#### Answer

$P = \frac{3}{r}~cos[\frac{2\pi~r}{4.9}- (1026)~(10)]$
We can see a sketch of the graph in the window $[0,20]$ by $[-2, 2]$
The sound pressure $P$ decreases as the radius $r$ from the source increases.

#### Work Step by Step

$P = \frac{a}{r}~cos(\frac{2\pi r}{\lambda}- ct)$
$\lambda = 4.9~ft$
$c = 1026~ft/s$
$a = 3~lb/ft^2$
$t = 10~s$
$P = \frac{3}{r}~cos[\frac{2\pi~r}{4.9}- (1026)~(10)]$
We can see a sketch of the graph in the window $[0,20]$ by $[-2, 2]$
The sound pressure $P$ decreases as the radius $r$ from the source increases.