Chapter 17 Sound Wave Hearing is one of our most important senses.

Sound is a disturbance of matter that is transmitted from its source outward. Sound is Longitudinal wave. Sound is a periodic wave, and the atoms undergo simple harmonic motion. 2 Sound

A vibrating string produces a sound wave. As the string oscillates back and forth, it transfers energy to the air. 3

Sound Waves Hearing is the perception of sound. The human ear can hear between about 20 Hz and 20,000 Hz sounds. Greater than 20,000 Hz are called ultrasonic. Less than 20 Hz are called infrasonic. Ultrasonic waves are familiar from medical applications; elephants and whales communicate, in part, by infrasonic waves.

Sound Wave Bats can use ultrasound to find small objects in the dark.

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PerceptionPhysical quantity PitchFrequency LoudnessIntensity and Frequency NoteBasic unit of music with specific names, combined to generate tunes 7 Sound perceptions

The wave equation Speed of a sound wave Unit [m/s]

18 The behavior of sound propagation is affected by three things: ■A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium. ■The propagation is also affected by the motion of the medium itself. if the medium is moving, the sound is further transported. ■The viscosity of the medium also affects the motion of sound waves. In solids, sound can be both transverse and longitudinal. 18 υ solids > υ liquids > υ gases

10 Speed of Sound Waves

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Practice Problems Show that the speed of sound in 20.0ºC air is 343 m/s. Air temperature in the Sahara Desert can reach 56.0ºC (about 134ºF ). What is the speed of sound in air at that temperature? Dolphins make sounds in air and water. What is the ratio of the wavelength of a sound in air to its wavelength in seawater? Assume air temperature is 20.0ºC. 12

13 Standing wave Because the observed wave pattern is characterized by points that appear to be standing still, the pattern is often called a standing wave pattern. 13

Standing Waves A standing wave is fixed in location. These waves are found on strings with both ends fixed, such as in a musical instrument, and also in vibrating columns of air.

In order for different strings to have different fundamental frequencies, they must differ in length and/or linear density. A guitar has strings that are all the same length, but the density varies and the variation in tension. Standing Waves

There must be an integral number of half- wavelengths on the string; this means that only certain frequencies are possible. Points on the string which never move are called nodes; those which have the maximum movement are called antinodes.

Standing Waves The fundamental, or lowest, frequency on a fixed string has a wavelength twice the length of the string. Higher frequencies are called harmonics.

Standing Waves Standing waves can also be excited in columns of air, such as soda bottles, woodwind instruments, or organ pipes. Height of the column decrease, the frequency increases and wavelength decreases.

Standing Waves In this case, the fundamental wavelength is four times the length of the pipe, and only odd-numbered harmonics appear.

Standing Waves If the tube is open at both ends, both ends are antinodes, and the sequence of harmonics is the same as that on a string.

Beats

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Check understanding The phenomenon of beats results from sound a.refractionb.reflection c.interference d.all of these A 1056 hertz tuning fork is sounded at the same time a piano note is struck. You hear three beats per second. What is the frequency of the piano string? a.1053 hertzb.1056 hertz c.1059 hertzd.2112 hertz e.not enough information given to determine 23

The Doppler Effect (shift) The Doppler effect is the change in pitch of a sound when the source and observer are moving with respect to each other. 1.When an observer moves toward a source, the wave speed appears to be higher, and the frequency appears to be higher as well.

The Doppler Effect Doppler effect for Moving Observer u- observer’s speed υ - speed of sound + Moving toward - Moving away

The Doppler Effect 1.When a source moves toward a observer, the wavelength appears to be shorter, and the frequency appears to be higher as well.

The Doppler Effect - Moving toward + Moving away Doppler effect for Moving Source:

Example The horn of a car attracts the attention of a stationary observer. If the car is approaching the observer at 16.7 m/s and the horn has a frequency of 512 Hz, what is the frequency of the sound perceived by the observer? Use 343 m/s for the speed of sound in air. a.488 Hz b.512 Hz c.538 Hz d.600 Hz 28

The Doppler Effect Combining results gives us the case where both observer and source are moving: The Doppler effect has many practical applications: weather radar, speed radar, medical diagnostics, astronomical measurements.

Check understanding If at a concert, a wind blows directly from the orchestra toward you, the speed of the sound you hear will be (a)decreased (b)increased (c) neither decreased nor increased If at a concert, a wind blows directly from the orchestra toward you, the wavelength of the sound you hear will be (a)decreased (b)increased (c) neither decreased nor increased If at a concert you run toward the orchestra, the frequency of the sound you hear will be (a)decreased (b)increased (c) neither decreased nor increased 30

Summary of Chapter 17 A wave is a propagating disturbance. Transverse wave: particles move at right angles to propagation direction Longitudinal wave: particles move along propagation direction Wave speed: Speed of a wave on a string:

Summary of Chapter 17 When two or more waves occupy the same location at the same time, their displacements add at each point. If they add to give a larger amplitude, interference is constructive. If they add to give a smaller amplitude, interference is destructive. An interference pattern consists of constructive and destructive interference areas. Two sources are in phase if their crests are emitted at the same time.