Reverberation evenness across frequencies is controlled by adding or removing absorbing materials

Evidence

The results of Example 6.8 beg the question: “How can we improve the evenness of the reverberation time?” The answer is to either add, or remove, additional absorbing materials into or from the room in order to achieve the desired reverberation characteristic. Here the concept of an Figure 6. — David Howard, Jamie Angus, coustics and Psychoacoustics, 1939