1. Sound 26.4-26.7

2. Objectives 1.Factors that affect the speed of sound 2.Loudness vs. sound intensity 3.Examples of forced vibration 4.How a sounding board works 5.Describe natural frequency

3. What’s Faster: Sound or Light? Light Any examples? What’s the speed of sound in normal air temp (@ 20C or 68F)? 340m/s –1 millionth the speed of light

4. How Far Away is the Storm? How far away is a storm if you note a 3- second delay between a lightning flash and the sound of thunder? d = vt –340 m/s x 3s = 1020m

5. Is the speed of sound always constant in air? Speed of Sound changed by: 1.Water vapor Why? H 2 O molecules (less mass) move faster than O 2 or N2; shortens collision time 2.Increased temperature Why? Warm air --> faster moving particles

6. What does the speed of sound depend on when traveling through materials? The speed of sound depends on the elasticity of a material. What is elasticity? Elasticity: –The ability of a material to change shape and then resume it’s initial shape Atoms close together & respond quickly to each others motion. Elasticity is NOT “stretchability.” –Is this rubber band elastic? –Steel is one of the most elastic material around

7. Speed of Sound 4 x’s as fast in water than in air 15 x’s as fast in steel than in air Just to keep things in perspective: light (in air) is 1million x’s faster!!!

8. Checkpoint 1.Is sound going to travel faster on a foggy day or a clear day, given all other conditions are the same? Why?  Foggy day. Moisture in the air, water molecules move faster than O 2 and N 2 molecules

9. Intensity vs. Loudness Sound Intensity: –Objective Measurable data –measured by instruments (oscilloscope) Loudness: –Subjective opinion –sensation sensed by brain

10. Sound Intensity Source of SoundLevel (dB) Jet engine140 Threshold of pain120 Loud Rock Music115 Subway Train100 Average Factory90 Busy Street Traffic70 Normal Speech60 Library40 Close Whisper20 Normal Breathing10 Hearing threshold0

11. Forced Vibrations & Sound Boards Acoustic guitar –String --> saddle --> soundboard--> ear Soundboard: –Larger surface whose vibrations reinforce sound of instrument Vibration of string sets larger surface in motion Acoustic guitar vs. electric guitar (unplugged). Which is louder? Why?

12. Demos Demos: –tuning fork on table –Music box –Record player with paper/pin –Talkie strips: How can we make this louder? I want you to write down in your notes how each of these things work