Pearson Prentice Hall Physical Science: Concepts in Action

1. Pearson Prentice Hall Physical Science: Concepts in Action Chapter 17 Mechanical Waves and Sound

2. 17.1 Mechanical Waves • Objectives: • 1. Explain what causes mechanical waves • 2. Name and describe the three main types of mechanical waves

3. What Causes Mechanical Waves • Def: A wave is a disturbance that carries energy through space or matter • Def: What a wave travels through is the medium (sound-air, earthquake-ground) • Def: Waves that require a medium to travel are called mechanical waves (majority of waves) • Mechanical waves carry energy from one place to another by using matter (a medium) • Mechanical waves are started with a vibration

4. Types of Mechanical Waves • The three main types of mechanical waves are transverse waves, longitudinal waves and surface waves • Particles in a medium can vibrate up and down or back and forth as a wave moves by • Def: If the particles move up and down they will move perpendicular to the direction of the wave-called a transverse wave

5. Transverse waves take the shape of sine curves (looks like an s on its side) • Def: High points are crests • Def: Low points are troughs • Def: Difference between high and low is called amplitude • Bigger amplitudes mean more energy • Def: in a longitudinal wave the medium moves in the same direction as the wave

6. Def: In longitudinal waves the bunched area is a compression (think of a slinky) • Def: The spaced out areas are called rarefactions • Def: Amplitude on a longitudinal wave is maximum deviation from normal density or pressure • At the boundary between two mediums (on the ocean for example) surface waves develop • Def: Surface waves are combinations of both types of waves • The particles in the medium of a surface wave move back and forth and up and down resulting in a circle, but they end up where they started

7. 17.2 Properties of Mechanical Waves • Objectives: • 1. Explain what determines the frequency of a wave • 2. Solve problems for frequency, wavelength and speed • 3. Describe how amplitude and energy are related

8. Frequency • Def: periodic motion is any motion that repeats at regular time intervals • Def: Frequency is how many waves pass by in a given time • Frequency = 1/period = 1 / T = f • It is measure in hertz (Hz) which is 1/s where s is seconds • Humans can hear 20Hz to 20000Hz • A wave’s frequency equals the frequency of the vibrating source producing the wave

9. Problems involving f, λ (wavelength) & speed • Def: Wavelengths of waves are measured from one crest to the next (crest to crest OR trough to trough) or from one compression to the next • It is represented by the Greek letter lambda whose symbol is λ • The period of a wave is how long it takes for a complete wave to go by a spot, symbol is T • Increasing the frequency (f) of a wave decreases the wavelength (λ)

10. Recall that speed = distance / time • For waves we can use wavelength for distance (in meters, m) and period for time (s for seconds) • Speed = wavelength X frequency v = λ * f • The speed of a wave depends on what it travels through (medium) • If the medium particles are closer together the energy from the wave can make vibrations easier • So waves travels best in solids, then liquids, and worse in air

11. Amplitude and Energy • Def: amplitude is the difference between crest and the rest position or point of origin • Def: the rest position or point of origin is an imaginary line through the middle of the wave that separates the crest from the trough • The more energy a wave has, the greater its amplitude

12. 17.3 Behavior of Waves • Objectives: • 1. Explain reflection and refraction and how they affect waves • 2. Identify several factors that affect the amount of wave diffraction • 3. Discuss two types of interference • 4. Explain what a standing wave is and detail the wavelengths that produce it

13. Reflection and Refraction • Def: Reflection occurs when a wave meets a boundary and bounces off • The wave cannot pass through the surface • Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down • Def: Refraction is the bending of a wave as it travels through different mediums • When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side (ex: pencil in water)

14. Diffraction and Interference • Def: Diffraction is the bending of a wave around an obstacle • A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle • The two types of interference are constructive interference and destructive interference

15. The combo of two or more waves at the same place at the same time causes interference • Def: When two crests meet the interference is constructive-the wave becomes more energetic-the amplitude increases • Def: When a crest and a trough meet they cancel each other and decrease the amplitude –destructive

16. Standing Waves • Interference may cause standing waves- • Def: Standing waves appear not to move along the medium • Has areas of destructive interference where there is no vibration called nodes • Def: a node is a point on a standing wave that has no displacement from the rest position

17. Areas of maximum interference called antinodes • Def: an antinode is a point where a crest & trough meet midway between 2 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

18. 17.4 Sound and Hearing • Objectives: • 1. Describe the properties of sound waves and explain how sound is produced and reproduced • 2. Describe how sound waves behave in applications such as ultrasound and music • 3. Explain how relative motion determines the frequency of sound as the observer hears

19. Properties of Sound Waves • Sound waves are longitudinal waves • They have compressions are rarefactions • Many behaviors can be explained by the properties of speed, intensity, loudness, frequency and pitch • At 20°C in dry air, the speed of sound is 342 m/s • Sound waves travel fastest in solids, slower in liquids and slowest in gases

20. Def: intensity is the rate at which a wave’s energy flow through a given area • The decibel (dB) compares the intensity of different sounds • Def: loudness is a physical response to the intensity of sound modified by physical factors • As intensity increases, loudness increases • Loudness also depends on the health of your ears and how your brain interpret sounds • Def: pitch is the frequency of a sound as you perceive it

21. How Sound Waves Behave & Relative Motion • Ultrasound is used in a variety of applications, including sonar and ultrasound imaging • Def: Sonar is a technique for determining the distance to an object under water • The pitch of a sound is determined by the frequency • Higher pitch means faster frequency • As the source of the waves moves it changes the frequency (this is the Doppler Effect) • As it moves toward you the pitch rises and away from you the pitch lowers = Doppler Effect

22. For us to hear, the outer ear gathers & focuses sound into the middle ear where the vibrations are received and amplified • The inner ear uses nerve endings to sense vibrations and send signals to the brain • Sound is recorded by converting sound waves into electronic signals that can be processed and stored • Sound is reproduced by converting electronic signals back to sound waves

23. Most musical instruments vary pitch by changing the frequency of standing waves • Def: resonance is the response of a standing wave to another wave of the same frequency • Musical instruments often use resonance to amplify sound