The level of speech and the sensitivity of the ear are not constant, they depend on the frequency. The sound reduction index for a structure does not appear to be linear, either; high frequencies can be reduced more easily than low frequencies. Thus the sound reduction index between two partitions is measured at the frequency range from 100 to 3150 Hz.
Pink noise is played through a loud speaker to produce a sound pressure level over 100 dB in the transmission room in order to determine the transmission loss. The sound pressure levels are measured in the transmission partition and in the receiving partition at 16 1/3 ?octave bands. The sound reduction index can be derived from the difference of the sound pressure levels, room volume and reverberation times. As a result a graph from which the weighted apparent sound reduction index R?w can be obtained from the measurement.
It would be confusing to announce the regulation for sound insulation in the form of graph, thus the result is concluded in one number, which is the weighted apparent sound reduction index, R'w. The graph obtained from the measurement is compared with the reference graph and the Sound Reduction Index will be read from the reference graph at the centre frequency of 500 Hz.
The airborne sound insulation between two partitions goes better as the value of the weighted apparent sound reduction index grows. The National Building Code of Finland (part C1 1998) sets the lowest allowed SRI as 55 dB.
However, the regulations are only for the structures and physical properties of the buildings. The real audible sound depends naturally from the sound sources also. Importantly, even though the regulations would be met, it cannot be guaranteed that nothing can be heard from the adjacent room.
