Question

The following passage is adapted from an article on early twentieth-century acoustic technology.

During the autumn of 1915, British artillery units on the Western Front faced a critical tactical disadvantage: German gun positions were thoroughly camouflaged and invisible to aerial reconnaissance. To locate enemy batteries, physicists were recruited to pioneer "sound-ranging"—a technique designed to determine an artillery piece's coordinates by recording the microsecond differences in arrival times of its muzzle blast across an array of calibrated microphones. The initial equipment, designed by French scientists, relied on standard carbon-button microphones. However, these devices proved inadequate in combat because they reacted violently to high-frequency rifle fire while remaining largely insensitive to the low-frequency acoustic waves generated by heavy artillery.

In 1916, Lieutenant William Lawrence Bragg was tasked with resolving this technical flaw. Bragg recognized that heavy artillery produced an infrasonic sound wave—a low-frequency air displacement below 20 Hertz that human ears experienced as a sudden pressure pulse rather than an audible sound. The breakthrough came when Corporal William Tucker invented the hot-wire microphone. Tucker replaced the rigid carbon membrane with a microscopic platinum wire, just 0.0006 centimeters in diameter, suspended across the neck of a small container. The platinum wire was continuously heated by a mild electrical current. When the low-frequency pressure wave from an enemy gun blast swept across the container, it displaced the air inside, causing a momentary cooling of the wire. This sudden drop in temperature altered the wire's electrical resistance, producing a precise electrical impulse recorded on a galvanometer film strip.

To prevent ambient environmental noises, such as wind gusts or infantry gunfire, from falsely triggering the apparatus, Bragg’s team modified the resonant chamber surrounding the wire. They discovered that an ordinary wooden ammunition box, lined with thick felt and punctured by a single narrow aperture, served as an effective low-pass acoustic filter. By tuning the internal volume of this container, only the low-frequency pulses of heavy artillery were permitted to cool the wire. By 1917, sound-ranging sections could pinpoint enemy guns within twenty-five yards in under three minutes, fundamentally transforming counter-battery warfare.

According to the passage, Corporal William Tucker's hot-wire microphone detected low-frequency artillery blasts through which specific physical phenomenon?