1. Sound
Longitudinal wave
requires a medium
(cannot travel in a vacuum)

2. Study of Sound
is known as
ACOUSTICS

3. Properties of Sound A sound is a vibration
The vibrating causes the air molecules near the movement to be forced closer. This is called compression
As the vibration moves on, the density and air pressure becomes lower than normal and is called rarefaction
Pressure wave – longitudinal
Frequency = pitch
v = 334 m/s in air at room temperature
Velocity is dependent upon the material

4. Do molecules move faster or slower as temperature increases?
Can
affect
speed
Do molecules move faster or slower as temperature increases?
So would sound travel faster or slower as temperature increases?

5. Waves travel fastest in solids, slowest in gases.
Type of Medium
Solid
Liquid
Gas
Waves travel fastest in solids, slowest in gases.

6. Speed of sound
Fastest in solids, slowest in gases.
Air = 340 m/s
water= 1440 m/s
steel = 5000 m/s
Supersonic: faster than the
speed of sound.

7. Sound travels faster in warm water than in cold water
Speed of sound
Sound travels faster in warm water than in cold water
By measuring the time it takes for sound to travel a known distance through the ocean the average temperature of the water can be calculated = ATOC (acoustic thermometry of ocean climate)

8. Speeds of Sound at T = 20 C Air Helium Hydrogen Water Sea water
Iron/Steel
Glass
Aluminum
343 m/s
1005 m/s
1300 m/s
1440 m/s
1560 m/s
≈5000 m/s
≈ 4500 m/s
≈ 5100 m/s

9. Pitch The highness or lowness of sound.
Depends on the frequency of sound waves.
High frequency = High pitch
Low frequency = Low pitch

10. Depends on the amplitude of sound waves. (amplifier)
Intensity
Also called LOUDNESS
Amount of energy
Depends on the amplitude of sound waves. (amplifier)
Large Amp. = Loud sound
Small Amp. = Soft sound

11. Intensity of Sound
Unit is the “Bel”. Named after Alexander Graham Bell
More commonly used is the decibel (dB)
= 0.1 Bel
 (in dB) = 10 log(I/I0)
I is the intensity ( A2)
I0 = 1.0 x W/m2 the “threshold of hearing”

12. Some Intensities (in dB)
Jet plane at 30 m
Threshold of pain
Indoor rock concert
Auto interior
Street traffic
Conversation
Whisper
Rustle of leaves
140
120 75 70 65
1x10-10
1x10-11

13. Reverberation Sound waves reflecting from hard surfaces
Ex.: Multiple echo resulting from the direct sound AND the reflected sound

14. Reverberation vs Echo Animations courtesy of Paul Hewitt
and borrowed from physicsclassroom.com

15. Measurement of loudness
Decibels
Near total silence - 0 dB
A whisper - 15 dB
Normal conversation - 60 dB
Lawnmower - 90 dB
Threshold of pain dB
A rock concert or a jet engine dB
Gunshot, firecracker dB

16. Sound is a pressure wave
Animations courtesy of Paul Hewitt
and borrowed from physicsclassroom.com

17. Ear

18. Human Ear 20Hz Hz
Infrasonic: below 20 Hz
(below our hearing)
Ultrasonic Sound:
above 20,000Hz
(above our hearing)

19. Tuning fork creating a sound wave
Animations courtesy of Paul Hewitt
and borrowed from physicsclassroom.com

20. Ultrasounds
Ultrasound can be used to create internal images of the human body
Ex. Pregnant woman gets a “picture” of her unborn baby

21. White Noise An equal mixture of all frequencies of sound
Used to calm stress
Used in office buildings

22. Noise Pollution Yep! There is such a thing Causes:
Damage to ear resulting in hearing loss
Stress

23. Dead Spots Caused by destructive interference
Result – hardly any or no sound
Bad for concert halls – designers be careful!

24. Used to locate underwater objects and distances.
sonar
Used to locate underwater objects and distances.
***Reflection**

25. Doppler Effect
Apparent change in frequency (pitch) of a sound from a moving source.
Source moving toward observer:
f’ = f / (1-vs/v)
Source moving away from observer:
f’ = f / (1+vs/v)
f = frequency of source,
f’=frequency heard by observer,
vs = velocity of source, v = velocity of sound

26. Change in pitch due to motion.
Doppler
effect
Change in pitch due to motion.

27. Figure 6. 11: The Doppler effect
Figure 6.11: The Doppler effect. (b) The clanging bell on a moving fire truck produces sounds that move outward (black circles). An observer ahead of the truck hears the clangs closer together, while an observer behind the truck hears them farther apart.

28. Figure 6. 11: The Doppler effect
Figure 6.11: The Doppler effect. (c) A moving source of light emits waves that move outward (black circles). An observer in front of the light source observes a shorter wavelength (a blue shift), and an observer behind the light source observes a longer wavelength (a red shift).

29. Doppler effect zoom *Moving towards increases the pitch
*Moving away decreases the pitch
*Think of sirens
zoom

30. Guitar String creating a sound wave
Animations courtesy of Paul Hewitt
and borrowed from physicsclassroom.com