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P1 Topic 4: Waves and the Earth
Sound can be measured in Hertz, which is the number of complete waves that pass a point in one second. The
frequency of a sound wave determines the pitch of the sound. All sound waves are longitudinal and
therefore need to pass through a medium to be heard. Light waves are transverse and therefore do not
need a medium.
The range of frequency humans can hear varies with age; younger people are more able to hear higher
sounds than older people. Generally we can hear from 20Hz to 20,000Hz. Sound with frequencies of less
than 20 Hertz are known as infrasound and we cannot hear them. However, microphones are used to detect
Animals can use infrasound for communication. Elephants, Giraffes and Whales can all use infrasound. This
passes over many kilometres as infrasound waves travel further in the air before they become too faint to
detect than higher frequencies of sound waves.
Infrasound is useful for studying animals. For example, biologists use infrasound to detect how and where
animals move throughout the year as it is difficult to track them in certain places like forests. Elephants and
giraffes move many kilometres per day so it can be difficult to track them. Microphones are used to detect
the infrasound made by the animals.
Infrasound is useful for detecting volcanic eruptions and meteors. Eruptions create infrasound waves which
can be heard from hundreds of kilometres away. Seismic waves can also be detected from far distances and
they travel through the ground.
This is useful as many volcanoes are in remote places so it is difficult to study them without infrasound.
Sensors many kilometres away can monitor the eruptions of remote volcanoes with the infrasound produced.
Infrasound can also be used to detect the passage of meteors through the atmosphere and also detect any
that explode. This helps scientists to work out how many meteors enter the Earth's atmosphere and what the
risks of meteor impacts may be.
Sounds made by waves with higher frequencies than 20,000 Hz are called ultrasound. Animals such as
dolphins and bats use ultrasound to sense their way around their environment.
Many animals e.g. frogs, insects, whales and rats use ultrasound to communicate with each other.
When a wave passed from one medium into another, some of the wave is reflected off the boundary
between the two media, and some is transmitted (and refracted). This is called partial reflection.
What this means is that you can point a pulse of ultrasound at an object and wherever there are boundaries
between one substance and another some of the ultrasound gets reflected back.
The time it takes for the reflections to reach a detector can be used to measure how far away the boundary
Bats can use ultrasound to detect obstacles and other objects around them. The ultrasound waves made by
the animals are reflected by these things and the animals then listen for the echoes.
Humans use a similar method on ships and submarines called sonar. It is used to find the depth of the sea or to
This works by this method: A loudspeaker on the ship emits a pulse of ultrasound which spreads out through
the water. Some of it is reflected by the sea bed, which is then detected by a special microphone on the ship.
The sonar equipment then measures the time between the sound being sent out and the returning echo.
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The distance travelled by the sound wave can be found by this equation (answer must be divided by two):
distance (metre, m) = speed (metre/second, m/s) x time (second, s)
Ultrasound can be used to make images of things in the body. One common use is for making detailed images
of unborn babies so that the doctors can monitor how well the foetus is developing.
A gel is put on the mother's stomach to prevent ultrasound reflecting from the skin.…read more
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L- wave Transverse N/A N/A (goes the long way
around the crust)
Investigating the Earth
Scientists investigate the layers of rock near the surface of the Earth by setting off small explosions or
dropping large masses from trucks. These occurrences produce seismic waves which are detected by
seismometers located at different distances from the source. The seismic waves are reflected and refracted
when they pass into different rocks. The travel times of these waves gives information about the rock.…read more
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The distance from three or more seismometers will all cross at one place which is the epicentre of the
earthquake. This method is called triangulation.
Predicting Earthquakes and Tsunamis
Scientists can use the plate boundaries to predict where earthquakes are likely to happen. However, it is not
currently possible to measure the forces trying to move the plates or the friction between them. As a result,
it makes it difficult to predict when a sudden movement will happen.…read more