of the narrow-band analysis of noise in the passenger cabin and in the electrician’s store are shown in Figure 3 (a) and (b). The noise level in the cabin was subject to pulsations as is particularly evident at the higher frequencies. The narrow band analysis of vibration on the partition between the two stores is shown in Figure 5. It can be seen here that there are very large contributions to the total noise or vibration levels from frequencies around 22, 44, 66 and 88 Hz. The same effects were observed in narrow-band analyses of
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SHIP NOISE REDUCTION BY DAMPING LAYER 193
vibration levels at other points in the electrician’s store. The frequency of 22 Hz is the funda- mental frequency of the propeller blades. There are four blades turning at roughly 340 rev/mm on each propellor and it is thus evident that the propellors are the major source of vibration and consequent noise in this part of this ship. The basic shaft frequency of 5.5 Hz is no doubt also present but is not detected by the instruments employed. Fortunately, it is not significant in itself. It may, however, also be responsible for exciting the higher frequencies detected.
Frequency (HZ)
Figure 5. Vibration level on partition between the two stores prior to treatment.
From the octave-band values of S.P.L. it can be deduced that a noise rating of NR 69 applies to the cabin. The value for the electrician’s store is NR 93. A calculation in phons from the narrow-band analysis gives 83.5 phons in the cabin and 106 phons in the store.
3. THE DAMPING TREATMENT
The treatment chosen to damp the vibration of the floor and walls of the store area was a graphite loaded viscoelastic material developed at I.C.I.‘s Petrochemical and Polymer Laboratory. The material, known as DC1 was produced in 4 in. thick tiles, mostly 14 inches square, for application to the walls and floor. It has a Class 2 “low spread of flame” rating according to British Standard 476 Part I and is not affected by oil or water. It was applied using a solvent-based Neoprene/Alloprenet adhesive supplied by I.C.I. Mond Division.
The store area is not well ventilated and in order to get the solvent away during application it was necessary to install an extraction fan and for the operators to wear breathing apparatus. Complete coverage of the walls was not possible because of heavy ribbing at 24 in. intervals but the gaps in between the ribs and between other protrusions were covered as well as possible. The areas covered amounted to approx. 800 ft2. They include the bulkhead, the floor and the partition wall between the two stores. The wall between stores and steering gear and the side of the ship were not covered.
7 Neoprene is a polychloroprene manufactured by E. I. Du Pont de Nemours Inc. Alloprene is a chlorinated rubber manufactured by X.1. Ltd., Mond Division.
194 A. E. TURNER
The loss properties of DC1 have been determined by observing the decay of an excited steel cantilever to which a coating of DC1 had been applied. This test is more fully described by Ball and Salyer [2]. The results as a measure of loss factor of the composite beam against temperature are shown in Figure 6.
In this test a weight of damping treatment of approximately 12 % of the weight of the steel was used. In practice, the damping coating was a greater percentage than this, more like 20 %. On the other hand, the actual surfaces in the ship were stiffened byribbingand byother stiffen- ing members which were welded in place. This would tend to make the damping action less effective. The graph gives an idea of the order of effectiveness of the damping material rather than an accurate value of its loss properties. It also shows that its maximum effectiveness at
/ I I I
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Figure 6. Composite loss factor of steel beam loaded with DCI. Loading = 11.4%; decay frequency = 28 to 35 Hz.
low frequencies (the frequency of this test was in the range 28 to 35 Hz) is greatest between -5 and +15”C. The temperature in the store area on the night crossings is not likely to be outside this range. At higher frequencies the maximum of the graph will be displaced to the right but only by a few centigrade degrees for a decade increase in frequency. At the frequencies of interest the damping material is thus at its optimum.
4. IMPROVEMENTS EFFECTED BY DAMPING TREATMENT
The first attempt to measure the sound and vibration levels subsequent to the application of the damping treatment was made on a crossing from Liverpool to Belfast on 6 May 1968. Unfortunately this was not successful as most of the noise and vibration levels were higher than previously! It transpired that this was due to moderately rough seas striking the side of the vessel, a point which became obvious when measurements were made at point E (Figure 2), an untreated area on the vessel side. The vibration level at this point had increased by 6 dB.
A second attempt was made the next evening on the return journey under calm conditions in Belfast Lough. The result are given in Tables 3 and 4 which also show the improvements obtained. The narrow-band analyses of noise in the two areas are shown in Figure 4 (a) and (b). To aid comparison the octave-band results from the cabin before and after treatment are shown in Figure 7.
SHIP NOISE REDUCTION BY DAMPING LAYER 195
The vibration level on the side of the vessel (position E) is within 2 dB of the measurements prior to treatment indicating that conditions of weather and ship speed were producing a very similar environment to those existing when measurements were taken previously. The im- provement in the vibration level of the floor is a considerable 17 dB. This is a large unsupported
TABLE 3
Soundpressure levels and improvements after treatment
Sound pressure levels (dB)
Frequency ’ Berths 436 to 439 Electrician’s store 0-W Meter reading Improvement Meter reading Improvement
31.5 63 125 250 500 1000 2000
Overall A 66 1 87 6 ,, B 78 4 97 5 3, c 95 -1 103 6 ,, linear 103 3 105 6
Vibration levels indicated by Briiel and Kjler sound level meter after treatment
Position Vibration level Improvement
(dB) (W
A 38 17 B 34 6 c 29 5 Dt 50 0 Et 38 -2
t These positions were not treated with damping material.
area and damping is very effective in these circumstances. The improvement in the vibration levels at positions C and D on the ribbed surfaces of the walls is 6 and 5 dB, respectively. The octave-band analysis of noise levels in the store and in the cabin shows improvements at the lower frequencies with some exceptions, notably at 31.5 Hz. The apparent worsening of noise level in the 3 l-5 Hz band is due to a very low reading of 95 dB during the octave band analysis in the cabin prior to treatment. According to the narrow-band analysis a value of 103 dB would seem more appropriate, giving a 0 dB improvement. Possibly the reading of 95 dB was due to a momentary reduction in vibration at the 22 Hz frequency. The improvement is seen more clearly in a comparison of the narrow-band analyses. The frequency peaks at 44, 66
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196 A.E.TURNER
and 88 Hz are significantly reduced. The noise rating curve for the new conditions is NR 65 (previously NR 69), a small but significant improvement. The fundamental peak at 22 Hz has not been appreciably affected. Fortunately this is at the lowest frequency of normal hearing and even although it is by far the largest peak it is not the most significant noise. It does not affect the noise rating curve for this environment. The new noise rating is now the same as that existing in some of the cabins further forward.
Frequency (Hz1
Figure 7. Octave-band noise levels in the cabin before (-o-o- ) and after (-x-x-) treatment.
For this type of sound field the “A” weighting of the sound level meter is not appropriate as all frequencies in the band centred on 31.5 Hz are given the weighting for that frequency, whereas a contribution at 22 Hz should be given a far lower weighting. Improvements at discrete frequencies at the lower end of the scale are thus likely to give unrepresentative answers on the meter.
More appropriate is the use of the phon scale where weighting factors for finer frequency intervals are given [3]. The noise levels in store area and cabin are improved from 106 to 100 phons and from 83.5 to 78 phons, respectively, using this method. The accuracy of these figures is estimated at f2 phons absolute. The improvement may also be in error by f2 phons.
5. CONCLUSIONS
The improvements effected are thus small but significant. It is hoped that although the environment in the cabins is not the most conducive to sound sleep, more passengers will be able to sleep easily now that the damping treatment is in place. The noise level in the noisiest cabins has been reduced to the same level as that existing in the cabins further forward, indicating that the largest contributor to the general noise level in the rear cabins has been reduced. to below the general background level for the area.
It is considered that further improvements could be obtained only by large-scale application of damping materials to the hull of the ship, much of which is inaccessible or by isolating the cabins on anti-vibration mounts, a very costly solution. The actual area covered in these trials is small compared with the area of the hull in this region of the ship.
The basic frequency of 22 Hz has been largely una&cted by the treatment, probably because this frequency is carried efficiently by the large untreated surfaces of the hull. The higher modes are excited in the store area and have been reduced considerably by damping.
SHIP NOISE REDUCTION BY DAMPING LAYER 197
It is also true that an unconstrained layer damping treatment depends on the relative stiffness of the treatment to the stiffness of the steel to which it is applied. At low frequencies the stiffness imposed on a structure like the hull of a ship or a partition by cross members or by ribbing can be considerable. At higher frequencies the wavelength of bending waves is small compared with the distance between ribbing and the damping layer is correspondingly more effective. Van To1 [l] also advanced this explanation for the failure of his damping treatments on the two-dimensional model at low frequencies. Indications on this full-scale test, however, have been that damping treatments can make significant improvements to the important frequencies of 44,66 and 88 Hz. The effect of a good damping treatment applied liberally on the hull and general structure of a ship at the time it is built could be considerable.
ACKNOWLEDGMENTS
The assistance of Mr W. D. Robinson in taking measurements and in assisting in the analysis of the results is gratefully acknowledged. Assistance is also acknowledged from Mr C. G. Reid, in the bonding of the damping material to the steel surfaces and advice on adhesive systems. The shipbuilders, Harland and Wolff Ltd. are thanked for the provision of Plate 1 and the shipowners, Coastlines Ltd., are thanked for permission to publish.
REFERENCES
1. F. H. VAN TOL 1967 International Symposium on The Damping of the Vibrations of Piates by Means of a Layer, Leuven, Belgium. Scale model investigations of damping layers applied for noise reduction aboard ships.
2. G. L. BALL and I. 0. SALYER 1966 J. acoust. Sot. Am. 39, 663. Development of a viscoelastic composition having superior vibration damping capability.
3. D. W. ROBINSON and R. S. DADS~N 1956 Br. J. appl. Phys. 7,166. A re-determination of the equal loudness relations of pure tones.
