Physics
Diffraction of Sound Waves
Quick fact
Sound waves can bend around a person's head, which is why we can still hear someone even when they are behind us—a form of diffraction around a comparatively small obstacle.
Why this is interesting
Have you ever heard someone talking from around a corner before seeing them? How does sound sneak around obstacles when light cannot?
Read the full explanation
Understanding Diffraction of Sound Waves
Imagine dropping a pebble into a pond—ripples spread outward and when they encounter a small rock, they curve around it. Sound waves behave similarly. When a sound wave meets an obstacle like a wall or a door, it doesn't just stop; it bends around the edges. This bending is called diffraction. The key factor is the size of the obstacle relative to the sound's wavelength. Low-frequency sounds (like bass) have long wavelengths—several feet—so they easily diffract around large objects. High-frequency sounds have short wavelengths (a few inches) and diffract less, casting sharper 'sound shadows'. This is why you hear the low thump of music from a distant room but not the high notes clearly.
A deeper explanation
At the heart of diffraction is Huygens' principle: every point on a wavefront acts as a source of tiny secondary wavelets. When a wavefront encounters an edge, those secondary wavelets spread into the 'shadow' region, interfering constructively to recreate the wave. The amount of bending depends on the ratio of wavelength to obstacle size. If the obstacle is much larger than the wavelength, the wave is mostly blocked (a sound shadow). If the obstacle is comparable or smaller, diffraction is strong. This principle explains why sound can travel around corners, why concert halls are designed to avoid unwanted diffraction, and why animals' ear shapes exploit diffraction to locate sounds. Understanding diffraction is essential for designing speakers, microphones, and noise barriers.