Stand in front of a picket fence, clap your hands, and listen to the musical quality of the reverberation.
The reverberation from a picket fence is made up of individual echoes of your hand clap from each of the slats in the fence. The echo from each slat reaches your ears a tiny fraction of a second later than the echo from a neighboring slat a few inches closer to you. That's because the sound's round- trip travel timefrom your hands, to a slat in the fence, and back to your earsis shorter for slats nearer to you than for slats farther away.
If the slats are evenly spaced, as they usually are in picket fences, then the echoes reach your ears at regular intervals and you perceive the train of echoes as a musical tone. Notice that the pitch of this tone depends on the spacing between slats in the fence, not on the quality of your hand clapping.
No Constant Pitch
But we need to refine this story by pointing out that the musical tone you hear will not have a constant pitch. The tone will start out high in pitch and quickly sweep down to a low pitch.
The high-pitched sounds come from the slats nearest you, which are all at just about the same distance from you. So echoes from those slats reach your ears very close together, giving you the impression of a high musical pitch.
The low-pitched part of the reverberation comes from the slats at the far ends of the fence. Each of those far-away slats is at a quite different distance from you than its neighbors. So echoes from those slats are separated by bigger time intervals, and you hear a lower pitch.