Toss a pebble in a calm pond and watch what happens to the ripples over time. Sound’s a lot like that.
First you hear it as the ripples pass you. Then, if you’re close enough to a fairly straight shore, you might hear it again as the ripples bounce back and pass under you a second time – this is an echo.
Eventually the ripples spread out from each other so far that you can’t see them (or in the case of sound, hear them). This can happen a lot faster if there’s a slight breeze that’s riffling the water, which is why background noise like the leaves on trees blowing in the wind makes it harder to hear distant sounds.
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