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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
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
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
Is the speed of sound always constant in air?
• Speed of Sound changed by:1. Water vapor
• Why?
• H2O molecules (less mass) move faster than O2 or N2; shortens collision time2. Increased temperature
• Why?• Warm air --> faster moving
particles
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
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!!!
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 O2 and N2 molecules
Intensity vs. Loudness• Sound Intensity:
– Objective• Measurable data
– measured by instruments (oscilloscope)
• Loudness: – Subjective
• opinion
– sensation sensed by brain
Sound IntensitySound Intensity
Source of SoundSource of Sound Level (dB)Level (dB)
Jet engine 140
Threshold of pain 120
Loud Rock Music 115
Subway Train 100
Average Factory 90
Busy Street Traffic 70
Normal Speech 60
Library 40
Close Whisper 20
Normal Breathing 10
Hearing threshold 0
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?