2.3
Analysing sound waves from musical instruments
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• Sound waves are vibrations of particles in a medium.
• Compressions relate to the crests of a transverse wave and rarefactions relate to the troughs of a transverse wave.
• The pitch of a sound wave increases as the frequency of the sound wave increases.
• The amplitude of a sound wave increases as the volume of sound grows louder.
• An echo is a reflection of a sound wave.
• Waves can interfere when they come into con- tact. That can result in the amplitude of the waves increasing if the waves are in phase or decreasing if the waves are out of phase. Addition of waves is called superposition. • Sound waves can be studied with a cathode-ray
oscilloscope (CRO) or cathode-ray oscilloscope simulator computer program.
• Different musical instruments produce sound waves that produce different shaped traces on a CRO.
• Tuning forks produce pure notes that result in a sine-wave trace on a CRO.
• The notes from tuning forks can be added to produce more complex sounds and wave traces on a CRO. This is an example of superposition of sound waves.
1. If sound travels in air at 330 m s−1 and a tuning fork producing sound travelling at that speed vibrates with a frequency of 256 Hz, calculate the wavelength of the sound wave produced by the tuning fork.
2. If two sound waves are travelling in the same medium but one sound wave has a frequency twice as high as the other, compare their wavelengths.
3. Much publicity has surrounded the develop- ment of sound-eliminator technology whereby a machine or headphone technology that gen- erates a noise is used to eliminate the loud noise of a machine or background sounds. Explain how this is possible.
4. Sound waves leaving one medium enter another medium where their speed is much greater. In both media, the frequency of the sound wave remains constant. Explain what is different between the sound wave in each medium.
5. Describe the principle of superposition. 6. Explain why high-frequency sound waves are
preferred for tasks such as echo location rather than low-frequency sound waves.
7. A baritone and a soprano were asked to sing the word ‘one’ into a microphone and the signal produced was fed into a CRO. Describe how you would assign each of these different traces to the correct person.
8. Calculate how many times per second a tuning fork vibrates if it is producing a sound wave in helium gas with a frequency of 384 Hz.
9. A CRO trace produced by a sound wave is shown in figure 2.20. Identify on the trace where the air particles are:
(a) undergoing rarefaction (b) undergoing compression
(c) in neither compression nor rarefaction.
Figure2.20 A CRO trace
10. Figure 2.21 shows an experiment often done by students learning about sound wave properties.
Figure2.21 An experiment used to study sound waves in a vacuum
The radio is switched on in a large sealed bell jar attached to a vacuum pump. As the air is pumped from the bell jar, the sound of the radio becomes progressively softer and finally disappears. The radio continues to work as normal throughout this experiment. Explain this phenomenon using your knowledge of the properties of sound.
SUMMARY
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To vacuum pump Glass valveCHAPTER RE
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11. Present diagrammatically (on graph paper) the following two transverse waves (that are initially in phase) and add the waves to pro- duce a resultant wave.
Wave 1: wavelength 2 cm, amplitude 1 cm Wave 2: wavelength 4 cm, amplitude 2 cm 12. Present, as diagrams on graph paper, the fol-
lowing two transverse waves (that are initially out of phase) and add the waves to produce a resultant wave.
Wave 1: wavelength 2 cm, amplitude 1 cm Wave 2: wavelength 4 cm, amplitude 2 cm 13. Two special tuning forks are used in an experi-
ment with a CRO to determine the trace pattern of a sound wave. One sound wave has a frequency of 250 Hz, the other 500 Hz. The screen of the CRO can capture 0.05 s of the sound. Draw labelled figures to show each of these screen traces.
14. An audio oscillator is a device that can pro- duce sound over a wide range of frequencies and amplitudes. Every frequency produces a sine wave trace on the CRO. A human voice box can also produce sounds over a wide
range of frequencies and amplitudes yet the CRO trace is rarely a sine wave. Explain the difference between the two traces.
15. Explain why you cannot hear an echo from a very close wall yet a microphone attached to a CRO can detect an echo as a shadow trace. 16. Describe the production of dead spots, where
sound cannot be heard in a room or stairwell despite the fact that a constant noise is being generated.
17. If two waves were superposed and the trace of the resultant wave was a trace along the base- line, what has happened to the sound?
18. Predict whether sound would travel faster in air or water based on the nature of the wave and the medium. Justify your prediction. 19. Explain why high frequency sound waves ‘see’
better than low frequency sound waves when using sonar.
20. Identify which type of sound wave would carry the most energy: a low amplitude sound with a frequency of 256 Hz or a high amplitude sound with a frequency of 512 Hz.
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Aim
To observe and collect sound traces from a CRO
Apparatus
at least two tuning forks of different frequency access to a CRO or a CRO simulation program for
the computer
a microphone to convert the sound wave into an electrical signal
Theory
The traces from a CRO can provide you with a snapshot of a number of different sound waves. The waves are a small time-grab of a much larger train of sound waves. These short interval grabs can show you some of the features of a sound.
Method
1. Connect the microphone to the input of the CRO or the microphone input on the computer if using a CRO simulation program.
Figure2.22 A microphone attached to a CRO
2. Tune and adjust the CRO so that when a single tuning fork is brought near to the microphone, a sine-wave trace is produced. Observe what happens to the amplitude of the wave as the tuning fork loses its vibrational energy and the sound becomes softer.
3. Check out all of the traces of all tuning forks you have. When doing this, keep the same CRO settings to make comparison easier. Note the frequencies and shape of the waves produced. If you are using a CRO simulation computer pro- gram, you should be able to freeze the CRO traces, save them and print them out.
4. Try striking two different frequency tuning forks and having the microphone collect the sound from both tuning forks. You will notice the shape of the CRO trace wave becomes more complex.
5. Try adding a third sound from another tuning fork to the input into the CRO. Observe the increasingly complex CRO trace.
Aim
To hear sound waves interfering with each other
Apparatus
tuning fork
Theory
Each of the vibrating tuning fork prongs acts as a coherent source of sound because it has the same frequency, amplitude and phase in relation to the other when producing a sound wave in air. Hence, there are two sound waves generated by the tuning fork prongs. Each one radiates from a slightly different position. As a compression is produced between the prongs, a rarefaction is produced outside each of the prongs and vice versa. The sound waves propagate outward from each tuning fork prong but on some paths they overlap. This is either because there is a full wave- length difference in the travel path length or because in some directions they meet at a point one half wavelength out of phase. In these direc- tions where the sound waves are exactly one half wavelength out of phase (compression meets rarefaction) the sound waves will add. If the amplitudes are the same, one sound wave’s com- pression is annulled by the rarefaction from the other. This produces a sound minimum.
2.1
ANALYSING
SOUND WAVES
FROM A TUNING
FORK
Oscilloscope Wave pattern Sound waves Microphone2.2
OBSERVING
WAVE
INTERFERENCE
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The sound waves can add to form a maximum if the path difference is equal to a whole number of wavelengths. The result is a higher amplitude