Chapter 26 Sound

1. Chapter 26 Sound Conceptual Physics Chapter 26

2. The Origin of Sound • All sound waves are produced by the vibration of a material object. E.g., the reed of a saxophone, the string of a guitar or the tines of a tuning fork. • Sound waves are longitudinal waves that transport acoustic energy through a medium by particle-to-particle interactions between neighboring molecules as they oscillate back and forth. • The frequency of the sound wave produced is equal to the frequency of the vibrating source. Conceptual Physics Chapter 26

3. The Origin of Sound • The transmission of sound requires a medium. There may be vibrations, but if there is nothing to compress and expand, there can be no sound. • Sound waves can propagate through solids, liquids and gases, but can not travel through a vacuum. Conceptual Physics Chapter 26

4. The Origin of Sound • Sound can be heard from the ringing bell when air is inside the jar, but not when the air is removed. Conceptual Physics Chapter 26

5. Sound in Air Consider sound waves in a tube. • When the prong of a tuning fork next to the tube moves toward the tube, a compression enters the tube. • When the prong swings away, in the opposite direction, a rarefaction follows the compression. • As the source vibrates, a series of compressions and rarefactions is produced. Conceptual Physics Chapter 26

6. Sound in Air Compressions and rarefactions are produced when opening and closing a door in a small room. • When you quickly open a door, you can imagine the door pushing the molecules next to it into their neighbors. • Neighboring molecules then push into their neighbors, and so on, like a compression wave moving along a spring. • A pulse of compressed air moves from the door to the curtain, pushing the curtain out the window. • This pulse of compressed air is called a compression. • When you quickly close the door, the door pushes neighboring air molecules out of the room. • This produces an area of low pressure next to the door. Nearby molecules move in, leaving a zone of lower pressure behind them. • Molecules then move into these regions, resulting in a low-pressure pulse moving from the door to the curtain. • This pulse of low-pressure air is called a rarefaction. • In both cases the pulse travels from the door to the curtain. • For all wave motion, it is not the medium that travels across the room, but a pulse that travels. Conceptual Physics Chapter 26

7. The Sonic Spectrum • All sound waves fall on the sonic spectrum. • Sound waves with a frequency below 20 Hz are called infrasonic. • Sound waves that fall between 20 Hz and 20000 Hz can be heard by most humans and are in the audible range of the sonic spectrum. • Waves above 20000 Hz are ultrasonic. • Humans can not hear infrasonic or ultrasonic sounds. • As people age, the hearing range shrinks, particularly at the high-frequency end. Conceptual Physics Chapter 26

8. Pitch • We describe our subjective impression of the frequency of sound waves as the pitch of the sound. • Higher frequency sound waves are heard to have a higher pitch. 300-Hz sound 500-Hz sound Conceptual Physics Chapter 26

9. Speed of Sound • Typically, sound travels fastest in solids (because of the high elasticity and greater density) and slowest in gases (because of the low elasticity and lesser density). • The speed of sound in a material is determined by the properties of the material, specifically the elasticity of the medium and the mass density of the medium. Conceptual Physics Chapter 26

10. Speed of Sound • At normal atmospheric pressure and a temperature of 20°C, sound waves travel at 343 m/s (about 750 mph) through the air. • Increasing the temperature of the air causes an increase in the speed of sound at a rate of 0.6 m/s for every degree above 20°C. • The speed of sound is much slower than the speed of light. You hear thunder after you see a flash of lightning. Conceptual Physics Chapter 26

11. Sound Intensity • Sound intensity is proportional to the square of the amplitude of the sound wave. • As sound waves travel away from their source, the energy is spread over a greater and greater surface area causing a reduction in the intensity of the sound wave. • Sound intensity is measured in Watts per square meter (W/m2). • The lowest intensity sound that can be detected by the human ear is 1 x 10-12 W/m2. This is referred to as the threshold of hearing. Conceptual Physics Chapter 26

12. The Decibel Scale • Since the range of intensities that can be detected by the human ear is very large, a logarithmic scale, based on powers of ten, is used to measure intensity levels. This intensity level is measured in decibels (dB). • A 50 dB sound is 100 times more intense than a 30 dB sound and 1000 times more intense than a 20 dB sound. Conceptual Physics Chapter 26

13. Loudness • While the intensity of a sound is a very objective quantity which can be measured with sensitive instrumentation, the loudness of a sound is more of a subjective response which will vary with human perception. • The sensation of loudness will roughly follow the decibel scale. Conceptual Physics Chapter 26

14. Forced Vibrations • The tendency of one object to force another adjoining or interconnected object into vibrational motion is referred to as a forced vibration. • Press the base of a vibrating tuning fork against a tabletop and the tabletop will be forced into vibration – the tabletop serves as a sounding board amplifying the sound. • The wooden body of an acoustic guitar will be forced to vibrate at the same frequency as a vibrating string on the guitar. Conceptual Physics Chapter 26

15. Natural Frequency • When an object composed of an elastic material is disturbed it will tend to vibrate at its natural frequency. • If the amplitude of the vibrations are large enough and if the frequency is within the audible portion of the sonic spectrum, then the object will produce sound waves which are audible. • The natural frequency is the frequency at which minimum energy is required to force the object into vibration. Conceptual Physics Chapter 26

16. Natural Frequency • A bell, a tuning fork and a flute all produce vibrations at a single frequency and are said to produce a pure tone. • Other instruments produce a set of frequencies that are multiples of one another to generate a rich sound. • Some objects produce a set of frequencies that have no simple relationship and generate a sound that is not at all musical – instead they produce noise. Conceptual Physics Chapter 26

17. Resonance • When one object vibrates at the same natural frequency of a second object and forces that second object into vibrational motion, resonance occurs. • Resonance can lead to a dramatic increase in the amplitude of the vibration. • A child on a swing can swing through a larger motion if she pumps her legs or if she is pushed by someone else. If the rhythm of the pushing matches the natural frequency of the swing, the amplitude of the swing can grow quite large. Conceptual Physics Chapter 26

18. Resonance • Two tuning forks of the same frequency are mounted on identical open wooden boxes and separated by a short distance. When one of the tuning forks is struck it will set the other tuning fork into vibration. • Air molecules surrounding the first tuning fork are set into vibration with the same frequency as that of the tuning fork. • With each compression that passes the second tuning fork, a tiny pushing force acts on the tines of this tuning fork. • Since the frequency of these pushes corresponds to the natural frequency of the second tuning fork, the pushes will successively increase the amplitude of vibration. Conceptual Physics Chapter 26

19. Resonance An air column in an open-ended tube or soda bottle can be made to resonate if the input frequency matches the natural frequency of the air column. Conceptual Physics Chapter 26

20. Resonance In 1940, the Tacoma Narrows Bridge in the state of Washington collapsed due to resonance! The wind speeds that caused the collapse were only 40 mph, but the frequency of the wind pulses matched the natural frequency of the bridge. Conceptual Physics Chapter 26

21. Interference • Interference occurs in longitudinal waves just as it does in transverse waves. • When sound waves interfere, the loudness of the sound is effected. • When two sound waves are in phase, compressions will match with compressions and rarefactions will match with rarefactions leading to constructive interference and increased intensity – the sound will be louder. • When two waves are out of phase, compressions will match with rarefactions leading to destructive interference and decreased intensity – the sound will be softer. Conceptual Physics Chapter 26

22. Interference • Destructive interference between reflected and non-reflected sound waves can cause dead spots in some poorly designed auditoriums or theaters. • Noise-canceling headphones produce a mirror image of any external noises to generate destructive interference Conceptual Physics Chapter 26

23. Beats • When two sound waves differing slightly in frequency are superimposed they will not maintain a constant phase relationship. • This leads to alternating reinforcement and cancellation of the sound energy. • The audible result is a series of pulsations called beats. 400 & 401 Hz sounds – 1 beat per second 400 & 403 Hz sounds – 3 beats per second 400 & 410 Hz sounds – 10 beats per second Conceptual Physics Chapter 26

24. Beats • The pattern of alternating constructive and destructive interference can be found from applying the law of superposition to the interfering waves. Conceptual Physics Chapter 26

25. Beats • The beat frequency is equal to the difference between the frequencies of the interfering sound waves. • Although the separate waves are of constant amplitude, we see amplitude variations in a superposed wave form. • This variation is produced by the interference of the two superposed waves. • Maximum amplitude of the composite wave occurs when both waves are in phase. • Minimum amplitude occurs when both waves are completely out of phase. Conceptual Physics Chapter 26

26. Beats • Beats can occur with any kind of wave and are a practical way to compare frequencies. • To tune a piano, a piano tuner listens for beats produced between a standard tuning fork and a particular string on the piano. • When the frequencies are identical, the beats disappear. • The members of an orchestra tune up by listening for beats between their instruments and a standard tone. Conceptual Physics Chapter 26