Because of the Pyramid's high staircase, the echo exhibits an additional auditory feature. I do not remember having heard such a long chirped echo elsewhere. An explanation for the downward chirp is easily obtained by a simple extension of the picket fence effect. This essentially amounts to a diffraction grating, familiar to physicists. Echoes arrive first from the lower stepfaces near the observer's head level.
Later echoes arrive from the upper steps.
The time between successive echoes gradually lengthens for the upper steps because the ray path becomes nearly parallel to the angle of staircase's rise. In the limit the time between successive echoes is proportional to the hypotenuse of the triangle consisting of the tread length and rise height. Because the hypotenuse is longer than either side of a triangle, the period of the tone is greater and the frequency is lower. The average riser height was measured at 26.4 cm. The actual frequency range also depends on one's distance from the pyramid.