Stair reflections diffract off stair edges, producing a descending chirped echo

Evidence

The pulses returning to the source at the bottom of the stairs have diffracted off the top edge of each stair. The sound pulses arrive after a round-trip from source to edge and back. Therefore, the time delay between a given returning pulse and the next one from the stair above it is twice the difference in distance divided by the speed of sound. It is seen from the inset in figure 23.26 that the difference goes from being close to a, the step width, for sound coming from near the bottom of the stairs, to close to (a2 + h2)1/2, where h is the rise of each stair, near the top of the stairs. This increased time delay for pulses arriving later accounts for the lower pitch. — Eric J. Heller, Why You Hear What You Hear- An…, 2143

Categorization

• XREF: Connects to the reader’s interest in sonic architecture and how built structures impose perception; parallels earlier discussion of refraction and wave propagation.