Through a series of Swedish research projects carried out over twelve years, the relation between the measured impact sound insulation and the corresponding subjective rating given by residents of multi-storey apartment buildings has been studied. Previous results, based on a limited number of building cases, suggested the need to include frequencies below 50 Hz in a frequency weighted single number quantity to get a reasonable correlation with the subjective ratings.

The purpose of this paper is to include an extended amount of data compared to the previous studies to secure more substantiated results. Data has been examined for 38 building cases, including a variety of lightweight and heavy constructions. More than 1200 questionnaire responses form the basis for comprehensive statistical analyses.

The results confirm the conclusions of the previous parts (I, II), stating that frequencies below 50 Hz are of importance when evaluating impact sound insulation, primarily in lightweight buildings. The correlation between measured weighted single number quantities and annoyance ratings increases significantly when the frequency range is extended down to 25 Hz. The strongest correlation was obtained when evaluating the impact sound insulation, again from 25 Hz, when using the weighted single number quantity L_nT,w + C_I,25€-2500 where the C_I-term is evaluated according to ISO 717-2 amended to the extended frequency range 25-€“2500 Hz. It is also suggested that in the lowest one-third octave bands, 25-40 Hz, there is no need to normalize the impact sound pressure levels with respect to the reverberation time.