Why does the thunder from close lightning produce a BOOM and then a slow fade off? Why does it fade off instead of just stop? Conversely, why is thunder from far away a slow fade-on instead of an instant but quieter boom?
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The sound waves bounce off of objects and as they get further away, the amplitude of those sound waves bouncing back towards you drops off as they dissipate through resistance.
Similar to how someone’s voice gets lower and lower the further they get from you, the sound waves from the thunder bouncing off of the ground and objects getting further and further away from you drops off too.
Often referred to as reflections. For the sound to just stop would require absolutely no reflective surfaces which is something that does not occur in nature. Outside of nature we have things like anechoic chambers which absorb almost all sound reflections. So you can’t hear someone unless their mouth is facing you (for the most part). If you were to recreate a small thunder strike sound in there, chances are it would be extremely brief. And it would likely sound nothing like thunder since most of the sound of thunder is the sound reflecting off of the ground and objects. The original sound I would venture to guess would only be milliseconds in length.
The sound you hear is a combination of the sound following different paths to your ears.
Imagine if the entire world was made of mirror. You’d see thousands of reflections of the same object, coming from all directions and distances. Every blade of grass, another tiny reflection of the world.
That’s the world for sound. Every blade of grass, leaf, tree, building, and rock reflects a bit of sound. Normally you can’t hear that reflection, but with a loud BANG you hear all of those little reflections adding together. The more distant reflections are weaker and reach you later, so you hear the initial noise and then a fading stretched-out echo of it.
The loudness of something is the pressure difference in compression/ratification of molecules in air, and as that pressure moves out from the source, its energy spreads out. The sound right next to a lighting strike is very high because most of the energy is not all that spread out yet, but at a great distance, a small fraction of the energy reaches your ears.
The “fade” is *reflection,* or reverberation, off of surfaces. Sound waves that moving away from you relative to the lighting strike take time to hit something hard, bounce, and come back toward you, so they come later, and the spreading out of the energy makes them quieter.
If it was just one thing it bounced off of, it would sound like a repeat, or echo, but because the sound is bouncing off of *everything,* it is more of a “smeared” sound, a rumble rather than a crack. Think of it as thousands of small, quiet echos at random and mixing with each other.
That’s also why far-off-thunder is less distinct, because lots of different echos mess with each other by the time any of the sound reaches you. Technically, if there’s a direct line from you and the lighting with nothing in between, it won’t “fade in” and still start with a boom, but most far-off lighting strikes have so many things in the way you actually only hear different echoes that are so mashed together it seems like it’s fading in first.
The Boom is what you are hearing. The slow fade off, is the echo of the boom off of everything around you. The sound you hear immediately after the boom, is the sound bouncing off houses around you, then after that it is the sound from buildings down the street, and it keeps going further until the distance gets too far to hear.
Sound travels like a wave. Think ripple in water. The further the wave moves away from the source, quieter it gets.
The shock-wave (thunder) forms along the entire lightning bolt, which can be several miles long. The initial crack comes from the closest part of the bolt. The rollng continues as the rest of the shockwave reaches you.
A shockwave is literally a wave of compressed air that travels at sonic speed. As the shockwave proceeds, it weakens and spreads out, lengthening the loud crack into a sinister roll.
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