Sound Preview Section 1 Sound Waves Section 2

Sound Preview Section 1 Sound Waves Section 2 Sound Intensity and Resonance Section 3 Harmonics © Houghton Mifflin Harcourt Publishing Company Section 1

Sound Section 1 What do you think? • What is sound? • What do all of the sounds that you hear have in common? • How do they differ? • Can sounds travel through solids? Liquids? Gases? • Is one type of material better for transmitting sound waves? • When race cars or emergency vehicles pass you, the sound changes. In what way, and why? © Houghton Mifflin Harcourt Publishing Company

Sound What is Sound? • Sound is a longitudinal wave. • All sound waves are produced by vibrating objects. – Tuning forks, guitar strings, vocal cords, speakers • The vibrating object pushes the air molecules together, forming a compression. • It then spreads them apart, forming a rarefaction. © Houghton Mifflin Harcourt Publishing Company Section 1

Sound Section 1 Graphing Sound Waves • The diagram shows compressions (dark) and rarefactions(white). If you measured the pressure or density of the air and plotted these against position, how would the graph appear? © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Sound Waves Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Characteristics of Sound • Frequency is the number of waves per second. • You have heard of ultrasound. What is it? • Frequencies audible to humans are between 20 Hz and 20 000 Hz. – Middle C on a piano is 262 Hz. – The emergency broadcast signal is 1 000 Hz. • Infrasound frequencies are lower than 20 Hz. • Ultrasound frequencies are greater than 20 000 Hz. © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Comparing Infrasonic and Ultrasonic Sounds Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Pitch • Pitch is the human perception of how high or low a sound appears to be. – Pitch is primarily determined by frequency. – Pitch also depends slightly on other factors. • Higher frequencies appear to have a higher pitch when played loudly, even though the frequency does not change. © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Speed of Sound • Sound waves travel though solids, liquids and gases. – In which would the speed generally be greatest? Why? • Solids. Because the molecules are more closely packed, the particles respond more rapidly to compressions. – How might the temperature of air affect the speed of sound waves? Why? • Higher temperature increases the speed of the waves because the particles are moving faster and colliding more often. © Houghton Mifflin Harcourt Publishing Company

Sound Speed of Sound © Houghton Mifflin Harcourt Publishing Company Section 1

Sound Section 1 Spherical Waves • Sound propagates in three dimensions. • The diagram shows: – Crests or wave fronts (blue circles) – Wavelength ( ) – Rays (red arrows) • Rays indicate the direction of propagation. • How would these wave fronts appear different if they were much farther from the source? © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Spherical Waves • Wave fronts and rays become more nearly parallel at great distances. • Plane waves are simply very small segments of a spherical wave a long distance from the source. © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Doppler Effect • Why are the waves closer together on the left? – Waves are closer because the vehicle moves to the left along with the previous wave. • How will the sound be different for observer A and observer B? – Higher frequency (pitch) for observer A • Continued on the next slide…. © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Doppler Effect • How would the wave pattern change if the vehicle moved at a faster speed? How would it sound different? – At a higher speed, waves would be even closer together and the pitch difference would be even greater. • The Doppler effect is the observed change in frequency due to the motion of the source or observer. © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Doppler Effect and Sound Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 1 Now what do you think? • What is sound? – What do all of the sounds that you hear have in common? – How do they differ? • Can sounds travel through solids? Liquids? Gases? – Is one type of material better for transmitting sound waves? • When race cars or emergency vehicles pass you, the sound changes. In what way, and why? © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 What do you think? • Members of rock bands generally protect their ears from the loud sounds to prevent damage to their hearing. • How do we determine the loudness of a sound? • What quantity is loudness measuring? • What units are used? • Name three ways you can reduce the loudness of the music heard by a person in the audience. © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Sound Intensity • Vibrating objects do work on the air as they push against the molecules. • Intensity is the rate of energy flow through an area. – What is “rate of energy flow” called? • E/t is called power (P). – Since the waves spread out spherically, you must calculate the area of a sphere. How? • A = 4 r 2 – So, what is the equation for intensity? © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Sound Intensity • SI unit: W/m 2 • This is an inverse square relationship. – Doubling r reduces intensity by ¼. – What happens if r is halved? • Intensity increases by a factor of 4. © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Intensity and Decibels • An intensity scale based on human perception of loudness is often used. • The base unit of this scale is the bel. More commonly, the decibel (d. B) is used. – 0. 1 bel = 1 d. B, 1 bel = 10 d. B, 5 bels = 50 d. B, etc. – The lowest intensity humans hear is assigned a value of zero. • The scale is logarithmic, so each increase of 1 bel is 10 times louder. – An increase in intensity of 3 bels is 1 000 times louder. © Houghton Mifflin Harcourt Publishing Company

Sound © Houghton Mifflin Harcourt Publishing Company Section 2

Sound Section 2 Classroom Practice Problems • The intensity of the sound from an explosion is 0. 10 W/m 2 at a distance of 1. 0 × 103 m. Find the intensity of the sound at a distance of 5. 0 × 102 m, 1. 0 × 102 m and 10. 0 m. – Answers: 0. 41 W/m 2, 1. 0 × 103 W/m 2 • Find the approximate decibel equivalents of these sound intensities using Table 2. – Answers: 110 d. B, 130 d. B, 150 d. B © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Audible Sounds • The softest sound humans can hear is called the threshold of hearing. – Intensity = 1 10 -12 W/m 2 or zero d. B • The loudest sound humans can tolerate is called the threshold of pain. – Intensity = 1. 0 W/m 2 or 120 d. B • Human hearing depends on both the frequency and the intensity. © Houghton Mifflin Harcourt Publishing Company

Sound © Houghton Mifflin Harcourt Publishing Company Section 2

Sound Section 2 Human Hearing Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Forced Vibrations • Sympathetic vibrations occur when a vibrating object forces another to vibrate as well. – A piano string vibrates the sound board. – A guitar string vibrates the bridge. • This makes the sound louder and the vibrations die out faster. – Energy is transferred from the string to the sound board or bridge. © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Resonance • The red rubber band links the 4 pendulums. • If a blue pendulum is set in motion, only the other blue pendulum will have largeamplitude vibrations. – The others will just move a small amount. • Since the vibrating frequencies of the blue pendulums match, they are resonant. © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Resonance • Large amplitude vibrations produced when the frequency of the applied force matches the natural frequency of receiver – One blue pendulum was the driving force and the other was the receiver. • Bridges have collapsed as a result of structural resonance. – Tacoma Narrows in the wind – A freeway overpass during an earthquake © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Resonance (Frequency) Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 2 Now what do you think? • Members of rock bands generally protect their ears from the loud sounds to prevent damage to their hearing. – How do we determine the loudness of a sound? • What quantity is loudness measuring? • What units are used? – Name three ways you can reduce the loudness of the music heard by a person in the audience. © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 What do you think? • A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different. • What is the same about the sound? • Are the frequencies produced the same? • Are the wave patterns the same? • Why do the instruments sound different? © Houghton Mifflin Harcourt Publishing Company

Sound Standing Waves on a String • There is a node at each end because the string is fixed at the ends. • The diagram shows three possible standing wave patterns. • Standing waves are produced by interference as waves travel in opposite directions after plucking or bowing the string. • The lowest frequency (one loop) is called the fundamental frequency (f 1). © Houghton Mifflin Harcourt Publishing Company Section 3

Sound Section 3 Standing Waves on a String • To the left is a snapshot of a single loop standing wave on a string of length, L. • What is the wavelength for this wave? – Answer: = 2 L • What is the frequency? – Answer: © Houghton Mifflin Harcourt Publishing Company

Sound © Houghton Mifflin Harcourt Publishing Company Section 3

Sound Section 3 Harmonics • n is the number of loops or harmonic number. • v is the speed of the wave on the string. – Depends on tension and density of the string • L is the length of the vibrating portion of the string. • How could you change the frequency (pitch) of a string? © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Fundamental Frequency Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Standing Waves in an Air Column • Wind instruments also use standing waves. – Flutes, trumpets, pipe organs, trombones, etc. • Some instruments have pipes open at both ends while others have one end closed. – Air is free to move at open ends so antinodes occur. – Closed ends are nodes. • The velocity of the wave is now the velocity of sound in air (346 m/s at 25°C). © Houghton Mifflin Harcourt Publishing Company

Sound Both Ends Open © Houghton Mifflin Harcourt Publishing Company Section 3

Sound Closed at One End © Houghton Mifflin Harcourt Publishing Company Section 3

Sound Section 3 Wind Instruments • Wind instruments are not as simple as organ pipes. – The shape is not always cylindrical. – The holes change the wave patterns as well. – The size of the “pipe” varies along the length. © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Classroom Practice Problems • One string on a toy guitar is 34. 5 cm long. – What is the wavelength of the first harmonic or the fundamental wavelength? • Answer: 69. 0 cm or 0. 690 m – The string is plucked and the speed of the waves on the string is 410 m/s. What are the frequencies of the first three harmonics? • 590 Hz, 1200 Hz, 1800 Hz • Note: The use of significant figures causes the multiples of 590 to be 1200 and 1800 because only two significant figures are present in the answer. © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Classroom Practice Problems • An organ pipe open at both ends is 34. 5 cm long. – What is the wavelength of the first harmonic or the fundamental wavelength? • Answer: 69. 0 cm or 0. 690 m – What are the frequencies of the first three harmonics if the air temperature is 25. 0°C? • Answers: 501 Hz, 1000 Hz, 1500 Hz – Answer the same questions if the pipe is closed at one end. • Answers: 251 Hz, 753 Hz, 1250 Hz © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Timbre or Quality of Sound • Instruments do not vibrate in a single mode. – Several harmonics are produced at the same time. – The particular harmonics and intensity of each vary with different instruments. • Timbre is the quality of the tone resulting from the combination of harmonics. • The fundamental frequency (1 st harmonic) determines the pitch. – Adding other harmonics changes the timbre. © Houghton Mifflin Harcourt Publishing Company

Sound Timbre © Houghton Mifflin Harcourt Publishing Company Section 3

Sound Section 3 Timbre Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Beats • The diagram shows two waves of different frequencies. Sketch the superposition or sum of these waves. • How would the combined wave sound? © Houghton Mifflin Harcourt Publishing Company Section 3

Sound Section 3 Beats • Produced by two waves with the same intensity and different frequencies – Generally the frequencies are nearly the same. • The sound pulses or changes from loud to soft and back. • Beats are used to tune instruments. – If a tuner hears beats, the instruments are slightly out of tune. • The number of beats heard per second is the difference in the two frequencies. © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Beat Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Sound Section 3 Now what do you think? • A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different. – What is the same about the sound? • Are the frequencies produced the same? • Are the wave patterns the same? – Why do the instruments sound different? © Houghton Mifflin Harcourt Publishing Company