The Harmonoise reference model has been developed in order to predict long-term average sound levels in road and railway situations that are geometrically relatively simple but physically complex. The present paper describes all steps of calculations with this powerful model which includes several advanced numerical propagation methods to calculate the coupled effects of atmosphere, ground surface and obstacles on sound waves. The reference model employs a statistical description of the atmosphere, based on local meteorological data, as well as various impedance models for ground surfaces and other absorbing surfaces. Validations against in situ longterm measurements have been achieved in several sites; agreement between reference model and experimental results ranges from excellent in flat terrain situations down to fairly good in more complex configurations (hilly, viaduct).