signal interference

two coherent waves of equal frequency add point-by-point. when phase aligns the sum doubles (constructive). when phase opposes the sum cancels (destructive).

• interference — every audible space is the sum of millions of reflected waves. two near-identical signals can null each other into silence (acoustic dead zones, noise-canceling headphones).
• beating — piano tuners listen for beat frequencies under 1 Hz to align two strings. the same effect makes ultrasonic chirps audibly modulated to bats.
• aliasing — a wagon-wheel turning forward in a movie appears to roll backward. digital cameras, audio, radar all rely on this trap and the filters that prevent it.
• modulation — every am/fm radio station, every screen pwm flicker, every fiber-optic blink is a fast carrier inheriting a slow envelope — the building block of every signal in this category.