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Chapter 17

Chapter 17. Mechanical Waves and Sound. 17.1 Mechanical Waves. A mechanical wave is a disturbance in matter that carries energy from one place to another. Medium - The material through which a wave travels. A solid, liquid or gas

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Chapter 17

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  1. Chapter 17 Mechanical Waves and Sound

  2. 17.1 Mechanical Waves • A mechanical wave is a disturbance in matter that carries energy from one place to another. • Medium- The material through which a wave travels. • A solid, liquid or gas • A mechanical wave is created when a source of energy causes a vibration to travel through a medium. • In general , a mechanical wave does not move the medium from one place to another. In a wave pool, the waves carry energy across the pool

  3. 3 Types of Waves • All waves need a source of energy to produce it. • 1. Transverse Waves- shake a rope up and down. • Crest- highest point • Trough- lowest point • A transverse wave is a wave that causes the medium to vibrate at right angles to the direction in which the wave travels.

  4. 3 Types of Waves • 2. Longitudinal Waves- • Compression- area where the particles in a medium are spaced close together • Rarefaction- area where the particles in a medium are spread out • longitudinal waves, the vibration of the medium is parallel to the direction the wave travels.

  5. 3 Types of Waves • 3. Surface Waves- • A wave that travels along a surface separating two media • Bobber moves up and down like a transverse wave and back and forth like a longitudinal wave • Most waves do not transfer matter. • Close to the shore, a surface wave topples over on itself because friction with the shore slows the bottom of the wave. The medium, and anything floating in it, travels to the shore.

  6. 17.2 Properties of Mechanical Waves • Frequency and Period • Periodic Motion- any motion that repeats itself at regular time intervals. • Period- time required for 1 cycle, a complete motion that returns to its starting point; for 1 complete wavelength to pass a fixed point. • Frequency- number of complete cycles in a given time. • For a wave – number of wave cycles that pass a point at a given time. • Frequency is measured in cycles per second or hertz. (Hz) • A wave’s frequency equals the frequency of the vibrating source producing the wave. 1 vibration = 1 Hz

  7. Frequency is the number of complete cycles in a given time. AA wave vibrating at one cycle per second has a frequency of 1.0 Hz. B A wave vibrating at two cycles per second has a frequency of 2.0 Hz

  8. Wavelength • Wavelength- distance between a point on one wave and the same point of the next cycle of the wave. • Transverse = crest to crest or trough to trough • Longitudinal = compression to compression or rarefaction to rarefaction • Increasing the frequency of the wave decreases its wavelength. They are inverses of one another/

  9. Wave Speed • speed = wavelength x frequency • (m/s) (m) (Hz) • The speed of a wave can change if it enters a new medium or if variables such as pressure or temperature change.  • If speed is constant, wavelength is inversely proportional to frequency. • Example Problem: One end of a rope is vibrated to produce a wave with a wavelength of 0.25 meters. The frequency of the wave is 3.0 hertz. What is the speed of the wave?

  10. Math Practice • 1.A wave on a rope has a wavelength of 2.0 m and a frequency of 2.0 Hz. What is the speed of the wave? • 2.A motorboat is tied to a dock with its motor running. The spinning propeller makes a surface wave in the water with a frequency of 4 Hz and a wavelength of 0.1 m. What is the speed of the wave? • 3.What is the speed of a wave in a spring if it has a wavelength of 10 cm and a period of 0.2 s? (v=λ/period) • 4.What is the wavelength of an earthquake wave if it has a speed of 5 km/s and a frequency of 10 Hz?

  11. Amplitude • Amplitude- two characteristics • The more energy a wave has, the greater its amplitude. • Maximum displacement of the medium from its rest position. • For longitudinal waves, more energy means more compression. . A The amplitude of a transverse wave equals the distance to the highest point above the rest position. B This wave's amplitude is one half the amplitude of the wave in A

  12. 17.3 Behavior of Waves The ripples visible on the bottom of the pool are caused by light shining through surface waves.

  13. Reflection • Reflection-a wave bounces off a surface that it cannot pass through. (like a ball thrown at a wall) • It does not change the speed or frequency of the wave, but the wave can be flipped upside down. • The only property in which the wave does not continue to move forward, it move back

  14. Refraction A lawnmower turns when it is pushed at an angle from the grass onto the gravel • Refraction-the bending of a wave as it enters a new medium at an angle. • When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side. • It only occurs when the two sides of a wave travel at different speeds.

  15. As an ocean wave approaches the shore at an angle, the wave bends, or refracts, because one side of each wave front slows down before the other side does

  16. Diffraction • Diffraction- the bending of a wave as it moves around an obstacle or passes through a narrow opening. • Water waves spread out as they pass through a narrow opening. • A wave diffracts more if its wavelength is large compared to the size of the opening or obstacle.

  17. Interference • Interference-waves can occupy the same region of space and then continue on. • There are two types of interference • Constructive Interference • Amplitudes of two waves add together. • occurs when two or more waves combine to produce a wave with a larger displacement • Destructive Interference • occurs when two or more waves combine to produce a wave with a smaller displacement. (reduced amplitude)

  18. Interference

  19. Standing Waves • Standing Waves - A wave that appears to stay in one place. It does not seem to move through the medium. • Node- point on a standing wave that has no displacement from the rest position. Complete destructive interference  • Antinode- a point where the crest and trough occurs. Midway between two nodes • A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord.

  20. Standing Waves

  21. 17.4 Sound & Hearing • Properties of Sound Waves • Sound waves are longitudinal waves—compressions and rarefactions that travel through a medium • Many behaviors of sound can be explained using a few properties—speed, intensity and loudness, and frequency and pitch.

  22. Speed of Sound Waves • speed of sound varies in different media • In general, sound travels fastest in solids, slower in liquids and slowest in gases

  23. Intensity & Loudness • Intensity is the rate at which a wave's energy flows through a given area. • intensity depends on both the wave's amplitude and the distance from the sound source • Sound intensity levels are measured in units called decibels. • The decibel (dB) is a unit that compares the intensity of different sounds. The decibel scale is based on powers of ten. Lengthy exposure to sounds more intense than 90 decibels can cause hearing damage

  24. Loudness is a physical response to the intensity of sound, modified by physical factors. • As intensity increases, loudness increases • Loudness also depends on factors such as the health of your ears and how your brain interprets the information in sound waves • Sounds too loud can damage hearing and you can develop tinnitus, a ringing in the ears. • Tinnitus (TIN-ih-tus) is noise or ringing in the ears. A common problem, tinnitus affects about 1 in 5 people. Tinnitus isn't a condition itself — it's a symptom of an underlying condition, such as age-related hearing loss, ear injury or a circulatory system disorder.

  25. Frequency & Pitch • The longer the tubing, the longer is the wavelength of the standing wave, and the lower is the frequency of the note produced. • Pitch is the frequency of a sound as you perceive it • Pitch, like loudness, also depends on other factors such as your age and the health of your ears • As a sound source approaches you, the pitch is higher than when the sound is stationary

  26. Ultrasound • Most people hear sounds between 20 hertz and 20,000 hertz • Infrasound is sound at frequencies lower than most people can hear • Ultrasoundis sound at frequencies higher than most people hear. • It is > 120 decibels. This high frequency doesn’t damage hearing. • Side note: Think of your ear drums as tuning forks. They get excited by certain frequencies but not by others. Ultrasonic frequencies are so many octaves above the natural frequencies of your ear drums that there's basically no exitation. That's why you can't hear it. With no exitation, there's no damage (well, unless you crank up the dB's to dangerous levels, but 2kW probably isn't capable of that). Even if you did do damage it would be to a range that you have no hearing for anyway. • Ultrasound is used in a variety of applications, including sonar and ultrasound imaging.

  27. Ultrasound • Sonar is a technique for determining the distance to an object under water. • sound navigation and ranging • Measures the distance to the object is calculated using the speed of sound in water and the time that the sound wave takes to reach an object and the time delay the echo takes to return • Ultrasound imaging • an image of the heart made by sending ultrasound pulses into a patient. • A pulse is a very short burst of a wave

  28. The Doppler Effect • Doppler effect—a change in sound frequency caused by motion of the sound source, motion of the listener, or both. • As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency. (The sounds sound different.) Observer A hears a lower-pitch sound than observer B because the wave fronts are farther apart for observer A

  29. Hearing and the Ear • The outer ear gathers and focuses sound into the middle ear, • Middle Ear receives and amplifies the vibrations. • The inner ear uses nerve endings to sense vibrations and send signals to the brain.

  30. Resonance • Resonance is the response of a standing wave to another wave of the same frequency. • Think of a child being pushed on a swing. If the pushes are timed at the right frequency, the child can swing higher and higher. • In the same way, one wave can “push” another wave to a higher amplitude. • Resonance can produce a dramatic increase in amplitude.

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