1. Mechanical Waves and Sound
Chapter 17
Mechanical Waves and Sound

2. Learning Targets Identify and determine properties and types of waves
Describe the relationship between frequency and wavelength
Investigate the use of technology in sound

3. A. Waves
What is a WAVE?
rhythmic disturbances that carry energy through matter or space from one place to another.
2 categories:
Electromagnetic: travel through space
Mechanical : travel through matter
Key Concepts
They travel through a medium (solid, liquid, gas, or combination)
Created when a source of energy causes a vibration to travel through a medium
electromagnetic waves don’t need a medium (e.g. visible light)

4. Waves
Types of Mechanical Waves-The particles in the medium can move in two different ways: either perpendicual or parallel to direction of the wave itself.
Transverse waves
Longitudinal waves
Surface waves

5. B. Transverse Waves Transverse Waves
medium moves perpendicular to the direction of wave motion, Example - long spring

6. C. Longitudinal Waves Longitudinal Waves (a.k.a. compressional)
medium moves in the same direction as wave motion. Example – slinky

7. C. Longitudinal Waves Wave Anatomy compression wavelength rarefaction
Amount of compression corresponds to amount of energy  AMPLITUDE.

8. C. Surface Waves Surface waves
Travels along the surface and separates two media. Combinations of transverse and longitudinal waves. Example - waves on surface of the ocean)

9. Wave Review

10. Wave properties depend on what (type of energy) is making the waves.
The basic properties of waves are amplitude, wavelength, frequency, and speed.
Wave properties depend on what (type of energy) is making the waves.
Wavelength: The distance between one point on a wave and the exact same place on the next wave.

11. The higher the frequency, the more energy in the wave.
2. Frequency: How many waves go past a point in one second; unit of measurement is hertz (Hz).
The higher the frequency, the more energy in the wave.
10 waves going past in 1 second = 10 Hz
1,000 waves go past in 1 second = 1,000 Hz
1 million waves going past = 1 million Hz

12. D. Measuring Waves Frequency ( f )
1 second
Frequency ( f )
# of waves passing a point in 1 second
SI unit : Hertz (Hz)
shorter wavelength  higher frequency  higher energy

13. 3. Amplitude: How far the medium moves from rest position (where it is when not moving).
Remember that for transverse waves, the highest point is the crest, and the lowest point is the trough.

14. Wave Anatomy

15. Parts of longitudinal waves:
Compression: where the particles are close together
Rarefaction: where the particles are spread apart

16. Speed = wavelength x frequency
D. Measuring Waves
Velocity ( v )
speed of a wave as it moves forward
depends on wave type and medium
v =  × f
Speed = wavelength x frequency
v: velocity (m/s)
: wavelength (m)
f: frequency (Hz)

17. D. Measuring Waves f v  GIVEN: WORK: v = ? v =  × f
EX: Find the velocity of a wave in a wave pool if its wavelength is 3.2 m and its frequency is 0.60 Hz.
GIVEN:
v = ?
 = 3.2 m
f = 0.60 Hz
WORK:
v =  × f
v = (3.2 m)(0.60 Hz)
v = 1.92 m/s  v f

18. D. Measuring Waves f v  GIVEN: WORK:  = 417 m f = v ÷ 
EX: An earthquake produces a wave that has a wavelength of 417 m and travels at 5000 m/s. What is its frequency?
GIVEN:
 = 417 m
v = 5000 m/s
f = ?
WORK:
f = v ÷ 
f = (5000 m/s) ÷ (417 m)
f = 12 Hz  v f

19. 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

20. 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

21. 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)

22. 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
constructive interference
destructive interference

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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