Building Acoustics

136 Building acoustics

Source

R

S 1

W

Figure 4.15 Example on image source not being visible in receiver positions R.

We shall put a source midway between the floor and the ceiling, initially assuming
that the absorption factor α is the same for these boundary surfaces. The energy density
w at a receiver position may then be expressed by

()

22

(^01)

1 1

2

4

n

n n

W

w

c rr

α

π

∞

=

,

⎡ − ⎤

=+⎢ ⎥

⎢⎣ ⎥⎦

∑ (4.63)

where W is the source sound power, r and rn are the distances between the source and the
receiver and between the receiver and the image source with index n, respectively.

S

R

S 2

S 1

S 2

S 1

Sn

Figure 4.16 Image sources in a “flat” room.