Home Engineering Building Acoustics
Experiencing Sound Insulation
Whoever has been living in flats certainly has firsthand experience in the notion of sound insulation when woken up in the middle of the night by noise from the next door flat! One may even have observed that according to the nature of the walls and the dimensions of the room, the perceived sound insulation was different.
Sound Reduction Index
The sound reduction index of a wall or floor is measured in a laboratory that features heavy slabs and walls with a framed 10 m2 opening in the middle. Due to the constitution of the envelope of the measuring rooms that are separated by an expansion joint, flanking transmissions can be considered negligible, and the sole contribution to the sound levels measured in the receiving room comes from the radiation of the floor or wall under test. The floor or wall under test is excited using a diffuse sound field that is generated using a couple of loudspeakers located in one room (labeled “emission room”) and directed toward the corners of the room opposite of the wall under test; the mean sound pressure level is measured in both the emission room and the receiving room.
The sound reduction index R is given by
where L1 is the mean sound pressure level in the emission room, expressed in dB, L2 is the mean sound pressure level in the reception room, expressed in dB, S is the area of the floor or wall specimen under test, expressed in m2, and A is the equivalent absorptive area in the reception room, expressed in m2.
Measurements are performed either in the 100-3150 Hz range (ISO)  or in the 125-4000 Hz range (ASTM) . More recently, EN/ISO standards have attempted to extend the range in the low-frequency region  down to the 50 Hz third octave band.
Of course, the measurement of the sound reduction index of a building component must be performed under laboratory conditions. But it may be possible to perform such a measurement in situ using sound intensity measurements as long as some conditions are met (mainly a low and stable background noise, and either a nonreverberant environment on the receiving side or reflective walls at least 3 m from the element under investigation) .
Note: When performing a sound reduction index measurement on a wall, roof, or floor sample, bear in mind that the dimensions of the sample should be as close as possible to the dimensions used in the project. Using smaller dimensions will usually result in overestimating the performance in the low-frequency range.
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