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

With structure borne sounds, i.e. impact and vibration the Impact Isolation Class (IIC) applies to floorconstruction, and is a single number rating the effectiveness of a floor system in providing insulationagainst impact noise such as footsteps.The IIC system of impact rating has now been replaced by the Weighted Standardised Impact SoundPressure Level (L nt.w ).Because building product information from some sources (includes Multi-Residential Timber FramedConstruction MRTFC 2) is still quoted in IIC the following relationship has been devised by the Association ofAustralian Acoustical Consultants to allow conversion.L nt.w= 110 – IICThe effect of holes, openings, and gaps will significantly downgrade the acoustic performance a wall.Even small air gaps between panels affect performance. Doors and windows (both closed) incorporated in awall system change its insulation rating quite dramatically.14.7 Subtraction and Addition of DecibelsSubtraction:When noise passes through a barrier, e.g. a plywood sound barrier, a transmission loss results. Assumethe sound source to be a truck producing 70dBA and the plywood barrier results in a transmission loss of21dBA then the noise received through the barrier is the algebraic difference:Addition:70 – 21 = 46 dBADecibels cannot be added algebraically. Addition of decibels requires the use of TABLE 14.1.For combining two decibel levels of sound with random frequency characteristicsDifference between levels (dB) Amount to be added to higher level (dB)0 or 10 0.0TABLE 14.1 : Addition of (dB’s) to be added for various (dB) differencesAs an example consider a person being exposed to a sound pressure level of 90dB from one source and88dB from another source.The resultant total sound pressure is not the algebraic sum, i.e. (90 + 88 = 178dB).To find the combined sources intensity subtract the smaller value from the larger to give:90 – 88 = 2dBFrom TABLE 14.1, the difference of 2dB (left column) results in 2.1dB being added to the higher value, i.e.90 + 2.1 = 92.1dBRounded to the nearest whole dB 92dB226