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Physics 3Physics 3

Sound

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How sound travels…How sound travels…As we know, sound waves are formed when something vibrates. But how does the sound reach our ears?

1) An object makes a sound by vibrating

2) The vibrations pass through air by making air molecules vibrate

3) These vibrations are picked up by the ear

Air molecules

21/04/23Sound can be reflected and Sound can be reflected and refractedrefracted

Bats use echoes to locate insects. They

time how long the echo takes to come back and use that to locate the

insect

• An Echo is a reflected sound wave

Echolocation is also used in the shipping industry

but remember Sound in air = 330m/sSound in water = 1500m/s

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• Sound is also refracted

In the day the hot ground warms the air and sound waves travel faster. This makes the waves refract

upwards

At night the air is colder by the ground so the sound

waves are refracted towards the ground

This is why you can here sound from a greater distance at

night

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Using an oscilloscopeUsing an oscilloscope

1) Quiet sound, low frequency:

2) Quiet sound, high frequency:

3) Loud sound, low frequency:

4) Loud sound, high frequency:

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1) Pre-natal scanning

2) Cleaning – it can be used to dislodge dirt

3) Detecting flaws or cracks

4) Medical treatment

UltrasoundUltrasoundUltrasound is the region of sound above 20,000Hz – it can’t be heard by humans. There are a number of uses for ultrasound:

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How does ultrasound work?How does ultrasound work?Ultrasonic waves are partly _________ at the boundary as they pass from one _______ to another. The time taken for these reflections can be used to measure the _______ of the reflecting surface and this information is used to build up a __________ of the object.

Words – depth, reflected, picture, medium

21/04/23Using an oscilloscope with Using an oscilloscope with ultrasoundultrasound

Consider a block of metal with a flaw:

20ms/div

Q. If the speed of the ultrasonic wave is 3,000m/s how far away is the flaw from the detector?