The absorbers and diffusers discussed in previous chapters have difficulty altering low-frequency sound. Low frequencies have long wavelengths, which means treatments have to be large to perturb or absorb the wavefronts. Active control technologies offer the possibility of bass absorption or diffuse reflections from relatively shallow surfaces, as well as a capability for variable acoustics. An example application for active absorption is the control of modes in small rooms. The cost and difficulties of implementation are considerable, however, and this is one reason why this technology has not been more widely applied.