Sound travels through the Eiffel Tower's iron structure 15 times faster than it does through the air.

The speed of sound is determined by the density and elastic properties of the medium through which it travels. In air at 20 degrees Celsius, sound travels at approximately 343 meters per second. However, in solids like the puddled iron used to construct the Eiffel Tower, the speed increases dramatically to about 5,120 meters per second. This significant difference occurs because solids have a much higher elastic modulus than gases.The Eiffel Tower was completed in 1889 for the Exposition Universelle and consists of 7,300 tons of iron. In this metallic lattice, the atoms are arranged in a rigid structure that allows mechanical vibrations to transfer with minimal energy loss. When a sound wave enters the iron, the closely packed atoms collide and pass the energy forward much more efficiently than the widely spaced molecules in the atmosphere.This physical principle is a cornerstone of non-destructive testing used by structural engineers today. By using ultrasonic sensors, maintenance crews can measure the time it takes for a sound pulse to travel through the iron beams. If the sound slows down or reflects unexpectedly, it indicates a hidden crack or corrosion within the 18,038 individual metallic parts. This ensures the 330-meter-tall landmark remains safe for millions of annual visitors.