Acoustic Value Rw for Building Materials
Rw index : The Rw index or sound reduction index (expressed in decibels) measures, in just one number, the acoustic performance of a specific unit. The higher the Rw index, the better the level of acoustic insulation offered by that composition. For instance, the Rw index of ordinary double glazing is around 29 dB where as a good acoustic interlayer offers an Rw index of around 50 dB.
Rw is a single figure rating for the airborne sound insulation of building elements (not just glass). It includes a weighting for the human ear and measures actual sound transmittance. Rw is measured in a laboratory, not on site (the site-measured equivalent value has the DnT,w). The Rw value is merely an average simplifying mutual comparison of various building components. That can be confusing some times. For instance, two glass units can have the same Rw index while one of them performs well at low frequencies and bad at high ones, and the other one performs just the opposite.
For laboratory measurements using sound pressure, the sound reduction index is calculated using: R = L1 – L2 + 10 lg (S/A) dB
L1 = average sound pressure level in the source room
L2 = average sound pressure level in the receiving room
S = area of the test specimen (m2)
A = equivalent sound absorption area of the receiving room
Sound Reduction Index Definition IEC 801-31-39 of a partition, for a specified frequency band, difference in decibels between the average sound pressure levels in the reverberant source and receiving rooms, plus ten times the logarithm to the base ten of the ratio of the area of the common partition to the total Sabine absorption in the receiving room.
C and Ctr factors : To slightly avoid this issue two spectrum adjustment factors: C and Ctr, have been added to modulate the Rw average. For sound waves featuring high frequencies, the factor C is added to the Rw value. For lower frequencies, factor Ctr needs to be added. The acoustic behaviour of a building component is hence defined by three numbers: Rw (C, Ctr). A building component with the values Rw (C, Ctr) = 40 (-1, -4) provides an average insulation performance of 40 dB. For higher pitched sounds the sound insulation is lessened by 1 dB (39 dB) and for lower pitched sound sources it is lessened by 4 dB (36 dB).
The table below, extracted from Saint Gobain, helps showing how these three numbers apply to different laminated units with acoustic interlayers :
Sound reduction index values for several laminated glass units with acoustic interlayers. The thickness shown at the right column is the total one. 13 mm means 6 mm + 6 mm + 0.8 mm interlayer. Taken from Saint Gobain Stadip Silence brochure.
C takes into account medium and high frequency noise sources such as TV, music, loud conversations or aircraft noise a short distance away. Ctr takes into account medium and low frequency noise sources such as urban traffic noise or aircraft noise a long distance away.