The main problem in the measurement of airborne sound insulation is the measurement of the sound power radiated by the barrier, in practice performed by measuring the sound pressure level and the acoustic absorption in the receiving room. Large variations of the sound pressure level in a reverberation room indicate the presence of dominating strong standing waves, so that it becomes necessary to install diffusing elements. In ISO 10140, the limits have been defined in which the reverberation time at frequencies at and above 100 Hz should be included. Sometimes, however, in the case of rooms with a large volume, obtaining the required parameters is difficult and sometimes even impossible. It should then be checked whether the measured sound insulation depends on the reverberation time.

The paper presents the results of sound insulation measurements at various reverberation time lengths in subsequent stages of diffusing elements installation in the receiving room. An analysis of diffusing materials amount and arrangement influence on the uniformity of the sound pressure level distribution and reverberation time in the room as well as the value of the measured sound insulation was carried out. Uncertainty of sound insulation measurement with partial uncertainties was adopted as a criterion supporting the assessment of the obtained results.

A set of sound power assessments was performed to determine measurement precision in specified conditions by the comparison method in a reverberation room with a fixed position array of six microphones. Six blenders (or mixers) and, complementary, a reference sound source were the noise sources. Five or six sound power calculations were undertaken on each noise source, and the standard deviation (sr) was computed as “measurement precision under repeatability conditions” for each octave band from 125 Hz to 8 kHz, and in dB(A). With the results obtained, values of sr equal 1.0 dB for 125 Hz and 250 Hz, 0.8 dB for 500 Hz to 2 kHz, and 0.5 dB for 4 kHz and 8 kHz. Those can be considered representative as sound power precision for blenders according to the measurement method used. The standard deviation of repeatability for the A-weighted sound power level equals 0.6 dB. This paper could be used for house or laboratory tests to check where their uncertainty assessment for sound power determination is similar or not to those generated at the National Metrology Institute.