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# Wind Turbine Noise Model

## Introduction

This Wind Turbine Noise Model is derived from the method documented by the International Energy Agency: Expert Group Study on Recommended Practices for Wind Turbine Testing and Evaluation, 4. Acoustics Measurements of Noise Emission from Wind Turbines, 3. Edition 1994. It is a simple model which assumes spherical spreading from a point source either in free space (spherical) or over a reflective plane (hemi-spherical). It can also take into account atmospheric attenuation, using an attenuation rate entered by the user. The source sound power and the absorption coefficient are both assumed to be broad band. Source to receiver distances are calculated by simple geometric means and the total received noise from each turbine logarithmically added.

Users should note that the model does not take account of:

• Uneven topography
• Large obstructions in the propagation path, e.g. barriers etc
• Refraction of noise, e.g. due to atmospheric effects such as temperature inversion
• Wind speed or direction effects
• Any change in the propagation with changing frequency

More sophisticated models, allowing some of these additional factors to be taken into account, are available. Those wishing to understand some of the other factors affecting propagation of sound outdoors should consult:

• ETSU W/13/00385/REP: A critical appraisal of wind farm noise propagation. Available from ETSU
• ISO 9613: Calculation of absorption of sound by the atmosphere
• Woods Practical Guide to Noise Control, Woods of Colchester
• Noise and Vibration Control, Edited by L. L. Beranek, ISBN 07-004841-X

This work is no longer supported by the current programme.

## Instructions

To use the model please enter the grid references (Easting & Northing) of the wind turbine and property locations to 1 meter resolution. Also enter the absolute height (H) of these locations above sea level. Don't forget to check the boxes (Inc) by the turbines and properties for those you wish to include when calculating the noise level. Any entries that are left blank or are not numbers will be assumed to be equal to zero.

Enter Lw, the sound power level of the turbine and the Source Height above ground level (usually the hub height). Atmospheric absorption can be included, check the box and enter the value (0.005dB/m is commonly used). The model will calculate both spherical and hemi-spherical source types, either can be selected using the option buttons.

That's it, press Calculate and the noise levels due to the turbines at each property should be calculated and displayed in the boxes on the right.

 Wind Turbines Properties Noise Levels E N H Inc E N H Inc 1 2 3 4 5 6 7 8 9 10
Turbine Details Atmospheric Absorption Propagation
 Source Height (m) Source Lw (dB)
 Include Attenuation Rate
 Spherical Hemi-Spherical

The model uses JavaScript and therefore only works if your browser is JavaScript enabled. This software has not been subjected to NPL's Quality Assurance procedures. No warranty or guarantee applies to this software, and therefore any users should satisfy themselves that it meets their requirements.

The propogation of the noise is calculated using the following formula:

Where:
r is the distance from source to receiver
a is the absorption due to the atmosphere (dB/m)
The first equation is for Hemi-Spherical radiation and the second for Spherical

In order to avoid problems in taking the Log10 of zero, if the combined distance from any turbine to any property is zero that combination will be ignored.