1. Chapter 12, Section 1
The Nature of Sound

2. Sonic Boom

3. What causes sound? All sounds are created by something that vibrates
Energy of these vibrations collides with nearby molecules of air and transfers this energy in all directions from the source
Sound is a compressional wave
A series of compressions
and rarefactions move
away from the source.
The result is a sound wave

4. Moving Through Mediums
Sound must have a medium (no sound in outer space!)
Sound can travel through any medium (solid, liquid, gas)
Speed of sound is determined by a medium’s density: the denser the medium, the faster the speed
Why?
The molecules in a dense medium are closer and transfer energy more rapidly
Air=437 m/s; Water=1498 m/s; Steel=5200 m/s

5. Moving Through Mediums
Careful: the speed of sound DOES NOT depend on loudness (amplitude)
Speed of sound is also determined by temperature
As temperature increases, the speed of sound increases
As temperature increases, molecules move faster
0°C Air=332 m/s; 20°C Air=344 m/s

6. Human Hearing

7. Human Hearing Outer ear: gathers sound waves
Middle ear: amplifies (makes louder) sound waves
Inner ear: converts sound waves into nerve impulses
Brain: decodes and interprets nerve impulses

8. Outer Ear
Outer ear parts: auricle (pinna), ear canal, eardrum (tympanic membrane)
Auricle directs sound waves into the ear canal.
Ear canal (2-3 cm long) is a passageway to the eardrum.
Eardrum (0.1 mm thick) is a tough membrane that vibrates.

9. Human Hearing

10. Middle Ear
Middle ear parts: hammer (malleus), anvil (incus), stirrup (stapes), oval window
Eardrum vibrations cause the 3 tiny bones (hammer, anvil, stirrup) to vibrate.
These bones make a lever system that amplifies the sound wave.
The stirrup is connected to the oval window which transfers the vibration to the inner ear.

11. Human Hearing

12. Inner Ear Inner ear parts: cochlea, hair cells
Oval window transfers vibrations to the cochlea, a spiral-shaped structure filled with fluid.
Within the cochlea’s fluid are tiny hair cells.
The hair cells vibrate and stimulate the auditory nerve that sends the information to the brain.

13. Human Hearing