1. Sound
AP Physics
Chapter 12

2. 12.1 Characteristics of Sound
Vibration and Waves
12.1 Characteristics of Sound

3. 12.1 Characteristic of Sound
Sound is a longitudinal wave
Caused by the vibration of a medium
The speed of sound depends on the medium it is in, and the temperature
For air, it is calculated as
12.1

4. 12.1 Characteristic of Sound
Loudness – sensation of intensity
Pitch – sensation of frequency
Range of human hearing – 20Hz to 20,000 Hz
ultrasonic – higher than human hearing
dogs hear to 50,000 Hz,
bats to 100,000 Hz
infrasonic – lower than human hearing
12.1

5. 12.1 Characteristic of Sound
Often called pressure waves
Vibration produces areas of higher pressure
These changes in pressure are recorded by the ear drum
12.1

6. 12.2 Intensity of Sound: Decibels
Vibration and Waves
12.2 Intensity of Sound: Decibels

7. 12.2 Intensity of Sound: Decibels
Loudness – sensation
Relative to surrounding and intensity
Intensity – power per unit area
Humans can detect intensities
as low as W/m2
The threshold of pain
is 1 W/m2
12.2

8. 12.2 Intensity of Sound: Decibels
Sound intensity is usually measured in decibels (dB)
Sound level is given as
I – intensity of the sound
I0 – threshold of hearing (10-12 W/m2)
– sound level in dB
Some common relative intensities
Source of Sound
Sound Level
(dB)
Jet Plane at 30 m
140
Threshold of Pain
120
Loud Rock Concert
Siren at 30 m
100
Auto Interior at 90 km/h 75
Busy Street Traffic 70
Conversation at 0.50 m 65
Quiet Radio 40
Whisper 20
Rustle of Leaves 10
Threshold of Hearing
12.2

9. Vibration and Waves
12.3 The Ear

10. Steps in sound transmission
12.3 The Ear
Steps in sound transmission
12.3

11. 12.4 Sources of Sound: Strings and Air Columns
Vibration and Waves
12.4 Sources of Sound: Strings and Air Columns

12. 12.4 Sources of Sound: Strings and Air Columns
Vibrations in strings
Fundamental frequency
Next Harmonic
12.4

13. 12.4 Sources of Sound: Strings and Air Columns
Vibrations in strings
Next Harmonic
Strings produce all harmonics – all whole number multiples of the fundamental frequency
12.4

14. 12.4 Sources of Sound: Strings and Air Columns
Vibrations in an open ended tube (both ends)
Fundamental frequency
Next Harmonic
(These are graphs of air displacement, pressure graphs would show the opposite, eg low pressure at the opening)
12.4

15. 12.4 Sources of Sound: Strings and Air Columns
Vibrations in open ended tubes
Next Harmonic
Open ended tubes produce all harmonics – all whole number multiples of the fundamental frequency
Examples include organ pipes and flutes
12.4

16. 12.4 Sources of Sound: Strings and Air Columns
Vibrations in an closed end tube (one end)
Fundamental frequency
Next Harmonic
12.4

17. 12.4 Sources of Sound: Strings and Air Columns
Vibrations in open ended tubes
Next Harmonic
Closed end tubes produce only odd harmonics
Examples include reeded instruments and brass instruments
12.4

18. 12.6 Interference of Sound Waves; Beats
Vibration and Waves
12.6 Interference of Sound Waves; Beats

19. 12.6 Inteference of Sound Waves; Beats
If waves are produced by two identical sources
A pattern of constructive and destructive interference forms
Applet
12.6

20. 12.6 Inteference of Sound Waves; Beats
path length difference
If the path length difference is = a whole number multiple of the wavelength, constructive interference occurs.
If the pld is = ½, 1 ½, 2 ½, … times the wavelength, destructive interference occurs.
12.6

21. Vibration and Waves
12.7 The Doppler Effect

22. Doppler Effect 12.7 The Doppler Effect
For a source moving towards a stationary observer:
𝑓 ′ = 𝑓 1− 𝑣 𝑠𝑜𝑢𝑟𝑐𝑒 𝑣 𝑠𝑜𝑢𝑛𝑑
For a source moving away from a stationary observer:
𝑓 ′ = 𝑓 1+ 𝑣 𝑠𝑜𝑢𝑟𝑐𝑒 𝑣 𝑠𝑜𝑢𝑛𝑑
12.7