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