NAME: SECOND YEAR: A. EXERCISES LESSON 11: Waves. Light and sound. Exercise sheet 1

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1 NAME: SECOND YEAR: A NATURAL SCIENCE 2º ESO EXERCISES LESSON 11: Waves. Light and sound READING 1: What is sound? Exercise sheet 1 Have you ever touched a loudspeaker as it is emitting sound? If so, you can notice that it vibrates. In the same way, the strings of a guitar produce nice sounds when the musician makes them vibrate and the stretched skin of a drum vibrates when it is hit. Put your hand on your throat and murmur something. What do you feel? Is anything vibrating? Sources of sound all have some part which vibrates. Does everything that vibrates produce sound? Not everything, for example, atoms that made up every substance are continuously vibrating and you cannot hear them. The Earth crust also vibrates, and so do the floor, walls and furniture in your house. It is clear, not all those vibrations are perceived as sounds. Otherwise, the world would be a terrible noisy place. In order to produce a sound it is necessary not only an emitter or sound source (the object that vibrates) but a material medium that the sound can travel or be transmitted through (solid, liquid or gas), and finally, a receptor, for example, our ear or a microphone. Fortunately or unfortunately, humans hear only sounds with frequencies between certain limits. These are the limits of audibility; the upper limit decreases with age. Exercise 1 Exercise 2 Exercise 3 Exercise 4 Exercise 5 Exercise 6 Exercise 7 What has to happen for a guitar string, a loud speaker, a violin string or a xylophone to produce sound? Is it possible for the sound to travel through vacuum? A body vibrates 60 times in 30 seconds. What is its frequency in Hertz? Can we perceive it? A fly flaps its wings 60 times every 5 seconds and a mosquito do it 100 times every 5 seconds. Can we hear them as they are flying? A body vibrates at a frequency of 500 Hz. How many oscillations does it produce in one minute? Using the concept of frequency, define infrasound and ultrasound What does it mean when we say that a wave sound is audible? 1

2 READING 2: To be transmitted sound needs a material medium A slinky spring can be used to show how sound waves travel through materials. As the wave moves along the spring, each part of the spring vibrates. The particles in air vibrate in a similar way when they transmit sound. Each wave consist of a compression or squash, where the particles are close together, followed by a rarefaction or stretch, where the particles are further apart. In the same way, when you make a ruler vibrate by pulling and then releasing it, the air molecules are pushed together and then pulled apart and a wave travels to your ear and then your eardrum vibrates and sends this vibration into your inner ear and to your brain which interprets this waves as sounds. Exercise 7 Match each material medium with its speed Material medium Air Water Iron Granite Vacuum Speed 0 m/s 340 m/s m/s m/s m/s Exercise 9 Imagine you are on the Moon. Could you hear the impact of a meteorite? Exercise 10 Explain why is different the speed of sound in solids, liquids and gases. Exercise 11 When a plane reaches the speed of sound, we say it travels at Speed Match 1. Find out the speed of a plane that travels at 14 Match. You need to look up the speed of sound in the air. 2

3 Loudness Sound source Level (db) Plane taking off 140 Noisy R&R 120 Pain threshold 120 Railway 100 mp4 player 80 Heavy traffic 70 Maximum recommended Normal conversation 60 Quiet office 50 Quiet house 40 Near murmur 5 m 30 Rustle of leaves 20 Normal breathing 10 Hearing threshold 0 Exercise 12 What sound property allows us to distinguish the musical note from two different instruments? Exercise 13 What are decibels (db) used for? They are used for measuring intensity of sound. Exercise 14 There exists a musical instrument called bass. Why do think it is called this way? Exercise 15 How does vary the pitch in the piano keyboard? Exercise 16 What sound property do you use to recognise a person by his voice? Exercise 17 Look the diagrams below obtained in an oscilloscope screen: Which sound is: a) the highest pitch, b) the lowest pitch, c) the most intense, d) the softest 3

4 READING 3: Speed of sound The speed of sound depends on the material through which it is passing. This material has to be elastic, that is, be able to return to its initial state after the disturbance has passed, it needs to vibrate. For example, plasticine is not elastic, it is plastic, and if we hit a big peace of plasticine on one end, the sound will not be transmitted to the other end, because plasticine become deformed and do not recover its original state. For this reason, the speed of sound is greater in solid bodies, because they are more elastic than liquids and liquids are more elastic than gases. The distance between the molecules that made up a body also influence the speed of sound through it.. Sound travels at a constant speed if the medium which it is travelling is homogeneous. In the air, the speed is 340 ms when the temperature is around 20 ºC. Exercise 18 A man shouts in the middle of the countryside. A cow is quietly eating 1000 metres from him. How long does it take for the cow to hear the man s shout. Exercise 19 Alba hits an abandoned rail from a railway track m from her is her friend Alvaro. If the rails are made of iron, how long does it take fort Alvaro to hear the sound? Consult the table of speed of sound. Exercise 20 A ship sailing 2 km away from the coast sounds its foghorn. If the sound of the foghorn is transmitted both through the air and the water, how long does it take for us to hear the sound at coast if we are: a) under the water and b) in the air? Speed of sound Substance Speed (m/s) Hydrogen 1270 Carbon dioxide 258 Air (20 ºC) 340 Water 1493 Salt water 1533 Iron 5130 Glass 5000 Rubber 54 Vacuum 0 Exercise 21 Convert the speed of sound in the air, 340 m/s, into km/h. Is it bigger or smaller in water? Why? Exercise 22 Why do we hear thunder after we have seen the lighting? Exercise 23 How can we know if a storm is going away or coming up? READING 4: Echo and reverberation 4 Sound waves are reflected well from hard, flat surfaces such as walls or cliffs. The reflected sound forms and echo or, sometimes, a reverberation. The human hear needs at least 0 1 seconds between the first sound and the reflected sound to hear them separately. This reflected sound is called echo. However, if the reflecting surface is nearer than 17 m from the source of sound, that is, if the time between the first and second sound is less than 0 1 seconds, we do not hear two separated sounds, we only hear one single prolonged sound called reverberation. Sonar (Sound Navigation And Ranging) is a device used in ships and submarines to detect how far an object is from the. This mechanism emits ultrasounds which are reflected by the object and detected by the mechanism.

5 Exercise 24 How far does the sound travel in 0 1 seconds? Exercise 25 John Smith shouts and 3 seconds later he can hear the sound again because it is reflected on a nearby wall. How far is the wall from John? Exercise 26 John Brown shouts Hello! from the summit of a mountain and three and five seconds later he receives the echoes from two mountains. How far away are they from John? REVISION ACTIVITIES Exercise 27 If you touch a bell when it is ringing or you put a finger on a xylophone bar when it is sounding, the sound stops immediately. Why is this? Exercise 28 An insect produces 2300 oscillations in 50 seconds when flapping. What is its frequency? Could we hear the flapping? Exercise 29 Is the sound a type of energy? Exercise 30 In what material medium does sound travel better? Exercise 31 In a storm, a few seconds after lightning strikes, thunder is heard. If you hear thunder four seconds after a bolt of lightning, how far are you from the storm? Exercise 32 In science-fiction films when an intergalactic space ship explodes we can hear the sound produced by the explosion. Is this situation really possible in physics? Exercise 33 Name the three sound properties and the magnitude they are related to. Exercise 34 The same note sounded by two different musical instruments can be clearly distinguished. Why? Exercise 35 What is the difference between a high pitch and a low pitch? Exercise 36 A person standing opposite a mountain makes a sound. She hears the echo two seconds later. How far is she from the mountain? Exercise 37 Why does the sound travel faster in solids than in liquids or gases? Exercise 38 Complete the following sentences: a) Frequency is the number of in a and its unit is the. b) The human being perceives as sound any vibrations with between 20 Hz and Hz. c) Sound waves need a to travel through and can never travel through a. d) is characteristic related to the intensity of sound and it is measured in e) is the sound characteristic related to the frequency emitted and it can be described as or f) Two sounds that have the same intensity and frequency are distinguishable by their if thy have a different wave shape. g) The human ear needs at least seconds to hear two separated sounds: this phenomenon of reflected sound is called an. is produced when there is less time between the sound and its reflection. 5