he diaphragm absorber, a wall surface is made to vibrate like a drumhead at the desired frequency. Soundwaves at this frequency tend to pass right through the wall with little resistance. Once the sound energy is outside the wall, it is kept out by absorbing it with massive layers of sound-absorbing material. Helmholtz absorbers trap soundwaves in tuned cavities. These can be recognized by their perforated or slotted faces.
Mid- to high-frequency soundwaves in the "diffusion region" can be dispersed with the use of diffusers. The most common diffusers are shaped like pyramids and attach to walls and ceilings. These diffuse soundwaves by scattering them in four different directions. Other diffusers look like flat sound panels, but are really an array of reflectors and absorbers, arranged in a mathematical pattern, which scatters soundwaves in many directions.
High-frequency soundwaves in the "specular reflection region" can be handled with sound absorbing panels and sound reflectors. Since, in small rooms, soundwaves only last a short time, it is critically important to place diffusers, absorbers and reflectors only where they are sure to intercept the soundwave that you want to control.
Classroom acoustics in the United States are so poor that students are not able to hear 70 percent of what their teachers say. One third of office workers say they are very disturbed by noise with a resulting loss in productivity.
It's interesting to read the complaints of Vitruvius, an architect from the first century B.C., that "improvements in architecture and the arts are always held back by baseless approval of inferior methods under the influence of social connections."