1. Sound
Holt Chapter 12

2. Sound is …. An oscillation Longitudinal wave
Cannot travel through a vacuum
Can be reflected, diffracted and can interfere

3. Types of Waves
Mechanical – a wave that propagates through a deformable elastic medium (needs a medium to travel)
2. Electromagnetic – does not need a medium to travel

4. Mechanical Waves
Longitudinal waves - Waves move parallel to the wave direction
EX: Sound Wave

5. Longitudinal Waves Anatomy

6. Longitudinal Waves
Compression- region of high density and pressure (become crests in electric waves)
Rarefaction- region of low density and pressure ( become troughs in electric waves)

7. Pitch
Pitch - a measure of how high or low a sound is perceived to be, depending on the frequency of the sound wave
Pitch is equivalent to frequency

8. Detection of Sound

9. Ear
A sound detector converts the energy of a sound wave into different forms of energy
3 parts of the ear
outer ear
middle ear
inner ear

10. Outer ear Pinna – ( auricle)- catches sound waves
Ear canal- channels sound waves ( the cilia and wax protect)
Ear drum- thin membrane that vibrates

11. Middle Ear 3 Bones malleus ( hammer) incus (anvil) stapes (stirrup)
Eustacian tube – equalizes pressure in the middle ear from the throat

12. Inner Ear Oval Window- thin membrane that vibrates
3 semicircular canals- contains fluid that vibrates
Cochlea- snail like project
has hair like cells tha when bent produce electrical impulse sent to the auditory nerve
Auditory nerve- sends electrical impulse to the brain

13. Human Hearing 20 vps – 20,000 vps Causes of Deafness Brain damage
Extended periods of loud noises
Calcification of bones
Damage to parts of the ear
explosions

14. Ultrasonic- above 20,000 Hz
bats, dolphins, ultrasound medical devices,…
Infrasonic- below 20 Hz
earthquakes, thunderstorms, wind and waves, explosions, compressors, turbines, low speed fans

16. Doppler Shift
Change in the wavelength of sound emitted by a moving source

17. Speed of Sound Depends on the medium
See page 410 table 1
What two statements can be made about the listings in Table 1?

18. Ultrasound See story on page 410
Ultrasonic waves can be used to produce images of objects inside the body
Sound waves are partially reflected when they reach a boundary between two materials of different densities

19. Sound Waves travel best in…
Materials having molecules close together
In solids the worst in gases
Warm heavier air – molecules are closer
Speed of sound in air at room temperature is 343 m/s

20. Lightning and Thunder
Light waves travel nearly 1 million times faster than sound waves in air
Explain the difference in timing of the lightning and thunder.

21. Speed of Sound Equation
v = f l
v = velocity in m/s
f = frequency Hz ( 1/sec)
l = wavelength ( m)

22. Sources of Sound Loudness indicates the amplitude of the wave
Intensity is measured in decibels
Sound waves are produced by vibrating objects
Echo is a reflected wave

24. Musical Instruments
Sound of a wind instrument is the result of a vibrating column of air
Sound of a Stringed instrument is the result of the vibration of a sounding board

25. Strings on Instruments
Higher pitch- tighten string, shorten string, thin strings
Lower pitch – loosen string, lengthen string, thick string

26. Resonance
Is a large motion of a system due to excitation

27. Resonance in pipes
Closed pipe resonates when its length is l/4 3l /4 5 l /4
Open pipe resonates when its length is
l/2 2l /2 3 l /2

28. Open Tube Harmonics or resonance lengths

29. Closed Tube Harmonics or resonance lengths

31. Resonance - closed
A Student is finding the resonance of a closed tube and finds the resonance is spaced by 34.7 cm. The air temp is 23.5 o . What is the speed of sound? What is the frequency of the tuning fork?
0o =331 m/s goes up by 0.6 for every degree
0.6 (23.5) = 345 m/s
Closed: L = l / so l = 4L or v = f (4L)
= 4 ( 0.347m) = m
V = f l f = v / l f = 345 m/s / m
= Hz = 249 Hz

32. Diameter of Resonance Tube
Changes the resonance length
Correction factor: L = l d
Example: 5 cm diameter tube with a resonate length of 46.3cm
L = cm (5cm) = 48.3 cm = 0.483m

33. Harmonics Example- Open
What are the first three harmonics in a 2.45 m pipe, that is open at both ends? Assume that the speed of sound in air is m/s.
fn = n v / 2L where n = 1,2,3,….
F1 = 1 ( 345m/s) / 2(2.45) = 70.4 Hz
F2 = 2 ( 345m/s) / 2(2.45) = 141 Hz
F3 = 3 ( 345m/s) / 2(2.45) = 211Hz

34. Resonance Example - closed
What are the first three harmonics in a 2.45m pipe, when one end is closed? Assume that the speed of sound in air is m/s.
fn = n v / 4L where n = 1,3,5,….
F1 = 1 ( 345m/s) / 4(2.45) = 35.2 Hz
F2 = 3 ( 345m/s) / 4(2.45) = 106 Hz
F3 = 5 ( 345m/s) / 4(2.45) = 176 Hz

35. OR
f1 = 1 ( 345m/s) / 4(2.45) = 35.2 Hz
f2 = 3 ( 345m/s) / 4(2.45) = 106 Hz
or f2 = n f1 = 3 ( 35.2) = 106 Hz
f3 = 5 ( 345m/s) / 4(2.45) = 176 Hz
or f2 = n f1 = 5 ( 35.2) = 176 Hz

36. Resonance ( at least one end is open in these types)