A single impedance mismatch in a chain splits energy into reflected and transmitted parts.

Evidence

If you line up 5 or 6 pennies perfectly on a slick surface and hit the end of the row head-on with another penny, you will notice the row stays intact, with the projectile penny adding to the row and the last penny popping off at the opposite end. The impedance matching works all the way down the penny chain, each penny for an instant carrying the momentum, giving as good as it got on its left, and then almost instantly giving and getting forces on its right that stop it cold and give the momentum to the next penny. Put a nickel in the chain of pennies and the first penny will rebound from the row; the last will still pop off the end but with less energy than before. All the energy of the first coin is not transferred down the chain; rather, part of the energy has been reflected and part transmitted, because of the impedance mismatch, which can be blamed on the interloping nickel. — Eric J. Heller, Why You Hear What You Hear- An…, 61

Categorization

• XREF: Direct mechanical analog for why sound reflects at boundaries where density changes — a core mechanism for the reader’s central question.