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

14.3 The “A – Weighted” Decibel (dBA)Because the human ear is not equally sensitive to all frequencies highway traffic noise ismeasured using an “A-Weighted” approach. A-weighting emphasizes sound within thefrequency range 1000 to 6300Hz and de-emphasizes sounds above and below these values.14.4 Sound Pressure Level (SPL)The SPL converts the sound pressure (energy) presented on a logarithmic scale todecibels (dB) given by Equations 14.1 and 14.2.In terms of pressure:where:SPLpp ref10.log⎛⎜⎝p⎞⎟⎠2= 10 (14.1)⎜ p ⎟ref= sound pressure;= reference sound pressure of 20µPaSound energy is related to SPL thus:⎛⎜⎝pp ref⎞⎟⎠2=10 (spl-10) (14.2)Because decibels are represented on a logarithmic scale they cannot be addedalgebraically. For example, say a source produces 50dB at a receiver and an additional50dB was added to the source their combined SPL would not be 100dB at the receiver, but53dB. How this result was arrived at will be discussed in some detail in Section 14.8.14.5 Transmission Loss (TL)Transmission Loss is the ability of a material to reduce or resist the transmission of soundby absorption as shown in FIGURE 14.2.FIGURE 14.2: Sound response on meeting a barrier224