Many enclosures require some form of ventilation as illustrated in Figure 10.21(a) and (b).
They may also require access for passing materials in and out. Such necessary permanent
openings must be treated with some form of silencing to avoid compromising the effectiveness
of the enclosure. In a good design, the acoustic performance of access silencing will match the
performance of the walls of the enclosure. Techniques developed for the control of sound
propagation in ducts may be employed for the design of silencers (see later section on mufflers).