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

2. 1. Explain what causes 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