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School-college Physics Notes: SOUND 8. INFRASOUND and uses

SOUND  8. INFRASOUND - very low frequency sound waves - definition, examples described, uses, infrasounds and animals, S and P earthquake waves

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8. INFRASOUND - definition, examples described, uses, infrasounds and animals, earthquake waves

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.

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.

Earthquake waves - seismic waves passing through the concentric layers of the Earth

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.

There are three types of earthquake waves (seismic waves)

P-waves are longitudinal waves and so can travel right through the Earth to the other side of the world.

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 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.

Keywords, phrases and learning objectives for sound waves

Know that infrasound has very low frequency sound waves which humans cannot hear.

Be able to describe and explain examples described on how infrasounds are used.

Know that some animals can hear heard by animals and that seismic detectors can monitor P earthquake waves through the Earth's layers.

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