Chapter 7 Sound:
Points to Note:
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Sound is a form of energy created by vibrations. We use sound to listen, learn and communicate with other in person.
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Musical instruments also use vibrations to create sound
Transmission of Sound:
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Sound cannot travel through a vaccum like space as it requires a medium. As a result there is no sound in space which is why astronauts have to use special communication devices
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Sound cannot travel without a medium because it moves by vibrating air particles, which vibrate other air particles and so on
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If there was no medium there would be nothing to pass the vibrations on
A picture demonstrating the need of a medium for sound to travel
Speed of Sound:
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Sound travels very fast from particle to particle in air at a speed of 340m/s1
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Sound travels instantly over short distances but over long distances there may be a time lag
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Light travels almost a million times faster than sound. We can see this in thunder and lightning
Reflection of Sound:
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In a cave or tunnel the noises seem louder inside than outside. This is due to sound vibrations, f in a cave the vibrations have no where to go as they simply bouce of the walls (echo) but outside the vibrations keep spreading out
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Echoes are sound that are reflected off a surface. Like light, sound will always be reflected when it meets a barrier
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For example, if you stand in front of a wall or cliff and shout, the sound you emit will go towards the wall bouce off and return as an echo. However, the further you are from the wall the longer it will take for the echo to return
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Echoes are proof that sounds can be reflected
Echoes as a nuisance and as a benefit:
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Echoes can be a nuisance in places like cinemas when a sound previously heard is echoed and can be heard again and again
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This is why cinemas have padded walls and carpets to prevent echoes
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However, echoes can also be useful. For example, ships use echo-sounding to find the depth of the water or a shoal of fish. The ships sends out a sound and the length of time it takes for the reflected seabed echo to return can inform sailors of the depth of water or the location of fish
Echo-sounding is one of the many uses of echoes
Thunder and lightning is a prime example of the difference in speed between light and sound
Ultrasound:
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The best type of echo sounding is ultra sound. This is a high pitched sound that cannot be heard by humans
Ultrasound is used for:
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Unborn babies in the womb can be seen using ultrasound. Skin, tissue, bone and muscle all reflect sound differently. Therefore the machine can create a picture of the baby
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The high pitched sound is used to shatter painful kidneys or gall stones that build up
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Ultrasounds can be used to find cracks in metal structures
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Geologists use ultrasound to find out about underground rocks and search for oil and gas
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Bats, dolphins and submarines use echosound. They send the echo out in front of them to see what is ahead
Sound detection:
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The ear acts as a sound detector
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The eardrum (a thin layer of skin) vibrates when the vibrating air particles hit it. These vibrations are carried from the eardrum to the middle ear to the inner ear. The nerve in the ear picks up the vibration and creates impulses sending them to the brain allowing us to pick up sound
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Different levels of sound are described on the decibel scale
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Anything above 85 d.b is harmful. This is why we need to protect our sensitive ears
Methods of protecting ears:
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Heed warning signs and stay away from higj sound areas
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Heed early warning signs such as a ringing in the ear
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Wear ear-muff when instructed to do so
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Keep away from high-level sound areas
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Carpets and soft wall padding absorb sound rather than reflect it.
