Page 136 - Master Handbook of Acoustics

FIGURE 6-2 In a specular reflection, the angle of incidence (θi) is equal to the angle of reflection
(θr).

When sound strikes more than one surface, multiple reflections will be created. For example,
images of the images will exist. Consider the example of two parallel walls as shown in Fig. 6-3.

Sound from the source will strike the left wall; this can be modeled as a virtual source located at I L
(a first-order image). Similarly, there is a virtual source located at I . Sound will continue to reflect
R
back and forth between the parallel walls; for example, sound will strike the left wall, right wall, and
left wall again, sounding as if there is a virtual source at location I LRL (a third-order image). We
observe that in this example, the walls are 15 distance units apart; thus the first-order images are 30

units apart, the second-order images are 60 units apart, the third-order images are 90 units apart, and
so on. Using this modeling technique, we can ignore the walls themselves, and consider sound as
coming from many virtual sources spaced away from the actual source, arriving at time delays based
on their distance from the source.

FIGURE 6-3 When sound strikes more than one surface, multiple reflections will be created. These
can be viewed as virtual sound sources. Parallel walls can pose acoustical problems such as flutter
echoes, reflections that are highly audible because of the regularity of the echo.