Free-Field Noise Transmission

When the noise transmission takes place in a free field (no reflective surfaces), it is possible to calculate the pressure levels at different distances from the source. For spherical propagation, the following formula can be used:

Lp = Lw — 20 • log r — 11, (9.168)

Where r is the distance (in m) from the source and Lw is its noise power level. The following formula can be used in the case of hemispherical propagation:

= Lw — 809809(20) ■ log ;, (9.169)

And in general

Lp = Lw — 20 • log r + 10 — log Q — 11, (9.170)

Where Q assumes the values indicated in Fig. 9.62, which is a characteristic of the geometry of floor and walls around the source.

IBB TABLE 9.18 A-Weighted Combined Noise

 Frequency (Hz) 63 125 250 500 1000 2000 4000 8000 Total level L total (dB) 63 65 63 66.2 63 .58.0 61 60 72.} A-wt. correction (dB) -26.2 -16.1 -8.6 -.3.2 0 1.2 1 -1.1 L total in dB(A) 36.8 48.9 54.4 6.3.0 63 .59.2 62 58.9 68.8

 NC curves Nominal frequency for octave band
 Sound pressure ievel for octave band, dB Sound pressure level for octave band, dB
 NC curves Nominal frequency for octave band
 NR curves Nominal frequency for octave band

 NR curves Nominal frequency for octave band

FIGURE 9.63 NC and NR curves and examples of application-The examined noise has an NR index of 28 and an NC index of 25.