4.1 Recall that sound with frequencies greater than 20 000 hertz, Hz, is known as ultrasound

Humans can hear only frequencies from 20 Hz to 20 000 Hz called audible range. But sound waves have frequencies much lower and higher than this range.

Sound frequencies below 20 Hz is called infra-sound and Sound waves frequencies above 20000 Hz is called Ultra-sound. Different frequencies have different uses

481px-Ultrasound range diagram.svg

(Image stolen from http://en.wikipedia.org/wiki/Ultrasound)

 

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4.2 Describe uses of ultrasound, including:
      (a) sonar
      (b) communication between animals
      (c) foetal scanning

 Why Ultra-Sound is useful?

Because of high frequency it can penetration deep inside the material without much bending (refraction and diffraction). So ultra-sound can travel as a beam of sound like beam of light. Due to this property we can use it to target pin pointedly on to the objects.

Applications of Ultra-Sound:

1- SONAR:

The word SONAR is an acronym which stands for SOund Navigation And Ranging ie it is an instrument used for Navigation purpose or to detect something.

Sound navigation and ranging, a technique used at sea for detecting and determining the position of underwater objects (e.g., submarines, shoals of fish) and for finding the depth of water under s ship's keel (see echo sounder. Sonar works on the principle of echo location: high-frequency sound pulses are beamed from the ship and the direction of and time taken for any returning echoes are measured to give the direction and range of the reflecting objects.

SONAR

(Image is stolen from:The American Heritage® Science Dictionary Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company.)

2- Communication Between Animals:

Baby rats call to their mothers with high pitched squeaks inaudible to humans. Dolphins use ultrasound to echolocate and find their way around in murky or dark water.

Dog's whistle is another example of animal communcation by ultra-sound.

 

3- Foetal Scanning:

Ultra-sound has been used to visualize muscles, tendons, and many internal organs, to capture size and structure of human and animal body for the past 50 yrs. It is frequently being used for foetal scanning.

foetal scanning

The picture above shows a pregnant mother is being scanned by ultra-sound. In foetal scanning echo-location technique is being used.

How does it works?

The ultrasound transducer (probe), the one which is held by the doctor in the picture above, functions as both a loudspeaker to create the sounds and a microphone to record them. When the transducer is pressed against the skin, it directs a stream of high frequency sound waves into the body. As the sound waves reflect from the body’s organs, the transducer records tiny changes in the incoming waves. These changes are measured and interpreted by the computer, which then creates a picture on the monitor. Materials (resembling a rubbery coating) on the face of the transducer enable the sound to be transmitted efficiently into the body. A water-based gel is placed between the patient's skin and the probe to further assist the transmission of the sound waves otherwise some ultrasound would be reflected back from the skin and would not transmit through.

Extra-Help pre-natal scanning!

Follow the link http://www.iop.org/education/teacher/resources/teaching-medical-physics/ultrasound/file_56277.pdf

 

Ultrasound sources may be used to generate local heating in biological tissue, with applications in physical therapy and cancer treatment. Focussed ultrasound sources may be used to break up kidney stones.

Ultrasound is a safe, non-invasive method of looking inside the human body. Ultrasounds does not emit hazardous radiations therefore much better to use than X-rays. Further, they can be reflected from muscles which X-rays can't do, so they can image muscles while X-rays can only image bones.

4- Metal Cleaning:

Ultrasonic cleaners, sometimes called supersonic cleaners, are used at frequencies from 20-40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments and industrial parts. These cleaners consist of containers with a fluid in which the object to be cleaned is placed. Ultrasonic waves are then sent into the fluid. The main mechanism for cleaning action in an ultrasonic cleaner is actually the energy released from the collapse of millions of microscopic bubbles occurring in the liquid of the cleaner.

 

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4.3 Calculate depth or distance from time and velocity of ultrasound

 The technique is called echo-location

sonarII

Image Stolen from: oscilatii2011.wikispaces.com

Question: A ship sends a signal to the bottom of the ocean to determine the
depth of the ocean. The speed of sound in sea water is 1450 m.s−1 If the signal is
received 1,5 seconds later, how deep is the ocean at that point?

Answer

Step 1 : Identify what is given and what is being asked:

s = 1450 m.s−1
t  = 1.5 s there and back
∴ t = 0.75 s one way
d = ?

Step 2 : Calculate the distance:

Distance = speed × time
d = s × t
d = 1450 × 0.75
d = 1087.5 m

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4.4 Recall that sound with frequencies less than 20 hertz, Hz, is known as infrasound

 see section 4.1

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4.5 Describe uses of infrasound, including:
      (a) communication between animals
      (b) detection of animal movement in remote locations
      (c) detection of volcanic eruptions and meteors

 

Animals have been known to perceive the infrasonic waves going through the earth by natural disasters and can use these as an early warning.
Infrasound may also be used for long-distance communication in African elephants.These calls range from 15–35 Hz and can be as loud as 117 dB, allowing communication for many kilometres, with a possible maximum range of around 10 km (6 mi). These calls may be used to coordinate the movement of herds and allow male elephants to find mates.

Whales, elephants,hippopotamuses, rhinoceros,giraffes, okapi,and alligators are known to use infrasound to communicate over distances—up to hundreds of miles in the case of whales. In particular, the Sumatran Rhinoceros has been shown to produce sounds with frequencies as low as 3 Hz which have similarities with the song of the humpback whale. The roar of the tiger contains infrasound of 18 Hz and lower, and the purr of felines is reported to cover a range of 20 to 50 Hz.It has also been suggested that migrating birds use naturally generated infrasound, from sources such as turbulent airflow over mountain ranges, as a navigational aid. Elephants, in particular, produce infrasound waves that travel through solid ground and are sensed by other herds using their feet, although they may be separated by hundreds of kilometres.

Scientist have been using infrasound to detect volcanic eruptions and to track the passage of meteors through the atmosphere. This helps in the prevention of any disaster.

 

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4.6 Recall that seismic waves are generated by earthquakes or explosions

 

When earthquakes or explosions are happen, earth is shaken and this create disturbances called Seismic Waves. This is exactly same as when you toss a pebble into a pond, it creates radiating waves in the water

As wave is an energy transfer phenomena so Seismic waves carry a lot of energy.

There are two types of Seismic Waves i- P Waves & ii- S Waves

 

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4.7 Investigate the unpredictability of earthquakes, through sliding blocks and weights

 

 

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4.8 Explain why scientists find it difficult to predict earthquakes and tsunami waves even with available data

 

 

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4.9 Recall that seismic waves can be longitudinal (P) waves and transverse (S) waves and that they can be reflected and refracted at boundaries between the crust, mantle and core

 

 Like light and sound waves, Seismic waves exhibit reflection and refraction behaviour.

 

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4.10 Explain how data from seismometers can be used to identify the location of an earthquake

 

 

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4.11 Demonstrate an understanding of how P and S waves travel inside the Earth including reflection and refraction

 

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4.12 Explain how the Earth’s outermost layer is composed of (tectonic) plates and is in relative motion due to convection currents in the mantle

 

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4.13 Demonstrate an understanding of how, at plate boundaries, plates may slide past each other, sometimes causing earthquakes