EWPAA Structural Plywood and LVL Design Manual - Engineered ...

The absorption process of a single leaf panel is a function of:• panel mass/m 2 ;• panel bending stiffness (or better its lack thereof);• frequency of the sound source;For a sound source of a given frequency, randomly incident to the panel, the transmissionloss is:TL = 20 log (M.f) – 47.2dB (14.3)where:M = mass of panel in kg/m 2 ;f = frequency of source;The relationship of Equation 14.3 is known as the Mass Law. From Equation 14.3:20 log 2 = 6 i.e.• doubling the mass increases TL by 6dB;• doubling the frequency increases TL by 6dB.The consequence of the influence of frequency response on TL for a single leaf partition isshown in FIGURE 14.3.FIGURE 14.3: Shows various regions of performance for single leaf partition14.6 Sound Transmission Reduction – Airborne & ImpactIt can be seen from FIGURE 14.3, because of the variation in magnitude of TransmissionLoss (TL) with frequency, difficulties are presented in the assigning of a single numberrating to characterise the TL of the partition. However, a single number rating is desirableand the Sound Transmission Class (STC) used in Multi-Residential Timber FramedConstruction 2, was to be such a number. The STC was a type of average value of TL overthe range of frequencies.The STC was limited in that it only applied to walls insulating against speech, i.e. airbornesound, or similar sound sources. STC was not really suited for external wall systems andeven for some internal sound sources.The STC concept was replaced by the Weighted Sound Reduction Index (R w ) for soundinsulation against airborne and impact noise on walls and floors separating sole occupancyunits. R w better accounted for the low frequency regime of the sound frequencydistribution than did the STC.225