Some rooms are much noisier than other, even with the same noise sources. For the design of low-noise indoor environments it is crucial that both the noise sources and the surrounding room surfaces are taken into account. Using a room acoustic computer model the first step is to model the room surfaces in terms of absorption and scattering properties. Next the sources are modelled, and while some sources may be sufficiently modelled by a simple point source, this is not sufficient for large noise sources. So, lines, surfaces and boxes should also be used for source modelling. In the design process the computer model offers a handful of tools like grid maps of noise distribution, reverberation time, spatial decay rate, identification of the most important sources, and possible effects of various noise control measures. Even the effect of sound insulating structures may be simulated. The methods and possibilities are demonstrated through a case of a large turbine hall with two turbines, each modelled by 17 surface sources and two point sources. The reliability of the computer modelling is confirmed by comparison of calculated and measured sound pressure levels.