Trane TRG-TRC007-EN manual = L p1 20 log, Distance Correction in a Free Field

period four

Equipment Sound Rating

notes

Distance Correction in a Free Field

Lp2	= Lp1	– 20 log10	r2
			r1

Figure 54

This type of relationship between distance and surface area provides the following simple mathematical model for estimating how sound will change as the distance from the source increases.

Lp2 = Lp1 - 20 log10 [r2 / r1]

where,

Lp2 = sound-pressure level at distance r2

Lp1 = sound-pressure level at distance r1

r1 = distance from the source where Lp1 was measured

r2 = distance from the source to where the sound pressure (Lp2) is desired

Using this expression, it can be shown that doubling the distance from the sound source results in a 6 dB reduction in the sound-pressure level. This is a handy fact to know when making estimates of outdoor sound levels.

Lp2 = Lp1 - 20 log10 [2 /1] = Lp1 - 6

TRG-TRC007-EN 49