Infrasound is sound waves with
frequencies less than 20 Hz and they are inaudible to the human ear.
We can't hear these infrasound
frequencies of such long wavelength.
However,
some animals can hear infrasonic
sound e.g.
like whales, elephants, rhinos, hippos, giraffes, alligators,
squid/cuttlefish/octopi, moles and even pigeons.
See "A
tale of the mole and its adaptations!"
What sort of
wavelengths are we talking about?
From the wave equation:
λ = v ÷ f
(i) Using the speed of sound in air
as 340 m/s, and a frequency of 20 Hz
wavelength = speed / frequency =
340 / 20 =
17 m (our little eardrums can't cope with this
!!!)
(ii) The speed of sound in seawater
is ~1520 m/s.
Whales can transmit and receive
infrasonic sounds down to 10 Hz.
λ = v ÷ f
= 1520 / 10 = 152 m
This is a very long wavelength
and low frequency sounds can travel up to 10,000 miles!
(iii) Blue whales can communicate
with each other over distances up to ~500 miles (~750 km).
How long goes it take for the
'message' to travel 750 km?
s = d / t, rearranging t =
d / s = (750 x 1000) / 1520 =
493 s (3 sf, 8.2 minutes)
Many animals can detect infrasound
In
some case the infrasounds are used by animals to communicate with each
other.
It
is possible for scientists to use infrasound to track some animals to help
in conservation projects.
There are several natural events that produce sound waves that travel
through the Earth.
Volcanic
eruptions release huge amounts of energy, some of it as infrasound waves
travelling through the Earth's crust - some infrasound waves are
detected prior to eruption and can be used to help in the prediction of
eruptions
The fall of
large quantities of snow in avalanches sets of infrasound waves.
The pounding of
large waterfalls, meteor strikes and the breaking up of large icebergs
all cause the emission of infrasound waves.
The biggest
source of infrasound waves is earthquakes. seismic waves are transmitted
through all the sections of the Earth - crust, mantle and core. These
are discussed in detail in the next section.
When an earthquake happens in the Earth's
crust it results in the spreading out of seismic waves ('shock waves').
Seismic waves result from the
huge amounts of potential energy stored in the stressed layers of rock being
released by tectonic plate movement.
These earthquake waves can be detected all around
the world using an instrument called a seismometer.
Some earthquake waves are infrasonic,
meaning their frequencies are less than 20 Hz.
The frequencies can be as low as 0.1
Hz and the speed of earthquake waves varies from 1800 to 7200 m/s (1.8 -
7.2 km/s) depending on the physical state and chemical composition of
the medium e.g. type of rock.
The speed of earthquake/seismic waves depends on the
material they are travelling through, in particular the density of the rock
layers.
When the waves meet a boundary they may be
partly/completely reflected or partly/completely absorbed,
they may continue in a direct line with a different speed
or the waves might change direction and speed (and wavelength) - refraction -
so things get
pretty complicated.
Because the density of the rock changes
gradually in a particular layer, so does the speed of the wave. If refracted,
the waves follow curved paths (see the diagram below).
However, at a boundary,
the speed may change more abruptly giving a bigger change in direction (just as
you see with light ray experiments with prisms.
Scientists (seismologists) study the
properties and pathways of seismic waves to deduce the internal structure of the
Earth.
From scientific studies of where
the different types of waves are detected, or not detected, due to
absorption, reflection or refraction you can work out the structure of
the layers of the Earth the seismic waves pass through.
These seismologists calculate the
time it takes for these shockwaves to reach every seismometer around the
world and, important to work out a specific earthquake below the Earth's
surface.
They are also called primary
pressure or compression
waves and are identical to sound waves but with much longer wavelengths
- see calculation below. The above diagram was used to illustrate a
sound wave!
The animation of a P-wave simulation
is shown on the right (from
https://en.wikipedia.org/wiki/P-wave ).
P-waves from an earthquake travel at ~5 to 8 km/s in the
crust, mantle or core and ~1.5 km/s (the same as 'sound') in water!
Weak to moderate earthquake waves
have a frequency of 0.1 to 2 Hz on the surface.
If a seismic wave has a speed of 5
km/s (5000 m/s) and a frequency of 0.5 Hz
the wavelength =
λ = v ÷ f = 5000 / 0.5 =
10 000 m (10 km),
~1000 x more than our audible sounds
S-waves are transverse waves and can only
travel through solids, so they cannot travel through the core.
S-waves cause an 'up and down'
shearing movement of the rock layers at 90o to the direction
of the wave. Secondary shear waves.
On the right is an animation of the
movement of an S-wave of an earthquake.
(from
https://en.wikipedia.org/wiki/S-wave )
L-waves are transverse move along the
surface of the Earth moving the ground up and down.
W =
atmosphere : From
seismic studies we can say ...
X = the relatively thin
Earth's crust - mainly consisting of hard rock 92/3rds overlaid
with water).
Y = about half the radius of
the Earth's is the mantle, consisting of rock that is almost
solid, it can flow very slowly under stress or in a convection current -
great plumes of rock can rise producing bands of volcanoes like the
'Ring of Fire' in the Pacific Ocean - lots of earthquakes too!
Z = the inner solid metal
core and an outer liquid metal core of the Earth, mostly iron
and some nickel and smaller amounts of other metals - the iron is the
source of the Earth's magnetic field.
For more details see
earthquake (seismic) wave analysis
notes in Earth Science section
Earthquake waves spread out in all
directions from the epicentre of an earthquake in the Earth's crust.
These vibrations caused by these
waves are detected by instruments called seismometers, which are
positioned in many locations on the Earth's surface.
P-waves (primary waves) and S-waves take
curved paths because of the ever changing density of the Earth's layers
producing a gradual refraction effect.
The longitudinal P-waves can pass
right through the centre of the Earth but due to refraction give two
small shadow zones (marked black on the diagram). They travel faster than
S-waves.
The transverse S-waves are absorbed by
the liquid outer core and give one much larger shadow zone (marked blue + black
on the diagram). They travel slower than P-waves.
All the wave paths are curved
because the density is changing continuously with depth (increase in
pressure), so they are continuously being refracted (change in
direction).
BUT, you get much greater refraction effects at
the boundaries between the crust/mantle, mantle/outer core and outer core/inner
core.
Boundaries are created by the
different physical states and densities of the rock and metal layers.
Seismometers pick up the vibrations of
earthquake waves from many seismographic stations around the world (over 2000
locations).
Analysis of the paths of waves in terms of
velocity and direction data has enabled geologists to work out the basic layered
structure of the Earth.
From the speed, absorption and refraction
of seismic waves scientists have worked out the number and depth of the four
layers of the internal structure of the Earth.
e.g. from the shadow zones you can work
out the depth of the mantle and the inner and outer layers of the core.
