Transcript Sound 26.4

Sound
26.4-26.7
Objectives
1.
2.
3.
4.
5.
Factors that affect the speed of sound
Loudness vs. sound intensity
Examples of forced vibration
How a sounding board works
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 3second 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 time
2. 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 Intensity
Source of Sound
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?
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