Dalkeith High School Level 4 Physics. Waves and Sound

Transcription

1 Dalkeith High School Level 4 Physics Waves and Sound

2 By recording and analysing sound signals, I can describe how they can be manipulated and used in sound engineering. SCN 4-11a

3 INSTRUCTIONS: Always put today s date and copy carefully each HEADING. Symbols used in this booklet: Copy The little pencil symbol means that you copy the passage neatly into your Physics Jotter. It is important that the Copy Passages are copied accurately since the content may appear in the End of Unit Test. Read The little book symbol means that you must read the passage carefully so you can extract the required information and so that knowledge is gained for the test. What to do This little symbol means you must collect apparatus and carry out an experiment or follow instructions in an activity. Remember, apparatus may be delicate and costly and should be treated as such. Please return all apparatus to its appropriate place of storage. Questions Answer in sentences This little question symbol means that there are some questions to be answered as best as you can. If you are unsure of an answer, your teacher may help or you can find out the answer from other sources like a text book or internet. More to do The little plus sign means that if you have the time there is more work that can be done. At all times, instructions should be carefully followed. Follow your instructions. When doing practical work: it should be carried out quietly and safely. equipment should be returned to the correct place. Page 3

4 Activity 1 What is Sound? Read In order for us to hear a sound something must make our eardrums move. You may remember from previously in Science that all moving objects have kinetic energy. Energy must be transferred to our ears. Sound waves carry this energy through the air. Sounds travel away from the source like ripples on a pond. The further from the source you are the less energy reaches you. This is why if you are too far from a sound you cannot hear it. A person can probably only shout over a distance of around 100m before the sound gets too quiet to hear. When the volcanic island of Krakatoa erupted it generated the loudest sound ever historically reported the massive explosion was distinctly heard as far away as the island of Rodrigues near Mauritius (approx miles or 4800 km). This is like us hearing an explosion that happened in New York. Nowadays if someone is a bit hard of hearing there are electronic hearing aids that they can wear. These are really like miniature tannoy systems. They have a microphone which picks up sounds and changes them into electrical signals, an amplifier which gives energy to the signal and a loudspeaker which changes the amplified electrical signal back into sound and sends the sound into the ear. In the olden days before electronics people used to make use of ear trumpets to help them hear. The ear trumpet collected sound waves and passed them down a tube into the person s ear. This is a bit like the satellite dish collecting radio signals and reflecting them to the aerial. Page 4

5 Activity 1 What is Sound? (continued) Questions a) What must happen for us to hear a sound? b) What do sound waves carry through the air? c) What was the loudest sound ever historically reported? d) Jeremy is hard of hearing. Write a short note to Jeremy to explain what he could do improve his hearing. Now watch the animation Sound Production on page 162 of Exploring Science Book 8 showing how different musical instruments are able to produce sound e) Explain briefly how sound is produced using each of the musical instruments shown. More to do The video Sounds on page 162 of Exploring Science Book 8 and try and identify the origin of each sound. When this man talks, his vocal chords vibrate to produce sound waves, which travel through the air. Page 5

6 Activity 2 Longitudinal & Transverse Waves What to do Follow the teacher demonstration and attempt to display the difference between longitudinal and transverse waves. Collect a transverse and longitudinal wave diagram and copy the following passage. In longitudinal waves the vibration of the particles is in the same direction as the direction of energy transport. In transverse waves the vibration of the particles is at right angles to the direction of energy transport. Sound is an example of a longitudinal wave. Light, water waves and waves on a string are examples of transverse waves. The simple demonstration experiment below can be used to show how sound energy is transferred through air. Loudspeaker Observe what happens to the candle flame when the sound emitted from the loudspeaker is directed towards it. Page 6

7 Activity 2 Longitudinal & Transverse Waves (continued) As the diaphragm of the speaker vibrates back and forth it disturbs the surrounding air molecules. The air molecules then pass on the disturbance to adjacent air molecules. In this way the originating disturbance from the speaker travels through air (the medium) via the air molecule as a sound wave. The air molecules do not themselves travel from the speaker to the ear rather they just vibrate to and fro. As the air molecules move in the same direction as the wave, sound waves are therefore longitudinal waves. The wavelength of a sound wave is the distance between successive compressions or rarefactions as shown in the diagram above. Page 7

8 Activity 3 Amplitude & Frequency Read In order for sound to be heard it requires a medium to pass through. Sound can travel (propagate) through solids liquids and gases at different speeds. This is due to the proximity of particles in these materials and their ability to pass on the sound vibrations. However, sound cannot travel through a vacuum, a volume of space with no (or very few) particles. Scientists use microphones to detect sounds. If a microphone is attached to an oscilloscope, you can see a representation of the sound wave produced. By looking at the wave (or trace) on the oscilloscope screen, scientists can make comments about the amplitude (volume) and frequency (pitch) of the sound. Copy and complete the following diagrams Page 8

9 Activity 3 Amplitude & Frequency (continued) What to do Part A: Oscilloscope Traces 1. Collect an oscilloscope trace sheet. 2. Follow the instructions on the sheet and attempt to complete all diagrams in the left hand column of blank traces using the apparatus provided. 3. Review the solutions and if required use the diagrams on the right hand column to make any corrections. More to do Use the Sound Wave animation page 164 of Exploring Science Book 8 and try and identify the correct oscilloscope trace before observing it. Page 9

10 Activity 3 Amplitude & Frequency (continued) Read The frequency of a sound produced can also be altered by adjusting the length of the object which produces the sound vibrations. What to do Part B: Test tube pipes 1. Collect four test tubes and a test tube rack. 2. In each test tube place a different volume of water, leaving an air column height of 2, 4, 6 and 8 cm in each. 3. Gently blow across the top of each one, noting down in a suitable table of height of air column (in cm) and frequency of sound (value 1 to 5, 1 being lowest frequency, 5 being highest) 4. Explain your results and make some valid conclusions based on them More to do Use the Pitch of a Guitar animation page 164 of Exploring Science Book 8 and try and determine the pitch of sound produced before moving the guitarist s hand. Page 10

11 Activity 4 The Wave Equation Read With the increase in mobile technology most of our day to day communications are completed using waves passing through the air, wires or fibre optics. We need to understand how to describe a wave so that we are able to talk about waves in a meaningful way. What to do Collect a waves diagram and stick it into your jotter. Listen to your teacher describe the main parts of the wave. Terms to describe waves A A Part Symbol Unit Description wavelength m It is the distance between two successive points on a wave in phase. It is measured from the centre line to the amplitude A m crest or trough. It is a measure of how much energy a wave carries. This is how many waves are produced each frequency f Hz second. This is the same as the number of waves that pass a point in one second. period T s This is the time to produce one wave. wavespeed v m/s This is the distance a wave travels in one second. Page 11

12 Activity 4 The Wave Equation (continued) Read the following statements We can find the speed (v) of a wave by measuring how far the wave travels (distance, d) in a known time interval (time, t). This uses the formula: It is also possible to calculate the speed of a wave if we know the wavelength ( ) and frequency (f) of the wave. This uses the formula: The frequency of a single wave can be expressed as the relationship between the frequency and the time taken for one wave, the period of a wave, (T). Similarly, the frequency of a number of waves (N) passing a given point in a fixed time (t) can be expressed as: Example: A water wave has a wavelength of 50 cm. Twenty waves pass a point in 10 seconds. Calculate: a) The frequency of the wave. b) The speed of the wave. Solution: (a) N f f? t 20 N 2.0 f 10 t 10 s f 2 Hz Solution: (b) v? v f f 2 Hz v m v 1 m / s Page 12

13 Activity 4 The Wave Equation (continued) Questions 1. If the speed of sound travels at 333 m/s, how long does it take to travel 3000 m? 2. If the speed of sound in water is 1500 m/s, how far does it travel in 3 mins 45 s? 3. Sound travels at 5050 m/s in railways tracks. A train is 4 km away. How long does it take the sound to reach a man with his ear to the tracks? Note 1 km = 1000 m 4. A wave has a wavelength of 34 cm and a frequency of 75 Hz. Calculate its speed. Note 1 cm = 0.01 m 5. A wave has a speed of 18 m/s ad a frequency of 9 Hz. Calculate: a) The wavelength of the wave. b) The period of the wave. 6. What is the wavelength of a microwave with a frequency MHz? Hint 1Mhz = Hz and the speed of radio waves is m/s waves arrive at a beach every minute. The speed of waves is 4.8 m/s. Calculate: a) The frequency of the waves. b) The wavelength of the waves. 8. A wave travels 60 m in 30 s. It has a wavelength of 5 cm. Calculate: a) The wavespeed. b) The frequency of the wave. c) The period of the wave. Page 13

14 Activity 5 The Speed of Sound Read You may well have noticed before that sound and light do not travel at the same speed. For instance, during a firework display or a thunderstorm, which do you observe first: the light or sound? Light travels extremely fast. In fact, the speed of light in a vacuum (3 x 10 8 m/s, or m/s) is the fastest speed anything in the universe can travel at. Sound travels much slower through air. We can use this difference between the speed of light and the speed of sound to attempt to measure the speed of sound in air using a variety of methods. What to do Method 1: Outdoor measurements 1. You and two partners will measure out a fixed distance of 50 metres (using either a surveyor s tape or trundle wheel). 2. One partner will stand at one end of this distance (d) and will make a sound. The other partner will indicate using a hand up or waving a flag the instant the sound has been made. 3. You will measure the time (t) using a stopclock taken until you hear the sound. 4. This distance and time should be noted in a suitable table. 5. You will then repeat this procedure twice, with each partner alternating their role in the group. 6. At the end of the practical activity, the group will calculate the average time ( taken and using the formula determine the speed of sound in air (v). Page 14

15 Activity 5 The Speed of Sound (continued) Method 2: Indoor measurements 1. Use the apparatus already pre assembled as shown in the diagram. 2. Hit the metal plate sharply. 3. Record the time (t) into your table. 4. Repeat twice and then calculate the average time ( taken and using the formula determine the speed of sound in air (v). Attempt distance Between microphones (m) time in milliseconds (ms) Average speed speed in metres per second (m/s) Questions 1. The approximate speed of sound in are can range between m/s. Which of the two methods used was most accurate? 2. What improvements could have been made to each experimental procedure to improve its reliability? More to do Use the Speed of Sound animation page 172 of Exploring Science Book 8 and use to try another method of measuring the speed of sound Page 15

16 Activity 6 Detecting Sound & The Range of Hearing Read The human ear has two functions: hearing and balance. The ear has three main parts: the outer, middle and inner ear. The outer ear is the part you can see and opens into the ear canal. The eardrum separates the ear canal from the middle ear. Three small bones in the middle ear transmit sound vibrations to the inner ear. The inner ear contains the cochlea which converts the vibrations into electrical signals. These electrical signals pass along the nerve to the brain. The semicircular canals in the ear have nothing to do with hearing. They are required for balance. What to do Collect a copy of the ear diagram and use the information from the passage above or the Hearing video page 169 of Exploring Science Book 8 to answer the following questions: 1. What is detected by the human ear? 2. What is the function of the three small bones in the inner ear? 3. Which part of the ear converts vibrations into electrical signals? Page 16

17 Activity 6 Detecting Sound & The Range of Hearing (continued) Although our ears are tuned to detecting a large range of audible frequencies, there is still an upper and lower limit to the range of human hearing. The following demonstration will help you determine your range of hearing. What to do 1. Your teacher will adjust the controls on the oscilloscope to a low frequency. 2. Listening intently, you should indicate by raising your hand when you begin to detect an audible sound. This is your lower threshold of hearing. 3. Your teacher will continue to increase the frequency. 4. Keep your hand raised until you can no longer detect an audible sound. You have reached your upper threshold of hearing. 5. Collect a Range of Hearing passage and complete using your data: Lowest frequency I can hear = Hertz Highest frequency I can hear = Hertz When listening to music, you hear sounds of many frequencies. On average, humans can detect frequencies between 20 Hertz and Hertz. These are audible frequencies for humans. As we get older, the upper limit gradually reduces to about Hz. Some animals can detect and communicate using frequency sounds either side of the range of human hearing. More to do Watch the BBC video How animals use sound to communicate video page 162 of Exploring Science Book 8 to find out more about the frequency ranges other animals can use to communicate. Page 17

18 Activity 7 Ultrasound Read As blind as a bat is a common saying. Bats are almost blind, but they can locate obstacles or insects by using very high frequency sound waves. Humans can hear frequencies up to Hz. Higher frequencies than this are called Ultrasounds. When these higher frequency waves are sent out and hit an object some are reflected like an audible echo. Bats, dolphins and whales are some mammals that use these echoes to find their way around, evade predators, or catch food. This process is called echolocation. Ultrasounds are used in medicine to scan unborn babies. This is a safe way to monitor the growth and health of an unborn baby. Ultrasounds are sent out from a transmitter and are reflected back from the baby to a detector. A picture or scan can then be viewed on a computer screen. Ultrasound can also be used in medicine to destroy kidney stones. An industrial application of ultrasound is in fishing, where fishermen use ultrasound at to locate shoals of fish. Ultrasounds are sent out and reflected by the sea bed. If there are fish in the way, the ultrasounds will reflect from them and be detected back at the boat more quickly. A military application of ultrasound is in the detection of enemy submarines, either through active or passive sonar. Active sonar is the method by which torpedoes use ultrasound pulses to echolocate targets. Passive sonar is a method by which boats use pulses todetect potential targets which cause the transmitted signal to be reflected differently. Page 18

19 Activity 7 Ultrasound (continued) Questions 1. What is ultrasound? 2. Describe a medical application of ultrasound. 3. Describe a non-medical application of ultrasound. Read Ultrasound numerical problems generally involve calculating the total distance travelled by a reflected ultrasound. The same principle should be applied when considering audible sounds being reflected, or echoed. The example below shows how to attempt these types of problems. Example: A sonar pulse was sent down from a ship looking for a shoal of fish and two pulses were reflected back, the first after 0.85s and the second after 2.3 seconds. If the speed of sound in water is 1500 m/s. Calculate: (a) The depth of the water. Solution: The pulse that takes the longest time to be reflected is the one which reflects off the seabed. v 1500 m / s t 2.3 s d? d v t d d 3450 m This is the total distance travelled. Therefore, the distance from boat to seabed is half i.e m. Page 19

20 Questions 1. An object is detected by ultrasound as long as it is equal to one wavelength of the ultrasound. If the frequency of the ultrasound is 50 khz, what is the size of the smallest object detected given that the ultrasonic pulses travel with a speed of 340 m/s in air. 2. A series of sonar pulses is used by fishermen to detect shoals of fish under the water. The speed of sound in sea water is 1200 m/s. a) An echo is received after 0.3 s. How far has the sound travelled? b) How deep is the water? c) A second echo is received after 120 ms. How far had the sound travelled. Note 1 ms = s. d) How deep is the shoal of fish? 3. The speed of sound in the human body is 1500 m/s and an echo from a foetus is detected 0.1 ms after an ultrasound transmission. The frequency of ultrasound is 250 khz. Note 1 khz = 1000 Hz. a) How many ultrasonic pulses are emitted every second? b) What is the period of ultrasound? c) How deep in the mother s body is the part of the foetus which provides the echo? 4. A man uses a silent dog whistle to call his dog. a) Explain why the dog can hear it but the man cannot. b) Suggest a possible frequency for the dog whistle. c) Another whistle emits a sound of wavelength 18 mm. Will this act as a silent whistle? Explain your answer. Note 1 mm = m. Page 20