3 Noise sources

Noise from ships in ports. Possibilities for noise reduction

The primary sources of noise on a ship at berth that give rise to noise in the environment can be divided into three categories:

Diesel generator engine exhaust
Ventilation inlets/outlets
Secondary noise sources, e.g. pumps, “reefer” refrigerated containers

The various noise sources are described in the following.

3.1 Diesel generator exhaust

The diesel generator is used to generate power on board the vessel. During port stay it will often be the most predominant source of noise radiating from the ship to the surroundings. The diesel engine exhaust is often placed at the top of a funnel which has a significant height compared to the surrounding landscape. The noise can therefore propagate over large distances without being reflected or absorbed by the surroundings. If the diesel generator exhaust noise is not sufficiently attenuated with a well-designed silencer it may easily cause high noise levels in the surroundings, even at large distances.

The unattenuated sound power level of the different diesel engine exhaust is in some cases supplied in the manufacturer’s engine project guide. An excerpt of the levels from project guides from two engine manufacturers is seen in the table below:

Table 4 An excerpt of the unattenuated exhaust A-weighted sound power levels, dB(A) re. 1pW, provided in engine project guides by the engine manufacturers MAN B&W and Wärtsilä as found on their respective websites reference [9] and [10]. ‘L’ and ‘V’ in the type description indicate whether the cylinders are in line or placed in a V-arrangement and the number indicates the diameter of the cylinder bore in centimetres.
		1/3-Octave frequency bands, Hz
Make	Type	16	31.5	63	125	250	500	1000	2000	4000	8000	Total
MAN B&W	L32/40	80	115	130	135	129	133	135	135	133	130	142
	V32/40	82	111	126	133	129	133	135	135	133	130	142
	L48/60B	88	119	124	126	129	133	135	135	133	130	141
	V48/60B	84	111	124	126	129	133	135	135	133	130	141
	L58/64	80	115	130	135	129	133	135	135	133	130	142
Wärtsilä	W26	-	122	132	135	131	125	124	118	112	102	138
	W32	-	107	115	127	130	129	127	121	109	-	135
	W38	-	101	119	122	127	131	134	129	126	118	138

The engine type number generally describes basic details, e.g. W6L32 is a 6 cylinder inline engine with a cylinder bore of 32cm. The data in Table 4 provides an idea concerning the magnitude of the unattenuated sound power of the exhausts of some of the commercially available diesel engines. The total sound power levels in the given selection of engines vary between 135dB(A) – 142dB(A) which is significantly higher than the maximum allowable sound power level of 107dB(A) found in calculation example 1. The silencer should, in this case, attenuate the exhaust noise from the engine by approximately 28 - 35dB.

The frequency content of the exhaust noise from a given engine depends on a number of factors. The most important are the number of cylinders, the speed of the diesel engine and if it is a 2 or 4-stroke engine. The firing frequency of the engine, and multiples or harmonics hereof, depends on these factors and can be determined by Eq.2.

n=rotational speed (rpm)

z=number of cylinders

a=1 for 2-stroke engines

a=2 for 4-stroke engines

In many cases the component corresponding to twice the firing frequency is the most significant. However, it is usual to see frequency components corresponding to half and whole harmonics of the engine as well. As an example the 4-stroke Wärtsilä engine, W6L32, has a rotational speed of 720 rpm and 6 cylinders. Therefore the first harmonic of the engine rotational speed is 12 Hz and the firing frequency 36 Hz. A narrowband frequency sound pressure level measurement at 5 meters distance from the exhaust stack outlet of a W6L32 engine exhaust is shown in the figure below:

Figure 2 Measured A-weighted sound pressure level, dB(A) re. 20 µPa, narrow-band frequency spectrum 0 – 100 Hz, at approximately 5 meters distance from a Wärtsilä W6L32 3000kW / 720 rpm engine exhaust stack outlet. An absorption type silencer is installed in the exhaust stack.

The narrowband frequency spectrum is seen to contain half multiples of the first harmonic. The component at twice the firing frequency is dominating the spectrum. The same measurement as shown in Figure 2 but analysed in 1/3-Octave bands, 25Hz – 4000Hz, is shown in Figure 3. The 1/3-Octave band measurement shows that the exhaust noise is mainly located in the frequency range 40 - 160Hz with the 80Hz-band dominating spectrum.

Figure 3 Measured A-weighted sound pressure level, dB(A) re. 20 µPa, in 1/3-octave bands, at approximately 5 meters distance from a Wärtsilä W6L32 3000kW / 720 rpm engine exhaust stack outlet. An absorption type silencer is installed in the exhaust stack.

Because diesel generators are used to generate electrical power on board the ship the engines run at fixed speed. The AC-current generated onboard by the diesel generators is either 50Hz or 60Hz. Typical diesel generator engine speeds for a 60Hz current installation are:

I.e. 1800 rpm, 1200 rpm, 900 rpm, 720 rpm, 600 rpm and 514.3 rpm. As a rule thumb higher speed engines are used on smaller vessels and slower speed engines in larger vessels. The frequency content of the diesel generator exhaust is dependent on the engine speed as described earlier. Smaller vessels are likely to have higher frequency content and larger vessels lower frequency content in the exhaust noise spectrum.

The actual sound power at the outlet of the exhaust stack with the silencer is dependent on a number of factors including the piping system layout, type, performance of the silencer and the location of the silencer in the exhaust stack.

3.2 Ventilation

Ventilation systems onboard ships may be a significant contributor to the noise generated by the ship in the surroundings if no noise reducing measures have been included in the design. Depending on the ship type different ventilation systems on board include engine room ventilation, cargo ventilation, AC-systems, galley ventilation, etc.

In some cases information on the sound power of the different fans is supplied by the manufacturers. An excerpt of the sound power levels of different engine room- and hold ventilation- fans as provided by a manufacturer are provided in the Table 5. The large hold ventilation fans are mainly used on RoRo ships for ventilating car decks. The sound power levels of the larger fans are seen to be quite significant.

*Table 5 The A-weighted sound power, dB(A) re. 1pW, of different engine room and hold ventilation fans as supplied by a manufacturer.*
		1/3-Octave frequency bands, Hz
	Volume flow, m³/h	63	125	250	500	1000	2000	4000	8000	Total
Engine room fans	120000	73	93	98	105	105	102	98	91	110
	70000	68	84	100	104	106	103	99	93	110
	50000	66	82	98	101	103	101	97	90	108
	33000	64	79	96	99	101	99	94	88	106
	15000	51	67	80	95	96	96	92	86	101
	12000	52	68	81	96	96	96	93	87	102
	1000	39	55	73	78	83	83	80	74	88
	1000	31	47	60	75	75	75	72	66	81
Hold ventilation	95000	75	93	97	100	100	97	91	83	105
	85000	69	89	94	101	101	98	94	87	106
	73000	67	83	99	102	104	102	97	91	109

The frequency content of the sound power of the fans in Table 5 is seen mainly in the intermediate frequency range, ca. 250 to 2000 Hz. Noise from different ventilators and noise mitigating measures is well described by Stampe in reference [19]. The sound power level of fans can be determined, with relatively good accuracy, based on empirical formulas which are also provided in [19]. Among these are:

q_V= volume flow, m³/s

Δp_t= fan total pressure difference, Pa

L*_w=specific sound power level

In Eq.3 it is presupposed that the fan is operated at the designed maximum efficiency of the fan. The specific sound power level of the fan is dependant on the type. For radial ventilators it is 25-30dB and for axial ventilators 25-35dB. The ventilators in Table 5 are of the axial type. Eq.3 shows that the sound power increases with the volume flow and the fan total pressure difference. If the fan is not operated at the designed maximum efficiency the sound power of the fan will generally increase.

Without any noise reducing measures included in the ventilation system the sound power at the outlet/inlet ventilation grid will be comparable to the sound power of the fan.

3.3 Secondary noise sources

The normally dominating noise sources on board a ship at berth are the diesel generators and different ventilation systems. However, a number of secondary noise sources on the ship also contribute to the noise in the surrounding environment, for example different hydraulic pumps, loading/unloading of cargo, winches, reefer (Cooling containers). On container ships reefers are used for temperature controlled transportation. The sound powers of two reefers are provided in Table 6

*Table 6 Typical A-weighted sound power, dB(A) re. 1pW, of Reefers. Measured by LR-ODS and data provided in [14].*
	1/3-Octave frequency bands, Hz
	63	125	250	500	1000	2000	4000	8000	Total	Note
Reefer	69	79	80	81	81	78	73	65	87	LR-ODS measurements
Reefer	66	78	82	84	84	84	78	69	90	Database

The sound power of the reefers is approximately 90 dB(A). Compared to the sound power levels of the diesel generators and the ventilation it is significantly lower. However, the total sound power will increase with the number of reefers approximately with:

n = the number of reefers

The total sound power of all the reefers onboard the ship can therefore be significant. If the ship is powered by on-shore power the reefers are likely to define the lower background noise limit of the container ship.

3.4 Environmental noise from ships in general

A relation between the dead weight tonnage, DWT, and the sound power level of ships has been sought to be established in [12], see Figure 4. The reported relation is based on measurements on 65 ships. The trend is that the sound power of the ship increases with the DWT. The diesel generators on smaller ships are often high speed engines with higher frequency content in the noise. Conventional exhaust silencers are often of the absorptive type with the most attenuation in the higher frequency range. Therefore, it may be speculated that the noise from the diesel engine exhaust noise on smaller ships is attenuated better than the exhaust noise on larger ships. Furthermore, the required ventilation in the engine room is less on smaller vessels, i.e. the engine room fans are smaller and have a lower sound power. However, the trend plot provided in Figure 4 also shows that there are large deviations of more than 20dB in the measured sound power on ships with the same DWT.

A similar comparison of the sound power as a function of lane length has been performed for RoRo-ships, [13]. No relation between the lane length and sound power was found. The reported source power of the RoRo-ships varies between approximately 108 – 125 dB(A).

Figure 4 The relation between Dead Weight Tonnage and Sound Power Level established in [12].

Figure 4 The relation between Dead Weight Tonnage and Sound Power Level established in [12].

In conclusion it is highly recommended that before estimating the sound power of a ship via the trend shown in Figure 4 that the individual noise sources and the noise reducing measures on the ship should be carefully evaluated.