1. Sound
Chapter 15

2. Sound Waves
Produced by vibrating object: strings, vocal cords, reeds, air columns
Frequency of sound wave equals frequency of vibration
Frequency directly related to pitch
Humans hear sounds between 20 Hz and 20,000 Hz

3. Sound Waves
Sounds below 20 Hz are infrasonic; above 20,000 Hz are ultrasonic
Sound waves are longitudinal waves
Composed of compressions and rarefactions, high and low pressure areas
Sound needs a medium; can travel through solid, liquid or gas but not in a vacuum

4. Speed of Sound Depends on the medium it is traveling through
Travels fast in liquids and much faster in elastic solids than in air
Speed in air depends on temperature and humidity

5. Speed of Sound
Ranges from about 330 m/s to 350 m/s (or about 1100 ft/s or 760 mph)
Faster in warm air due to more kinetic energy and humid air due to more density
In aviation, speed of sound is called Mach 1, twice speed of sound Mach 2, etc.

6. Sound Intensity & Loudness
Intensity is measurable quantity proportional to square of wave amplitude
Loudness is physiological measure of sound intensity
Depends on hearing of listener
Varies logarithmically with intensity
Measured on decibel scale (dB)

7. Sound Intensity and Loudness
Decibel change is equal to 10 times log of the ratio of two sound intensities
Increase in loudness of 20 dB means intensity increases by factor of 10 squared or 100 times
Change of 30 dB means factor of 10 cubed or 1000

8. The Doppler Effect
The apparent change of frequency due to motion between wave source and receiver
Heard as change in pitch of sound of moving objects
Occurs with all types of waves
Used by police radar to check speed
Astronomers measure color shift in starlight

9. Forced Vibration and Resonance
Forced vibration is setting an object into vibration by contact with another vibrating object
Principle behind sound production in stringed instruments
Most objects have a natural frequency of vibration that depends on size, shape, elasticity

10. Forced Vibration and Resonance
Natural frequency is also resonant frequency of object
When forced to vibrate at its natural frequency, resonance occurs and wave amplitude (loudness) is greatly increased
Important in design of buildings, bridges, musical instruments
Principle behind TV and radio tuners

11. Resonance in Tubes
Air column in a tube or pipe will vibrate—organ pipes, wind instruments, blow across bottle
Resonance occurs when wave fits in length of tube

12. Interference of Sound Waves
Can occur between 2 sources (like speakers) or between one source and its reflection
Constructive and destructive interference affects loudness of sounds heard at different locations
Destructive interference can be used to cancel noise

13. Beats
Caused by interference between two sounds of nearly the same frequency
Heard as pulsing loudness
Number of beats heard per second (beat frequency) equals difference between two original frequencies sounded
Can be used to tune musical instruments to a reference pitch

14. Review: Sound produced by vibration
longitudinal wave-compressions & rarefactions
must have medium
speed depends on medium, temperature and density
loudness measured on logarithmic decibel scale

15. Review: Sound
Resonance due to forced vibration at natural frequency increases wave amplitude & energy
Interference can occur changing loudness
Beats are caused by interference between two sound sources of nearly identical frequencies

16. Important Terms infrasonic ultrasonic longitudinal wave compression
rarefaction
medium
pitch
intensity
loudness
decibel scale
forced vibration
natural frequency
resonance
beats