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# Sound Waves

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• As an ambulance speeds towards you, sirens blazing, the sound you hear is rather high in pitch. This is because the sound waves in front of the vehicle are being squashed together by the moving ambulance. This causes more vibrations to reach your ear per second. As you know, more vibrations per second results in a higher pitched sound. When the ambulance passes you, the sound becomes lower in pitch. Behind the ambulance there are fewer vibrations per second, and a lower sound is heard. This change in pitch is known as the Doppler Effect.
• ### Sound Waves

1. 1. Unit 1 - Waves Sound Waves
2. 2. Unit 1 - Waves • Sound is a wave, which is created by vibrating objects and propagated through a medium from one location to another. A sound wave is similar in nature to a slinky wave for a variety of reasons. – First, there is a medium which carries the disturbance from one location to another. – Second, there is an original source of the wave, some vibrating object capable of disturbing the first particle of the medium. – Third, the sound wave is transported from one location to another by means of the particle interaction. • Since a sound wave is a disturbance, which is transported through a medium via the mechanism of particle interaction, a sound wave is characterized as a mechanical wave. Sound Waves
3. 3. Unit 1 - Waves • Sound waves are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction, which the sound moves. • There are regions of compressions and rarefactions. The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. Sound Waves
4. 4. Unit 1 - Waves • The diagram below depicts a sound wave created by a tuning fork and propagated through the air in an open tube. Sound Waves
5. 5. Unit 1 - Waves • Even though a sound wave is a longitudinal wave, it can be mathematically represented as a transverse wave. The crests correspond to areas of high pressure (compressions) and the troughs correspond with the rarefactions or areas of low pressure. Sound Waves
6. 6. Unit 1 - Waves Sound Waves
7. 7. Unit 1 - Waves Check Your Understanding: • A sound wave is a pressure wave; regions of high (compressions) and low pressure (rarefactions) are established as the result of the vibrations of the sound source. These compressions and rarefactions result because sound a.is more dense than air and thus has more inertia, causing the bunching up of sound. b.waves have a speed which is dependent only upon the properties of the medium. c.is like all waves; it is able to bend into the regions of space behind obstacles. d.is able to reflect off fixed ends and interfere with incident waves e.vibrates longitudinally; the longitudinal movement of air produces pressure fluctuations. Sound Waves
8. 8. Unit 1 - Waves • Frequency for any wave can be defined as the number of cycle of the wave that pass by a given point every second. The sound wave is a longitudinal wave. • Each cycle of the sound wave includes one compression and one rarefaction. If, for example, a speaker vibrates back and forth at a frequency of 500 Hz, then 500 compressions, each followed by a rarefaction, is generated every second. A sound with a single frequency is called a pure tone. Pitch & Frequency of Sound Waves
9. 9. Unit 1 - Waves • A healthy young person hears all sound frequencies from 20 Hz to 20 000 Hz (20 kHz). This audible range of frequencies is called the audio spectrum. • Infrasonic sound waves are those below the range of normal human hearing. These frequencies occur below 20 Hz. • Ultrasonic frequencies occur above the range of normal human hearing and therefore are above 20 kHz. Pitch & Frequency of Sound Waves
10. 10. Unit 1 - Waves • Frequency is an objective physical property of a sound wave and can be measured using an electronic frequency counter. The listener’s perception of sound is subjective. • The brain interprets the frequency detected by the ear as a subjective or perceived quality called pitch. • A pure tone with a high frequency is interpreted as a high-pitched sound. A pure tone with a low frequency is interpreted as a low-pitched sound. Pitch & Frequency of Sound Waves
11. 11. Unit 1 - Waves Pitch & Frequency of Sound Waves
12. 12. Unit 1 - Waves • The diagram below shows a pure-tone sound wave travelling down a tube.        • In the areas of rarefaction, the air pressure of the sound wave is low. In the areas of compression, the air pressure is high. Sound Pressure, Amplitude and Loudness
13. 13. Unit 1 - Waves • An air pressure versus distance graph has the appearance of a transverse wave even though sound itself is a longitudinal wave. • The pressure amplitude shown in the graph above is the magnitude of the maximum change in pressure measured relative to undisturbed or Sound Pressure, Amplitude and Loudness
14. 14. Unit 1 - Waves • Loudness is a characteristic of sound that depends primarily on the amplitude of the wave. The larger the amplitude, the louder the sound. The pressure amplitude is an objective property of the sound wave since it can be measured with an instrument. Loudness, on the other hand, is subjective. Each individual determines what is loud, depending on the acuteness of his or her hearing. Sound Pressure, Amplitude and Loudness
15. 15. Unit 1 - Waves • Sound waves carry energy with them as they move from one place to another. • The intensity of a sound wave is the energy that is transported past a given area per unit of time. When the amplitude of a sound wave increases, the energy of the wave increases and therefore the intensity is also larger. As the sound wave travels away from its source, the surface area is larger and the intensity of the wave is less. The intensity is less because the wave is spreading out over a larger area. Sound Intensity and Decibels
16. 16. Unit 1 - Waves • The decibel (dB) is a measurement that is used to compare two sound intensities. • The threshold of hearing is assigned a decibel level of 0 dB. – A sound that is 10 (101 ) times more intense than the threshold is assigned a sound level of 10 dB. Rustling leaves would have a level of 10 dB. – A sound that is 100 (102 ) times more intense than the threshold is assigned a sound level of 20 dB. This is the sound of a whisper. – A sound that is 100 000 (105 ) times more intense than the threshold has a sound level of 50 dB. This is the sound of an average classroom with students actively working with each other. – Other examples of sound levels are a car without a muffler at 100 dB, a live rock concert at 120 dB, and the threshold of pain is at about 130 dB. Sound Intensity and Decibels
17. 17. Unit 1 - Waves Sound Intensity and Decibels
18. 18. Unit 1 - Waves • Wave interference is the phenomenon, which occurs when two waves meet while traveling along the same medium. Recall, there is two types of interference; constructive and destructive. • Sound is a pressure wave, which consists of compressions and rarefactions. The interference of sound waves causes the particles of the medium to behave in a manner that reflects the net effect of the two individual waves upon the particles. Wave Interference & Production of Beats
19. 19. Unit 1 - Waves • If a compression (high pressure) of one wave meets up with a compression (high pressure) of a second wave at the same location in the medium, then the net effect is that that particular location will experience an even greater pressure. This is a form of constructive interference. • If two rarefactions (two low pressure disturbances) from two different sound waves meet up at the same location, then the net effect is that that particular location will experience an even lower pressure. This is also an example of constructive interference. Wave Interference & Production of Beats
20. 20. Unit 1 - Waves • If two sound waves interfere at a given location in such a way that the compression of one wave meets up with the rarefaction of a second wave, destructive interference results. • The net effect of a compression (which pushes particles together) and a rarefaction (which pulls particles apart) upon the particles in a given region of the medium is to not even cause a displacement of the particles. The particles would remain at their rest position as though there wasn't even a disturbance passing through them. This is a form of destructive interference. Wave Interference & Production of Beats
21. 21. Unit 1 - Waves • Constructive and destructive interference occur when the waves are of the same frequency. In this section we will study what happens when sound waves have slightly different frequencies. This gives rise to a phenomenon called beats. • Sound beats are the periodic and repeating fluctuations heard in the intensity of a sound when two sound waves of slightly different frequencies interfere with one another. Wave Interference & Production of Beats
22. 22. Unit 1 - Waves • The sound changes from loud to softer, then loud again, and so on. The diagram below illustrates the wave interference pattern resulting from two waves with slightly different frequencies. Wave Interference & Production of Beats
23. 23. Unit 1 - Waves Wave Interference & Production of Beats
24. 24. Unit 1 - Waves • The beat frequency is the rate at which the sound alternates from loud to soft and equals the difference in frequency of the two sounds. If two sound waves with frequencies of 440 Hz and 442 Hz interfere to produce beats, a beat frequency of 2 Hz will be heard. • The human ear is only capable of hearing beats with small beat frequencies (e.g., 8 Hz or less). Wave Interference & Production of Beats
25. 25. Unit 1 - Waves Wave Interference & Production of Beats
26. 26. Unit 1 - Waves • Recall, a standing wave pattern is formed when reflected waves interfere with incident waves to form a standing wave that appears to be standing in place. • The frequency at which the standing wave exists is called the resonant frequency. Each resonant frequency is a whole-number multiple of the lowest resonant frequency called the fundamental frequency. Standing Wave Patterns
27. 27. Unit 1 - Waves • The word resonance comes from Latin and means to "resound" - to sound out together with a loud sound. Resonance is a common cause of sound production in musical instruments. • This is known as resonance - when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion. Resonance
28. 28. Unit 1 - Waves Resonance
29. 29. Unit 1 - Waves • The way our ears separate various frequencies is based on the principle of resonance. An object that, when struck, can vibrate with a certain frequency, will also start vibrating in response to a sound wave of this frequency. Resonance
30. 30. Unit 1 - Waves Speed of Sound
31. 31. Unit 1 - Waves • Like any wave, the speed of a sound wave refers to how fast the disturbance is passed from particle to particle. While frequency refers to the number of vibrations, which an individual particle makes per unit of time, speed refers to the distance, which the disturbance travels per unit of time. • Always be cautious to distinguish between the two often-confused quantities of speed (how fast...) and frequency (how often...). Speed of Sound
32. 32. Unit 1 - Waves • Since the speed of a wave is defined as the distance, which a point on a wave (such as a compression or a rarefaction) travels per unit of time, it is often expressed in units of meters/second (abbreviated m/s). In equation form, this is speed = distance/time Speed of Sound
33. 33. Unit 1 - Waves • The speed of any wave depends upon the properties of the medium through which the wave is traveling. Typically there are two essential types of properties which effect wave speed - inertial properties and elastic properties. Speed of Sound
34. 34. Unit 1 - Waves • The density of a medium is an example of an inertial property. The greater the inertia (i.e., mass density) of individual particles of the medium, the less responsive they will be to the interactions between neighbouring particles and the slower the wave. If all other factors are equal (and seldom is it that simple), a sound wave will travel faster in a less dense material than a more dense material. Speed of Sound
35. 35. Unit 1 - Waves • Elastic properties are those properties related to the tendency of a material to both maintain its shape and not deform whenever a force or stress is applied to it. • Steel versus plasticine. Speed of Sound
36. 36. Unit 1 - Waves • In general, solids have the strongest interactions between particles, followed by liquids and then gases. For this reason, longitudinal sound waves travel faster in solids than they do in liquids than they do in gases. Even though the inertial factor may favor gases, the elastic factor has a greater influence on the speed (v) of a wave, thus yielding this general pattern: vsolids > vliquids > vgases Speed of Sound
37. 37. Unit 1 - Waves • The speed of a sound wave in air depends upon the properties of the air, namely the temperature and the pressure. The pressure of air (like any gas) will affect the mass density of the air (an inertial property) and the temperature will affect the strength of the particle interactions (an elastic property). At normal atmospheric pressure, the temperature dependence of the speed of a sound wave through air is approximated by the following equation: v = 331 m/s + (0.6 m/s/C)*T where T is the temperature of the air in degrees Celsius. Speed of Sound
38. 38. Unit 1 - Waves • Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, there seemed to be a change in the pitch of the siren; and after the car passed by, the pitch of the siren sound changed again. Doppler Effect   In small groups discuss this phenomenon, explaining what you would here before car reaches you and after the car passes you. Try and explain why this may occur.
39. 39. Unit 1 - Waves Doppler Effect
40. 40. Unit 1 - Waves Doppler Effect
41. 41. Unit 1 - Waves • The Doppler effect is a phenomenon observed whenever the source of waves is moving with respect to an observer. The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for the observer as the source is approaching and an apparent downward shift in frequency when the observer and the source is receding. Doppler Effect
42. 42. Unit 1 - Waves • When a vehicle travels faster than the speed of sound, a sonic boom can be heard. As the vehicle overtakes its own sound, the sound waves spread out behind in a shockwave, or sonic boom. Doppler Effect