Efecto de la aw, concentración de sorbato de potasio (SK) y tratamiento

A noise profile of a vessel is a representation of how a vessel acts as a noise source in the water. There are different ways to evaluate a ship as a noise source and how this is considered has implications for suitable measuring distances, affects assumptions about uniformity in the noise field and the decay of ship noise with distance. The following section reviews the theory, standards and reported characterisation of other vessels to establish the key parameters for describing ship noise. Urick [56] divides ship noise into three classes: machinery, propeller and hydrodynamic noise. In our case the ship is stationary and the propeller is disengaged so the ship noise being produced is generated by the on-board machinery or plant. This steady state machinery noise can be described as a tonal noise source as the machinery creates a consistent noise with distinct frequencies [68]. Noise from ship machinery is expected to be primarily between 10 Hz to 1 kHz [64]. There are currently two standard measurements of ship noise, a whole vessel noise profile and a sonar self noise measurement.

Three international reports exist for the measurement of whole of vessel far field noise profiles, a vessel noise classification produced by Det Norske Veritas (DNV) [69], an American National Standards Institute (ANSI) [70] for measurement procedures, and an International Council for the Exploration of the Sea (ICES) [71] cooperative research re- port on vessel noise. These reports provide noise limits for moving vessels, and guidelines for measuring and reporting the radiated noise of vessels. They only consider moving vessels and assume that the measured vessel noise is in the acoustic far field. The noise data is required to be presented using third-octave band filters for frequency to reduce the effect of the moving ship by integrating with time to obtain a power spectrum [72, 73]. Noise profiles that apply the above standards to specific vessels are available for the RV

3.1. INTRODUCTION 29

Oscar Dyson [74], and theRV Sharp [73, 75]. A noise profile represents the steady state radiated noise of the ship as a whole measured in the far field, and the sonar self-noise is a near field measurement of ship noise as received by an on-board echo-sounder at a particular location within the near field.

Every sound source has near and far field characteristics. In the acoustic far field a sound source appears as a single source of sound, and can be modelled as a point source that decays uniformly with distance from the source with cylindrical or spherical spreading depending on the environment. In the near field the different parts of the source are still constructively and destructively interfering in a way that varies strongly with location within the field. In the near field the decrease of the sound levels with distance is less than the spherical spreading of the far field. In the near field the individual noise contributors, such as motors, compressors or steam pipes, can be considered as a collection of sources, whereas in the far field the ship can be considered a single noise source. Urick [56] suggests that in the case of ship noise there is sufficient evidence to simply use spherical spreading, even for lower frequencies, shallow depths and short distances. As this work compares noise measurements made from lateral positions with aft measurements it is worth considering the assumptions being made about this near and far field boundary. Lurton [76] defines the near field boundary or Fresnel distance as:

DF =

L2

4λ (3.1)

where L is the characteristic dimension of the radiating source, and λ is the wavelength which can also be expressed as the ratio of sound speed over frequency. Beyond this boundary he defines a range where the field is stable but not yet decaying as it does in the far field, and then a far field boundary of approximately 4DF. If the ship’s hull

is considered to be a single radiating source of noise, then the largest characteristic dimension would be ship length, so for the Aurora Australis L is 94 m. Combining this with an approximate sound speed in Antarctic waters of 1,440 m/s and a frequency range of 10 Hz to 1 kHz gives DF = 15 m to 1.5 km and a corresponding far field boundary

of 60 m to 6 km depending on the frequency being measured. Because a ship is not a uniform shape the noise field it generates will not be uniform in all directions. As the measurements used in this experiment are from the aft deck the ship’s beam of 20.3 m may be a more appropriate characteristic dimension, giving an expected far field boundary of 1.42 m to 142 m.

Measuring ship noise too far from the source would introduce errors due to ship noise being lost in background noise. This requirement needs to be balanced with the need to

3.1. INTRODUCTION 30

Table 3.1: Recommended distances for noise measurement and the theoretical far field boundary for characteristic dimensions of ship length of 94 m, and ship beam of 20 m.

Reference Frequency Range Distance

DNV Silent-E 10 Hz - 100 kHz 150 - 250 m

ANSI 20 Hz - 25 kHz greater of ship length or 100 m

ICES 10 Hz - 20 kHz ship length

Calculated L = 20 m 10 Hz- 1 kHz 1.4 m - 1.4 km Calculated L = 94 m 10 Hz- 1 kHz 60 m - 6 km

measure in the far field. The theoretical measurement of far field from the texts does not appear to be considered in the recommendations for measuring distance for noise profiles. Tab. 3.1 shows a summary of published recommendations for measuring distances and the theoretical far field boundary. In the noise profile of theRV Oscar Dyson, the measuring distance recommendations from the ICES report are used with measurements between 100 m and 200 m. This is noted to be considered in the far field for a ship length of 64 m [74]. It is possible that these considerations of far field are done assuming a much smaller source size, rather than the full length of the vessel hull. Using the equations above for a 1 kHz noise source, the source size can be up to 19 m in characteristic dimension to be measured in the far field at 250 m. The methods for noise profiling presented in the international reports request information on frequencies up to 50 kHz. For this frequency the source of noise would need to be less than 2.6 m to be measured in the far field at 250 m. There is existing work that considers different noise producing sources on the ship individually to create a mathematical transmission loss algorithm for the ship noise as a whole as described in Hall [72]. ANSI [70] defines the geometric far field as being the horizontal distance from the ship where there is less the 1 dB re 1 µPa error when normalising to a 1 m reference distance using the assumption of a single source location. In the experiments detailed in this paper measurements were made between 10 m and 350 m. For the purposes of creating a noise profile as close to those presented for other vessels and comparison to the noise limits, distances over 250 m will be considered in the far field. Also for comparative purposes noise data will be presented in one-third octave band representation. Additional to this standard noise characterisation, far field and near field data will be presented for a subset of frequencies. Narrowband noise data is used to identify the effect of particular plant and present the noise signature of the vessel [77], and near field data as an indicator of field strength and variability in the near field.