By exploiting resonance it is possible to get absorption at low to mid-frequencies. It is difficult to achieve this with porous absorbers, because of the required thickness of the material. Furthermore, treatments are often placed at room boundaries where porous absorbers are inefficient as the particle velocity is low. For many resonant absorbers, placing the device at the boundaries will improve their effectiveness. The absorption characteristics of these resonant devices are a peak of absorption, as shown in the thick line in Figure 6.1. Unlike porous materials, wide band absorption is difficult to achieve in one device, and so one of the frequent challenges in the design of resonant structures is to extend the bandwidth.