P199: Introduction & Start of Sound
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Transcript of P199: Introduction & Start of Sound
P199: Introduction & Start of SoundLecturer Stacy McCormackPhysics Department, Indiana University
Topic 1: SoundVIBRATIO
N
Sound in space?
MISCONCEPTION!Molecules in solid, liquid, gas
Sound Waves & TermsLongitudinal wave-individual vibrating objects move right or left and the disturbance also moves right or left (ex. sound wave)Transverse wave-individual objects move up and down, but disturbance moves right or left (ex. water wave)
Amplitude-height from rest position (determined by the amount of energy), loudness
Wavelength-distance between two like molecules (humans hear from 0.65 in to 54 ft, musical note A is 31 in)
Frequency & Pitch• Frequency-how quickly the vibration is occurring,
how much time is needed to complete a cycle• High frequency-very fast, many cycles per second• Low frequency-slower, less cycles per second• Measured in Hertz (Hz)-known as cycles/second• Musical note A has a frequency of 440 Hz• Humans can hear 20 Hz-20,000 Hz
• Pitch is our perception of frequency
• High pitch=high frequency=soprano singer
• Low or deep pitch=low frequency=bass singer
Frequency vs. Wavelength
Why are some sounds high-pitched and others low-pitched? Node-a place where
there is NO VIBRATION-the molecule always remains in the same location
Antinode-a place where there is MAXIMUM VIBRATION-the molecule is moving
NOTE: Distance between adjacent node and antinode is ¼ wavelength
Tuning fork
Antinode: MAXIMUM VIBRATION
Node: NO VIBRATION
Large or long distance between node and antinode=large wavelength=small frequency=low pitch
Small or short distance between node and antinode=small wavelength=large frequency=high pitch
Pitch and Tuning ForksLarge distance between node and antinode
Large wavelength
Small frequency
Low pitch
Small distance between
node and antinode
Small wavelength
Large frequency
High pitchRemember: distance between node and antinode is ¼
wavelengthSo to find wavelength, take N-A distance and multiply by
4!
N N
A A
Drinking straw demonstration
• Two antinodes at the ends because molecules have maximum vibration
• Since antinodes are always separated by nodes, there must be a node in the center
• Can determine wavelength
• Longer straw=higher or lower pitch?
A AN
Demonstration with two twists
Closing off one end makes a node
Makes wavelength longer than if just left open
Lowers pitch
N A
Using scissors, add a hole at the halfway point
Creates an antinode
Shortens wavelength
Higher pitchN NA AA