I am reading Joshua Reiss's Audio effects: theory, implementation and application. He states
"Reverberation is also more than a simple delay device with feedback. With reverb the rate at which the reflections arrive will change over time, as opposed to just simulating reflections that have a fixed time interval between them. In reverberation, there is a set of reflections that occur shortly after the direct sound. These early reflections are related to the position of the source and listener in the room, as well as the room’s shape, size, and material composition. The later reflections arrive much more frequently, appear more randomly, usually decay exponentially, and are difficult to directly relate to the physical characteristics of the room. These late reflections give rise to diffuse reverberation. An example impulse response for a room is depicted in Figure 11.2. Each vertical line marks when the original sound is repeated, and the height of each of these lines is the amplitude of the sound at that time.
He shows the following plot:
However, I'm confused why the density of times "when the original sound is repeated" increases. Intuitively, it looks like the amplitudes decrease over time. I imagine this is because as the sounds reflect off the walls, they lose energy and so their amplitudes decrease. But you can see from Reiss's plot that it looks like the number of times when the original sound is repeated increases in density.
Why does the number of times when the original sound is repeated go up?