Transcript Sound - Spring Branch ISD

Sound
Sound is caused by vibrations which sets air molecules in motion
Crests correspond to compressions and
troughs correspond to rarefactions. The x-axis
represents time and y-axis represents density
Frequency
• Audible – what we hear (20 – 20000Hz)
• Infrasonic – less than 20Hz (Elephants)
• Ultrasonic – greater than 20000Hz (Dogs)
– higher frequencies have smaller wavelengths
so they can be absorbed by smaller objects
– Can produce images
• Pitch -is determined by frequency
– Higher f, higher pitch
– High or lowness of a sound based on
frequency of sound waves
Speed of Sound
• Depends on the medium
– State of matter
• Travel fastest in solids due to density
• Travel slowest in gases due to density
– Temperature
• High temperature means more motion due to more
kinetic energy
-Sound travels in
3D
-Assume sound
waves are
spherical
-crest or dark
lines of circle
represent the
compressions
-lines between
circles are
rarefactions and
represent the
trough
Doppler Effect
Stationary Source
Moving Source
Doppler Effect - Apparent change in the object’s frequency
due to the relative motion of the source and the observer
-it is an APPARENT not real change. The source
would not hear a change in frequency
Sound Intensity
Intensity: W/m2
Power: W
r: distance from source (m)
Audible Sound
• Determined by frequency and intensity
– Threshold of hearing
• I = 1.0 x 10-12 W/m2
– Threshold of pain
• I = 1.0 W/m2
• Middle can be heard at lower intensities
•
Higher and lower frequencies can be heard if intensity is
greater
Relative Intensity
Intensity (W/m2)
1 x 10-12
1 x 10-11
1 x 10-10
1 x 10-9
1 x 10-8
1 x 10-7
1 x 10-6
1 x 10-5
1 x 10-4
1 x 10-3
1 x 10-2
1 x 10-1
1 x 100
1 x 101
1 x 103
Decibel level (dB)
0
10
20
30
40
50
60
70
80
90
100
110
120
130
150
Examples
Threshold of hearing
Rustling leaves
Quiet whisper
Whisper
Mosquito buzzing
Normal conversation
Air conditioning at 6 m
Vacuum cleaner
Busy traffic, alarm clock
Lawn mower
Subway, power motor
Auto horn at 1 m
Threshold of pain
Thunderclap, machine gun
Nearby jet airplane
• Decibel increases by 10 which corresponds to a 10
factor on intensity.
– Ex) Decibel change of 30, means intensity changed by 1000
Tacoma Narrows Bridge
http://www.youtube.com/watch?v=3mclp9QmCGs
(Link is also on my website)
Resonance = when the natural frequency of a on
object is met by a forced frequency on the object.
-Forced frequency can be made by people,
weather, or other factors
-Sound
travels from Outer ear to
middle ear to inner ear.
-Sound travels from ear canal to
the outer ear. Ear canal ends at
eardrum which vibrates and
transfers vibrations to the
middle ear.
-The 3 bones in middle ear then
vibrate which then transfer the
vibrations to the inner ear.
-The cochlea in the inner ear
has membranes of different
frequencies at different
positions.
-These create impulses in
different nerve fibers which are
sent to the brain to interpret.
Standing
Waves
-Wave created when
a wave is traveling
from 2 different
directions
-Harmonic Series:
series of frequencies
that include
fundamental and
multiples of that
fundamental
Open pipe has
antinodes at
both ends due to
complete
freedom
Closed pipe
(like
instruments)
has a fixed
end on one
side which
represents
the node.
Open end is
the antinode
Keys on an instrument change column length which changes the
harmonics (notes)
-Beats are formed by the interference of two waves of slightly different
frequencies moving in the same direction
-Difference in frequency = number of beats per second