Audio Engineering

98 Chapter 3

3.12.6 Outdoor Acoustics

If, for example, the ambient noise level measured 70 dBA (not an unreasonable reading
outdoors) and the mostSPL you could generate at 4 ft was 110 d B LP, how far could you
reach before your signal was submerged in noise?

110 70 40

20

4

40

410

400

40 20

LL

x

x

PP







dB

log dB

ft.

/

The problem actually is more complicated than this outdoors, but this serves as an
illustration of how to begin.

We have now touched on the most important basics of the acoustics environment outdoors.
Before going indoors, let us apply some of this knowledge to a series of ancient outdoor
problems. A simple rule of thumb dictates that when a change of 10 dB occurs, the
higher level will be subjectively judged as approximately twice as loud as the level 10 dB
below it. While the computation of loudness is more complex than this, the rule is useful
for midrange sounds. Using such a rule, we could examine a sound source radiating
hemispherically due to the presence of the surface of the earth. Figure 3.8 shows sound in

4884

Arbitrary loudness units

100 50 0 50 100

Noise Noise

Distance (ft)

s

Noise

Figure 3.8 : Sound in an open fi eld with no wind.