Why does thunder rumble instead of making one sharp crack?
Why does distant thunder sound like a long rolling rumble rather than a single sharp bang, even though the lightning stroke that causes it happens almost instantaneously?
History and references
Settled in 1975 by Established acoustics of tortuous/branching lightning channels producing many spatially distributed shock sources (reviewed e.g. by A. A. Few, 1975).
Each kinked, branching segment of a lightning channel generates its own cylindrical shock wave, and because the channel is not straight, these segments lie at different distances from a listener; sound from the near, high, or directly overhead segments arrives sooner than sound from far or obscured segments, and each segment's shock is also stretched by nonlinear propagation and by refraction/reflection off terrain, buildings, and temperature layers. The listener's ear sums these many overlapping arrivals into a continuous rumble rather than discrete claps. This 'tortuous path' explanation was established well before Walker's book and elaborated with modern acoustic modeling in subsequent decades (e.g. Applied Acoustics, 2018).
The single most common natural loud sound turns out to encode the entire crooked geometry of a lightning bolt in its rhythm.
Explanations that circulated
- Rumbling is purely an echo off clouds
- Thunder rumbles because sound itself slows down and speeds up in air
References
- Thunder signatures, A. A. Few, Eos, Transactions American Geophysical Union, 1974
- A brief study on thunder claps, reviewed in, Applied Acoustics, 2018
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