Teacher Sound

8/2/2019 Teacher Sound

1/12

A m p l i t u d e

W a v e l e n g t

h

SOUNDAVELENGTH RESONANCE AMPLITUDE FREQUENCY WAVELENGTH

SPLACEMENT VIBRATION ANTINODES HARMONICS TENSION

AVELENGTH RESONANCE AMPLITUDE FREQUENCY WAVELENGTH












TEACHERS NOTES ON

in association with theSchool of Education, Trinity College Dublin


2/12

SOUND INVESTIGATIONS

BRIEF DESCRIPTIONThis unit of work is designed to introduce basic ideas about sound using several (relatively simple) practical investigations. Thestudents observations are used as foundations upon which to build the theory of sound. There is an emphasis on group work, andwith students making predictions before they undertake the various tasks.

TEACHING ORDER1 . Set up two matched tuning forks, preferably on sounding boxes. One fork is to be the focus of the students attention,

the other put some distance to one side.

2. Students make predictions as directed in the Student Notes part A (reproduced below).

The surprising thing is that the sound should still be heard even after the students stop the vibration of the main tuning fork.This acts as an introduction to resonance as a phenomenon.

Set students tasks as per Student Notes part B. Key ideas to come out of this task is the link between vibration and sound,extent of vibration and volume of sound.

Their observations and results can be used to introduce in a qualitative way the idea of pitch and frequency of vibration,volume of sound and amplitude of vibration. (The detail is the subject of the next lesson.)

TIME ALLOCATIONOne double period.

RESOURCES NEEDEDTwo matched tuning forks. Variety of musical instruments and other objects, e .g. rulers, coke bottles, (indeed, anything thatcan be made to make a sound).

PRACTICALSAs indicated in student notes.

USEFUL WEBSITESThis site has a simple animation of vibrations from a tuning fork:

http://www.physicsclassroom.com/mmedia/waves/tfl.html

A 2D impression is to be found at:

http://www.kettering.edu/~drussell/forkanim.html

Animation courtesy of Dr. Dan Russell, Kettering University, USA.

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3/12

A. DEMONSTRATION(FROM STUDENT NOTES)Watch the demonstration with a tuning fork set up by your teacher.

You will hear a tuning fork give out a sound.

Below are some questions for you before the demonstration begins. Work with your partners in your group to make yourpredictions. You must write down your answers.

1. How would you make the tuning fork give out a sound?Why would your method work (what would happen to the tuning fork)?

2. Once the tuning fork makes a sound, how would you stop it?

Now your teacher will guide you through getting the tuning fork to sound, and stop ringing.

Were your predictions confirmed? Or were there any surprises?

After going through the demonstration with your teacher:

1. Draw diagrams of what you saw (your teacher will assist).

2. Explain what you did and what happened.

B. INVESTIGATING WAYS OF MAKING SOUNDSYou should have various items available to you (or your group); e.g. a guitar, violin, metre rule, pencil, test-tubes in a rack,glass containing water.

1. Make each of the items make a sound. For each one, write down what you did to make the sound, and what thesound was like, e.g. high note, low note, loud, soft etc.

It would be best if you draw up a table or chart in which to put your results.

2. Can you make the items change the sound they make? Try out your ideas, and draw up another table/chart in whichto put your results.

When you are finished you will be asked to share your findings with the rest of the class.

Your teacher will help you summarise your findings.

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4/12

MAKING SOUNDS

KEY POINTS(FROM STUDENT NOTES) THESE DIAGRAMS ARE IN THE POWERPOINT FILE.

ms -1

1Hz = 1s -1

m/s or ms -1

v = f

5. You need to know the meaning of speed, amplitude, frequency and wavelength when describing wave motion.

The formula speed = frequency x wavelength .In symbols this is often written

(The symbol is the Greek letter lambda .)

The standard units for measuring the speed, frequency and wavelength of a wave are:SPEED FREQUENCY WAVELENGTH

The Hertz, Hz is often used as the unit for frequency; i.e.

Displacement

Time

Wavelength

Amplitude

Displacement

Time

Amplitude

Wavelength

BRIEF DESCRIPTIONThis topic follows on from the investigations lesson. It develops the key quantities used to describe sound waves

TEACHING ORDER1. Ask students for main things they discovered in the investigation lesson.2. Demonstrate the bell jar experiment to show that sound needs a medium through which to travel.

3. Demonstrate longitudinal waves on a slinky.

4. Summarise: key points as in student notes and in PowerPoint file

TIME ALLOCATIONOne double period.

RESOURCES NEEDEDBell jar apparatus. Tuning forks, sound generator (if required). Slinky.

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5/12

Displacement of layers

These layers are displaced to the leftThese layers are displaced to the right

Displacements plotted above

Distance

(the negative direction).(the positive direction).

(to a different scale).

The dotted lines show the positions of the layers of air as the sound wave passes.

These lines show the equilibrium positions of the layers of air; i.e. when no sound

Notice that some layers remain in their equilibrium position while others are displaced

wave passes.

0

to the left or the right of their equilibrium positions.

FIGURE 1Illustration ofthe movementof layers of airparticles for asound wave

KEY PRACTICALSBell jar experiment showing sound needs a medium through which to travel.

Use of applets to illustrate sound waves of different frequencies and wavelengths.

Demonstration of longitudinal waves on a slinky.

USEFUL WEBSITESThe following sites are useful for various aspects of sound:

einstein.byu.edu/~masong/HTMstuff/WaveTrans.html

www.fearofphysics.com/Sound/dist.html

The following site has some good applets about the wave nature of sound:

surendranath.tripod.com/Applets.html

(You need to click on the Applets button at top left to see the menu of choices.)

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6/12

PUZZLES/QUESTIONS1. Look at the diagram opposite. If someone spoke into the

hole in the plastic cup, what would you expect to see onthe screen?

What would be the point of doing this experiment? (Youmight like to try it for yourself. You can see a video of theexperiment at www.scispy.ie/Default.aspx .)

2. A student thought he had understood how sound waves

pass through air. He said this: A trumpet works bypushing many layers of air out of the mouth of thetrumpet. These layers travelled into our ears, so wecould hear the sound. If you could see the particles of air coming from the trumpet, you would see them flyingacross the room as if they were thousands of tiny bullets. Write down reasons why you agree, or disagree, with thestudent s explanation.

3. What do you think is the explanation of why an explosion is always accompanied by a loud bang ?

1. Movement of spot of light in time with movement of the cellophane membrane as it vibrates.2. The particles do not move far from their equilibrium positions; i.e. they do not behave like bullets of sound or

particles shot out of the trumpet.

3. Sudden expansion of gas caused by the release of gases in the explosion cause huge pulse of movement of layers ofair to be passed on.

ANSWERS

Screen

B e a m o f l a s e r l i g h t

Hole cut in side

Plastic cup

Cellophane stretched over mouth of cup

Light, shiny mirror

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7/12

SOUNDS & VIBRATIONS

1. The loudness of a sound depends on the amplitude of the wave.

2. Very loud sounds can damage your hearing permanently, even not very loud sounds can cause damage over long periods.

3. The pitch of the sound depends on the frequency of the vibration.

4. In stringed instruments, the frequency, f , depends on the lengthof the string, L, its mass per unit length ( ) and on the tensionof the string, T . The formula is

5. In other instruments, e.g. trumpet, organ, whistle, flute, it is the length of a vibrating column of air inside the instrument thatdetermines the pitch/frequency of a note.

KEY POINTS(FROM STUDENT NOTES)

BRIEF DESCRIPTIONThis topic extends the students knowledge of sound to include the detail of standing waves on a stretched string.

TEACHING ORDER1. Revise basic terms, amplitude, frequency and wavelength.2. Show standing wave on a stretched string and introduce terms nodes and antinodes.

3. Describe standing waves as the combination of two travelling waves.

4. Introduce the frequency of a stretched string experiment by asking students in their groups how they would find out ifthe frequency of vibration of a stretched string varied with its length and/or with its tension. Groups report back on theirsuggested methods. Use responses to focus on:

(i) need to change one variable at a time, (ii) need to keep experiments simple. Discuss the standard practical set up.

5. Students carry out practical.

6. Students practice drawing graphs to show the relations and(Video to be available as support.)

7. Ask students why listening to very loud music can be dangerous. Set groups the task of researching the connectionsbetween hearing loss and loud sounds (see Puzzles/Questions).

TIME ALLOCATIONTwo double periods, or perhaps one single and one double.

RESOURCES NEEDEDString or rubber tubing that can used to set up a standing wave, and show reflection of a wave pulse. A slinky could alsobe used.

Standard apparatus for investigating frequency of waves on a stretched string.

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8/12

Investigating relation between frequency and length and tension of a stretched string.

PRACTICALS

USEFUL WEBSITESThe following site has an excellent set of animations to show fundamentals and harmonics:

http://www.mta.ca/faculty/science/physics/suren/Harmonics/Harmonics.html

This site has pure notes of various frequencies that can be played:

http//www.fearofphysics.com/Sound/sounds.html

PUZZLES/QUESTIONS1. Together with other members of your group, carry out research to discover the links between (i) the physics of sound

and hearing loss caused, for example, when people are exposed to very loud sounds, (ii) what physicists have done tohelp people with hearing problems.

2. Here is a diagram of a human ear. Together withyour partners in your group, find out how your earswork; i.e. how do your ears convert sound into thesort of things we recognise as music or speech.Write a brief report that you can present to the restof the class. Your report should include (at least) thenames for the parts labelled as A to E in thediagram, and what they do.

3. The intensity of a sound is measured in decibels(dB).

(i) Find out the origin of the idea of measuring theintensity of sound in decibels. Hint: it has aconnection with bels and telephones.

(ii) What is odd about the decibel scale? Forexample, if a sound has an intensity of 20dB, howdoes it compare to a sound of intensity 10dB?

(iii) Find our what happens to the intensity of asound if its intensity changes by 3dB.

6. Standing waves can be set up on strings and in columns of air. Nodes occur where there is no overall movement of the string(or air). Antinodes occur where the displacement of the string (or air) is a maximum.

7. Sound can also travel in other mediums, e.g. a metal bar, through water etc. Sound also travels as a longitudinal wave in thesesubstances.

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9/12

4. Here is some data obtained from a stretched string experiment.

Frequency/Hz 280 300 325 380 410 440 475 500 520

Tension/N 2 4 5 10 14 18 25 31 38

Plot a graph of (i) Frequency (vertical axis) against Tension, (ii) Frequency against the square root of Tension.Which plot gives a straight line graph? What does this result show?

5. Look at the chart that shows various choices for plotting frequency against length in the stretched string experiment.Which combinations would you expect to give a straight line graph?

Frequency against:

A. Length B. (Length) 2 C. 1/Length D. 1/(2xLength)

Pure note Noise Resultant

6. Why is it important that you should not turn up the volume on an iPod or other MP3 player so loudly that other people canhear it even though you are wearing the earphones?

7. The last question was partly about noise pollution . What do you think makes a noise a form of pollution? Write down atleast three types of noise pollution. What should be done to prevent noise pollution?

8. You may have heard of noise cancelling headphones. They show how a knowledge of interference of waves can be put topractical use. The idea is that the electronics inside the earphone samples the noise coming from outside. It then createsa sound that is exactly the opposite to that signal. The created sound is then added to the actual sound of the music (orspeech) that the person is listening to. The outside sound and created sound should cancel one another out because ofdestructive interference, and just leave the music.

In the diagram are waves that represent a pure note, and noise.

Here is your task: find out more information about noise cancelling headphones; e.g. which companies make them, howmuch do they cost, why would some cost more than others, what might you get extra with the more expensive models?Why is it very hard to cancel out noise completely?

Noise turned upside down (i.e. inverted)

Inverted noise added to resultant give the pure note

FIGURE 1Interference of wavesin noise cancelling headphones

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10/12

1. There is a vast number of possible outcomes to this task. Among the resources that have a base in physics are analogueand digital hearing aids. Also, some implants that boost hearing are all based on the application of digital electronics.

2. A = canal; B = drumskin; C = hammer; D = Anvil; E = stirrup.

3. (i) The bel and decibel was invented by the American company called the Bell Telephone Laboratory in the 1920s.The name was given in honour of Alexander Graham Bell, the inventor of the telephone. That company has beenresponsible for an enormous range of important discoveries in physics, and their application in everyday life.

(ii) The scale of the bel is known as a logarithmic scale. Such a scale measures changes in powers of ten but usingwhole numbers. For example, if the intensity of a sound goes up by 10, it increases by 1 bel (1B), if the intensity goesup by 100, the change is 2 bels (2B), and so on. However, increases in intensity of 10 or 100 is rather unusual, andit is much easier to use a smaller scale - the decibel, dB. Using these units means that 1B = 10dB, 2B = 20dB, andso on.

(iii) The formula for calculating decibels is:

where I 1 and I 2 are the intensities of the two sounds. So,

To find out the value of you have to take the antilog of 0.3.

You will find this function on a scientific calculator, or on a calculator on a PC or Mac. The result is

That is, a change in 3dB means a sound has doubled (or halved) its intensity, or loudness.

4. The plot of frequency against the square root of the tension should give a straight line. Well, it is not a perfect straightline; we never get perfect results in any experiment!

The straight line shows that frequency is proportional to the square root of the tension. i.e.

5. Only C and D would give straight lines.

6. If the sound output is so loud that other people can hear it, it is almost certainly means that the intensity of the soundin your ears is dangerously high.

7. Noise pollution can be thought of as sound where it is unwanted . Examples: noise from traffic, aircraft and loudlyplayed music from houses. The solutions vary and are fairly obvious.

8. Several manufacturers of such earphones are to be found on the web. Perhaps the first to go to market was Bose (atthe usual high price for their equipment). You pay for more sophisticated electronics and, e.g., comfort of theheadphones. Noise can be random and it is very hard to both capture truly random signals sufficiently fast and to cancelthem out by the time the pattern has been matched, the noise has changed again.

ANSWERS

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11/12

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