1. Sound Waves
1986 world cup in mexico

2. Announcement
Quiz will be tomorrow, NO EXCEPTIONS!
Chapter 11

3. Goal of the class
To understand sound waves, frequency and pitch, and the Doppler Effect
Question of the Day: What is infrasonic sound?
Previous Answer: At a fixed boundary they are reflected and inverted, otherwise just reflected at a free boundary.
Previous question: What happens to reflected waves at a boundary?
Label equation if needed

4. Sound Wave Production
The production of sound waves is caused by vibrations.
When a tuning fork vibrates the prongs bump into nearby air molecules causing them to move.
The sound spreads out from the source in a circular pattern.

5. Sound Waves
As a result of superposition of waves, interference can be observed. Interference is an effect caused by two or more waves. Waves can interfere constructively or destructively.
Show compression and rarefractions.
Sound waves are longitudinal (direction of vibrations are parallel to the wave motion)

6. Characteristics
The audible frequency of the average human is between 20 Hz and 20,000 Hz (but can vary slightly)
Humans lose the ability to hear high frequencies with age.
Show siren to drive off youths from shopping mall.
Ask if they know any animals that can hear outside of this range.
Elephants can hear lower frequencies, dogs higher.
Mention dog whistles.

7. Frequencies Higher frequencies than 20,000 Hz are called ultrasonic
Lower frequencies than 20 Hz are called infrasonic.
Kidney stones

8. Frequencies The frequency determines the pitch of the sound.
The pitch of a sound refers to how humans perceive the sound.
Pitch can depend on loudness and background noise

9. Speed of Sound
The speed of sound depends on the medium it’s travelling through.
The sound can travel as fast as the vibrations with in the medium
The speed of sound is generally faster in solids than liquids, and liquids generally faster than gases.
Elbow on the ear trick
Bones of the ears vibrate
Or hammer drill

10. Speed of Sound Speed of sound sometimes called mach
Draw waves for each!
Speed of sound sometimes called mach

11. Sound Waves Waves propagate in three dimensions.
The wave fronts are approximately spherical from the source.
We can draw this as 2D by using concentric circles.
When you listen to music your position in the room doesn’t matter too much
Waves spread out in 3d

12. Sound Waves
As the circles get bigger the sound ‘spreads out’ causing a decrease in intensity
At far enough away the spherical waves approximate plane waves.
Quieter the further you are away.
Inverse square law.
We did plane waves (1D)

13. Doppler Effect
You hear this everyday when a police car goes past with its siren on.
The frequency of the sound changes with relative motion
What does it sound like inside the police car?
Austrian Christopher Doppler
Also used in astronomy with visible light (red shift) to see the motion of stars / galaxies. p428

14. Homework Read chapter 12 – Section 1
Questions on P432 Q1, 2, 6, 9, 11, 12