Because of standing waves, the sound pressure level within a room may vary as much as 20-30 dB at low frequencies. Principal properties of low-frequency sound in rooms are illustrated by simulations, and sound pressure distributions as well as the performance of Swedish and Danish measurement methods are studied by measurements in three rooms. For assessment of annoyance, mainly areas of the room with high sound pressure levels are of interest, since persons present in such areas are not helped by the existence of lower levels in other areas. The level that is exceeded in 10% of the room (L₁₀) is proposed as a rational and objective target for a measurement method. The Swedish method showed results close to the target, but, because of a doubtful use of C-weighting in the scanning, it may give results below the target in case of complex sounds. The Danish method was found to have a high risk of giving results substantially below the target, unless complainants can precisely appoint measurement positions, where the sound is loudest/most annoying. An alternative method using measurements in four three-dimensional corners of the room is proposed. This easy and straightforward method seems to give reliable results close to the proposed target.