The Hague, 2 January 2025. During your New Year’s celebration, did you notice how the sound of a firework changes as it moves through the sky? When a firework is launched and explodes, there’s a distinct rise in the pitch of the sound as it moves upwards. This fascinating auditory phenomenon is known as the Doppler Effect. In this article, we’ll dive into why the pitch rises as the firework goes off, and how the Doppler Effect changes the way we experience sound in motion.
What Is the Doppler Effect?
The Doppler Effect occurs when a sound source is moving relative to an observer. As the sound source approaches, the sound waves in front of it are compressed, making the pitch higher. As the source moves away, the waves are stretched, lowering the pitch. This shift in frequency happens with all kinds of waves, including sound, light, and even electromagnetic waves.
Named after physicist Christian Doppler, this effect is what makes the siren of an ambulance or police car rise in pitch as it approaches you and then drop as it passes and moves away.
How the Doppler Effect Works with Fireworks
During a fireworks display, the explosion sends sound waves in all directions. But if the firework is moving at the moment of the explosion, the sound waves are compressed as they travel. This compression shortens the wavelength of the sound and increases its frequency, which is what causes the pitch to rise.
Here’s a breakdown of what’s happening:
Compressed Sound Waves: As the firework moves toward you, the air molecules in front of it are pushed together. The result is that the sound waves reach your ears more frequently than if the firework were stationary. This is perceived as a higher pitch.
Rapid Approach, Noticeable Shift: The faster the firework moves toward you, the more noticeable this pitch shift becomes. While the effect can be subtle depending on how quickly the firework is moving, you may still hear a distinct rise in pitch just before the boom reaches its peak.
This rise in pitch is most pronounced when the firework is launched at a steep angle or directly toward the observer. Once the firework reaches its peak or begins to move away, the pitch will drop, but the initial approach is where the effect becomes most noticeable.
At RESONIKS, we take inspiration from wave mechanics, like the Doppler Effect, in our Acoustic Resonance Testing (ART) systems. While the Doppler Effect focuses on the shift in sound caused by motion, our ART systems use controlled sound waves to detect imperfections in materials. By studying how sound interacts with objects, we can uncover hidden flaws and ensure quality in manufacturing processes. The next time you watch a fireworks display, listen closely to the sound as the firework approaches its peak.