The loudest known sound, recorded in modern times, was the
eruption in 1883 of the volcanic island of Krakatoa, located in
the Dutch East Indies. The volcano erupted with such force that
it could be heard in Australia, two thousand miles away. The explosions
lasted for 36 hours, and blew off half the island. The final outburst
had enough sound energy to circumnavigate the Earth 7 times before
it eventually dissipated.
The softest sound which can be detected by the human ear
occurs in the range of about 0 to 5 decibels, which is equivalent
to the sound of air molecules moving inside the mouth of a drinking
glass. By definition, the softest sound that can be detected by
an acoustic sensor requires the minimum amount of energy
which is necessary to produce a vibrating wave motion in a medium.
The highest sounds, known as hypersounds, are generated
at frequencies which vibrate at billions of times per second. These
microcycles include the oscillation of the tiniest known particles
of matter.
The lowest sounds are produced by large objects, and are
known as infrasounds. Infrasounds have very long wavelengths
with a single vibration occurring within a period of millions of
years. These include planetary, interplanetary, stellar, galactic,
and intergalactic cycles. The very lowest sounds are generated
in large intergalactic gas clouds caused by the galactic wind and
stellar explosions.