During the work with the draft standard ISO/DIS 19488 for the sound classification of dwellings, several studies were presented with recommendations on whether 50 or 100 Hz would be the most appropriate lower frequency limit when evaluating single numbers of airborne sound insulation between dwellings. It was observed that 100-3150 Hz is the range used in almost all national building regulations, but including lower frequencies has been considered in some countries and are recommended for higher sound classes in some standards. The Swedish regulations began including evaluation from 50 Hz in 1999, which means there is a long-term experience from the field to support this discussion. In this paper, several single numbers based on field measurements are compared to subjective ratings given by the residents in a variety of building types. The questionnaire surveys were distributed in total 46 building objects with light-weight or heavy walls and floors. Several single number quantities according to ISO 717-1 were correlated against the ratings given by the residents. The statistical evidence for a 50 Hz limit was found to be small on the average, but analysing light-weight buildings separately showed an importance of including sound reduction indices from 50 Hz. Currently, we consider measurements from 50 Hz the best choice when good sound protection against music at low frequencies may occur, since it will also protect from noise at mid and high frequencies. If protection against disturbing speech is sufficient, measuring from 100 Hz is enough. In 2019-2021, about 15 surveys will be made in buildings with timber joist, CLT- or concrete floors.