I can answer the first part of your question,
what is sound? When you speak or a speaker
vibrates or a loud motorcycle drives by, you
perceive a sound. What happens is the air near
the source of the sound gets compressed and then
expands again only to compress the air next to it.
This compression wave travels away from the
source in a sphere like after a rock in thrown in
a pond. After a short time this compression
wave hits your ear drum and little bones, and
nerves in your inner ear change this into a signal
that your brain learns to interpret as sound.
Sound can travel through other materials as well,
not just air. You can hear noises under water, or
by tapping on a metal pipe or through a string if
you have ever attached two paper cups to a tight
As for the other part of your question you
would have to ask a psychologist or evolutionary
biologist. I would say loud noises are disturbing
because they may be unexpected. If your blood
pressure increases, it is likely to make more
blood flow to your brain making you more alert so
you can react to any danger.
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