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) |
63.0 |
65.0 |
63.0 |
66.2 |
63.0 |
.58.0 |
61.0 |
60.0 |
72.} |
|
A-wt. correction (dB) |
-26.2 |
-16.1 |
-8.6 |
-.3.2 |
0.0 |
1.2 |
1.0 |
-1.1 |
|
|
L total in dB(A) |
36.8 |
48.9 |
54.4 |
6.3.0 |
63.0 |
.59.2 |
62.0 |
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 |
|
|
  |
|
|
|
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.
Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK