2SoundAims:To describe how to measure the speed of sound in air by a simple direct method.To use an oscilloscope to determine the frequency of a sound wave and appreciate that the pitch of a sound depends on the frequency of vibration,To appreciate that the loudness of a sound depends on the amplitude of vibration,
3Sound cannot travel though a vacuum Sound is made up of the vibrations of atoms or molecules passing through a substance, and so without atoms or molecules (i.e. in a vacuum) sound cannot travel
4When the pump is turned on, the bell sounds quieter and quieter as the air is pumped out. Eventually, it can hardly be heard at all - thought he gong can still be seen striking the bell. This is because the sound from the bell cannot travel though a vacuum, but light can. The bell does not become totally silent, since some of its vibrations are transferred to the container - the foam rubber helps to reduce this effect.
6START STOP You need a quiet open space at least 100 m long to perform this investigation.STARTSTOP100 m1. When you see the cymbals crash, press START.2. When you hear the cymbals crash, press STOP.
7 Record the results of your sound experiments in a table. 1 2 3 4 100 distance (m)time(s)speed(m/s)12341000.34294How are these values used to estimate the speed of sound?distancetimespeed ==1000.34=294 m/s
8340 m/s The speed of sound in air is about… Use the results of the cymbals experimentto calculate your average speed of sound.How does your calculation for the average speed of sound compare with the real speed?What errors could have affected the results of your cymbals experiment?Do you think the speed of sound in water is the same as it is in air?
9Measuring speed with echos START150 mSTOPStand at least 100 m from a large, flat wall with a stop watch.1. Use a starting pistol (or clapper board) to make a sound.2. Measure the time taken between firing the pistol andhearing the echo. How far does the sound travel?
10distance time speed = = 300 0.92 = 326 m/s The sound of the starting pistol takes 0.92 sto travel a distance of 300 m.How can you use this result to estimate the speed of sound?distancetimespeed ==3000.92=326 m/sRepeat the experiment several times to obtain an average.How does your calculation for the average speed of soundcompare with the real speed?
11Draw a labelled diagram of the arrangement of suitable apparatus for the experiment.
13Speed of sound in different materials The speed at which it travels though a material will therefore depend on how close together the molecules are, and so upon the density of a material - the more tightly packed the molecules, the more quickly vibrations can be transferred from molecule to molecule, i.e. the faster the sound can travel though the material.
14Speed of sound in different materials The speed of sound is approximately:340m/s in air - gas molecules are relatively far apart, and the transfer of vibrations (sound) is relatively slow1400m/s in water - liquid molecules are much closer together, and transfer of vibrations is much more rapid6000m/s in steel - solid molecules are even closer together, and transfer of vibrations is even more rapid
15Speed of sound in different materials vgas<vliquid<vsolidSoft materials dampen sound; hard materials reflect it (echoes and reverberations).
16Visualising soundDescribing a sound wave with diagrams or equations makes it easier to communicate our ideas.
28OscilloscopeOne button can change the width of the wave on the screen so that you can measure the frequency even if the wavelength is very large or very small.An oscilloscope can show an electrical signal on its screen.One button can change the height of the wave on the screen so that you can measure the amplitude even if it is very large or very small.
29Looking at Sound Waves A low frequency sound at a medium volume Trace MicrophoneA low frequency sound at a medium volumeOscilloscope
30What is the difference between the sound wave of a quiet sound and a loud sound? The loud sound has taller waves.The louder the sound, the greater the amplitude.What would the sound wave of a very loud sound look like?
31low pitch sound high pitch sound What is the difference between the sound wave of a low pitch sound and a high pitch sound?low pitch soundhigh pitch soundThe high pitch sound has a shorter wavelength,so more waves are visible. It has higher frequency waves.What would the sound wave of a very low sound look like?
32Looking at Sound WavesA higher frequency sound at a lower volume
33Looking at Sound WavesA low frequency sound at a low volume
34Looking at Sound WavesA high frequency sound at a high volume
35A B Which trace represents the loudest sound? Sound A is the loudest. Sound A has the largest amplitude, which means the wave has more energy and so the sound is louder.
36A B Which trace represents the sound with the highest pitch? Sound B is the highest pitched.Sound B has the shortest wavelength and the most number of waves visible, so it has the highest frequency.
37Match the description to the oscilloscope pattern: BCD1. Low-pitched sound, very loudD2. Loud, high-pitched soundC3. Medium sound with medium pitchB4. Quiet, high-pitched soundA
38Match the oscilloscope traces to the source of the sound. BCD1. A flute playing a single clear noteB2. A dog whistleC3. A milk bottle dropped onto a floorD4. A rumble of thunderA
40Amplitude and Frequency The human can hear sound between about 20Hz and 20,000Hz.Amplitude & Frequency
41Ultrasonic wavesA signal generator can produce electrical signals which can be converted to sound waves by a speaker. The sound waves can be greater than the hearing range for humans (ultrasonic waves).Ultrasonic waves can be used in industry for cleaning, quality control, and for pre-natal scanningUltrasonic waves
43Uses of sounda. Acoustics – the study of sound.Soft materials dampen sound; hard materials reflect it (echoes and reverberations).b. SONAR – Sound Navigation and Ranging (echolocation).c. Ultrasound imagingd. Kidney stones & gallstones.
44Oscilloscopes in detail Oscilloscopes can be complicated but we need to know how to work out frequency from the image on the screen.
46Oscilloscope diagramThe height of the wave on the screen is the amplitude of the signal and can be controlled by the ‘volts/div’ dial.The width of the wave on the screen is the frequency of the signal and can be controlled by the ‘time base’ or ‘time/div’ dial.
47Reading the screenThe height of the screen shows the amplitude or voltage of the signal.The width of the screen can be used to determine the frequency of the signal if it is changing.
481 cm squares Each large square is one centimetre wide. Each little division is two millimetres apart.The middle horizontal line displays zero voltage.
49d.c. signals 1If the scale of the oscilloscope is set to 4V per division.This signal would be -8 volts.
50d.c. signals 2If the scale of the oscilloscope is set to 0.2V per division.This signal would be volts.
51d.c. signals 3 If the scale of the oscilloscope is set to 1V/division. This signal would be -2.8 volts.
52d.c. signals 4 The scale has been set to 1V/division. The signal is still d.c.It is changing from 1 V to 3 V but is always positive.
53a.c. signals 1 This is a correct a.c. signal. At 1 cm per volt it has an amplitude of 2 volts.It contains both positive and negative voltages.
57Calculating frequency To work out the frequency of the wave displayed on the screen you need to do three things:Measure a complete wave on the screen in cm,Multiply this number by the ‘Timebase’ setting to get the period,The frequency = 1 period.
58Example calculation 1The wavelength of a signal on an oscilloscope screen is 3 cm long. The timebase setting is 0.1 s/div.Wavelength = 3 cmPeriod = 3 × 0.1 s = 0.3 sFrequency = 1 0.3 = 3.33 Hz
59Example calculation 2The wavelength of a signal on an oscilloscope screen is 4 cm long. The timebase setting is 0.01 s/div.Wavelength = 4 cmPeriod = 4 × 0.01 s = 0.04 sFrequency = 1 0.04 = 25 Hz
60Example calculation 3The wavelength of a signal on an oscilloscope screen is 5 cm long. The timebase setting is 0.1 ms/div. (1 ms = s)Wavelength = 5 cmPeriod = 5 × s = sFrequency = 1 = 2000 Hz
61Summary – Picturing Sound A starting pistol can be used to measure the speed of sound by using the difference in speed between light and sound, the bigger the distance the better the result.An oscilloscope can be used to display the frequency and amplitude of a sound wave. If the wavelength on the screen in cm is multiplied by the timebase we can work out the period of the wave. 1/period = frequency.The bigger the amplitude, the louder the sound.The closer the waves, the higher the pitch.