Noise Weather

Article Index

Sound weather

Definition

As pointed out in the previous chapter, the sound weather is basically a level difference. Equation 1 gives the formulation of any sound weather typically evaluated for an arbitrary hour.

Let denote LSW,n(r, φ) the sound weather level at the distance r and the compass angle φ for a certain hour n. Let denote LI any relevant receiver level of interest at the given point (r,φ). The index ‘ref’ indicates the level of the normal propagation situation whatever is appropriate to be termed as ‘normal’.

It is clear that the sound weather requires a weather forecast for all relevant features of the atmosphere that influence the sound propagation. The second requirement is a sophisticated sound propagation model to predict the receiver levels around a source up to the distance of interest and at all directions considering the weather forecast.

Profile forecasts

It is well-know that close to the source the ground weather is sufficient to make reliable level prediction. The wind direction yields the most import influence. This is because in such cases the sound propagates close to ground. For typical noise sources and receiver heights, relevant sound rays of the direct transmission and of the ground reflections will not reach a height above ground where the change of wind speed or temperature plays an important role. However, if it comes to larger distances, the rays will reach heights where these features take over. Hence, the profiles – the height dependency – of wind speed, wind direction, temperature and humidity are the important meteorological input parameters of the propagation models. Forecasts of the sound weather requires forecasts of these profiles.

The forecast of profiles is not a standard theme of weather models. For sound weather purposes, MeteoGroup (3) developed such a profile forecast as a special service using their sophisticated weather models to make that prediction. For a given location, this service provides a profile forecast for the upcoming next 36 hours on an hourly basis. The following discussion relies on such forecasts every 12 hours.

Sound propagation model

Technical sound propagation schemes are widely used for noise assessment purposes. They do not take into account weather profiles and do not apply to the current weather situation. They are designed and restricted to average sound levels and to close range propagation using a simple decibel adjustment to account for favorable, neutral or unfavorable sound propagation conditions. The ISO 9613-2, for example, introduces a correction called cmet to adjust its downwind propagation level to a long term prediction level.

For sound weather purposes, a sophisticated propagation model is needed that considers the profiles of wind speed and temperature and that is capable to predict the height dependent refraction due to the resulting sound speed profile. The sound weather in the present paper uses a sophisticated ray tracing model that fulfill these basic requirements.

The meteorological profile forecast gives hourly average values of the wind speed and temperature at selected heights. The sound refraction however does not depend on these parameters but on their change with height. This change is an orthogonal information. For example, even if the average wind speed does not change with height, it does not mean that there is no refraction. Let’s assume for a moment that the radius of curvature has a symmetric Gaussian distribution around the radius of curvature of 0 m, which is the average radius of a not – on the average – changing profile. In addition, let’s assume that the level predictions depend linearly on the radius of curvature than the whole effect is not really important. However, the second assumption does not hold.

As a conclusion, an hourly averaged sound weather needs level averaging and not the averaging of weather conditions. Therefore, the sound weather discussed here is the result of level prediction based upon the averaging of a distribution of single prediction levels as a result of varying input parameters each based on a reasonable estimation of its relevant range of uncertainty.

The sound weather depends, of course, on some general settings. The scenario under consideration is given in Table 1. The source radiates a pink noise in the frequency range of 100 Hz to 8 kHz. Source and receiver heights are both set to 4 m above the overall plane ground. The ground is something like grass given here as flow resistance to meet the requirements of the used propagation model. The air humidity is not taken from the profile forecast but introduced as a scenario parameter.

Table 1 also indicates the range of uncertainty for each parameter to perform the level averaging. Of course, the level averaging includes the distance and compass angle ranges for each value of the noise rose. In the present paper, the level predictions of the model for a non-refracting atmosphere are used as reference levels in Equation 1.

Input parameter

Mean

lower limit

Upper limit

Noise

Pink noise 100 Hz to 8 kHz

No variation

Source height

4 m

3,8 m

4,2 m

Receiver height

4 m

3,8 m

4,2 m

Ground

400 kPa s/ m²

200 kPa s/ m²

600 kPa s/ m²

Relative humidity

70%

50%

90%

Table 1 - Scenario parameters and its uncertainties for the sound weather evaluation

The calculation of the sound weather is realized using look-up tables for the scenario. Note that the sound propagation model will still have a significant range of uncertainty. However, the sound weather only relies on the level difference of two predictions. Therefore, any systematic errors will not be relevant in zero order. This is the reason why the source needs no source strength but only a source spectrum.

Sound weather also depends on the choice of the type of receiver level under consideration. In this paper an A-weighted sound level is used. Using a different scenario, for instance impulse noise and an A-unweighted exposure level for a single event as receiver level, the sound weather will change. For aircraft noise or in particular for noise from wind turbines the sound weather yields complete different results with respect to the meteorological correction cmet of the ISO 9613 -2.

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