Chapter 17

Mechanical Waves & Sound

Waves

A repeating disturbance or movement that transfers energy through matter or space.

A wave will travel as long as it has energy.

Mechanical Waves

Mechanical Wave: is a disturbance in matter that carries energy from one place to another.

Mechanical waves require matter to travel Medium: is a material (matter) that

mechanical waves travel through solid liquid or gas. Ex. Air, water, aluminum, copper,

The speed of mechanical waves changes with different mediums

Mechanical wave is created when a source of energy causes a vibration to travel through a medium

Vibration: a repeating motion that follows a pattern

Sound is created by vibrations Vocal cords

There are 3 types of mechanical waves Transverse waves Longitudinal waves/ Compressional Surface waves

Longitudinal Transverse

Transverse waves: is a wave that causes the medium to vibrate at right angles (perpendicular) to the direction in which the wave travels

Up & Down, or Side to Side motion Draw a picture of a transverse wave Ex.

Water Rope Slinky Electromagnetic (radio waves, infrared, etc)

B. Transverse Waves

Transverse Waves medium moves

perpendicular to the direction of wave motion

Crest: is the highest point above the resting position (top of the wave)

Trough: is the lowest point below the resting position (bottom of the wave)

Resting position: is the flat position of a wave before it starts moving Slinky demo

B. Transverse Waves

Wave Anatomy

crests

troughswavelength

wavelength

amplitude

amplitude

corresponds to the amount of energy carried by the wave

nodes

Longitudinal waves: is a wave in which the vibration of the medium travels parallel to the direction of the wave\ Slinky demo

Compression: a part of a longitudinal wave where the particles are pushed closely together

Rarefaction: a part of a longitudinal wave where the particles are spaced farther apart

C. Longitudinal Waves

Longitudinal Waves (a.k.a. compressional) medium moves in the same direction as wave

motion

C. Longitudinal Waves

Wave Anatomy

rarefaction

compression

wavelength

wavelength

Amount of compression corresponds to amount of energy AMPLITUDE.

Draw a longitudinal wave

Ex. of longitudinal waves Sound

Surface wave: is a wave that has characteristics of both transverse and longitudinal waves

Up & down movement like a transverse Parallel movement of energy like longitudinal Ex.

Ocean Waves Earthquakes (waves through Earth’s surface)

Properties of Waves

Periodic Motion: is motion that follows a repeating pattern

Period: the time period for one interval of movement

Frequency: is the number of complete cycles that pass a point in a given amount of time Frequency of waves are measured in hertz

(Hz)

http://www.kettering.edu/physics/drussell/Demos/waves/wavemotion.html

Wavelength: is the distance of a complete cycle (either crest to crest or trough to trough)

Long wavelength = low frequency Short wavelength = high frequency

D. Measuring Waves

Frequency ( f ) # of waves passing a

point in 1 second

Hertz (Hz)

shorter wavelength = higher frequency = higher energy

1 second

D. Measuring Waves

Velocity ( v ) speed of a wave as it moves forward depends on wave type and medium

v = wave λ × f V:velocity (m/s) λ:wavelength (m)

ƒ: frequency (Hz)

WORK:v = λ × f

v = (3.2 m)(0.60 Hz)

v = 1.92 m/s

D. Measuring Waves

EX: Find the velocity of a wave in a wave pool if its wavelength is 3.2 m and its frequency is 0.60 Hz.

GIVEN:

v = ?

λ= 3.2 m

f = 0.60 Hz

λ

v

f

WORK: f = v ÷ λ

f = (5000 m/s) ÷ (417 m)

f = 12 Hz

D. Measuring Waves

EX: An earthquake produces a wave that has a wavelength of 417 m and travels at 5000 m/s. What is its frequency?

GIVEN:

λ = 417 m

v = 5000 m/s

f = ?

λ

v

f

Wave Speed = wavelength X frequency

Wave speed changes in different mediums

If waves are traveling the same speed, then wavelength and frequency are INDIRECTLY related

Amplitude: is the distance from the resting position to either a crest or trough

Energy and amplitude are DIRECTLY related High energy = high amplitude Low energy = low amplitude

Amplitude in sound is called volume

Light waves travel faster than sound Sound waves travel faster in liquids and

solids than gas Light waves travel faster in gases and

vacuums than in liquids an solids.

Behavior of Waves

Reflection: is when a wave bounces off a surface it can not pass through

Reflection does not change the speed or frequency (the wave can be flipped upside down or side to side) Ex. Mirror

Law of Reflection: the angle of incidence (incoming wave) = the angle of reflection (outgoing wave)

All waves can be reflected The reflection of sound is called an echo

reflection

Reflection terms

normal

Refraction: is the bending of a

wave as it enters a new medium Ex. light waves

Ruler in a beaker of water Ex. sound waves

Listening to sound underwater

Diffraction: is the bending of a wave as is moves around an obstacle or passes through a narrow opening

Page 510 Eddy: is an area behind a mid-stream boulder

where the water flows in a reverse direction (provides safety for rafters)

Chute: is an area of a river where the water is constricted to a narrow passage

defraction

Constructive Interference: is when 2 or more waves combine to form a wave with a larger displacement (amplitude)

Destructive interference: is when 2 or more waves combine to form a wave with a smaller displacement (amplitude) add together

Standing wave: is wave or waves that appear to stay in the same place

Plucking a guitar string Waves in a river Node: is the point on a standing wave where

there is no displacement (amplitude) Antinodes: are the crests and the troughs on

a standing wave

17.4 Sound Waves

Sound waves are longitudinal waves The speed of sound changes due to different

types of mediums Chart 514 Speed: is the distance traveled in a certain

amount of time Meters/second: m/s

Intensity: depends on the amplitude (volume) and the distance from the source

Decibels: (dB) is the unit for sound intensity Chart on 515 Damage to ears around 120 dB

Frequency: is the number of wave cycles to pass a given point in one second

Measured in hertz (Hz) Pitch: is the perceived frequency of sound Different notes in music All the different notes have a unique

frequency

Ultrasound: use sound to locate objects or create pictures

SONAR, fish finders, radar Animals use “echo-location” Bats, dolphins, whales Pregnant ladies get ultra sounds to check the

baby’s health

Doppler Effect: pitch changes due to the object creating the sound moving closer or farther away

Pic on 516

Human Ear

Picture on 517 Outer ear: the collect and funnel the sound

waves into the middle ear Middle ear: amplifies the the vibrations Inner ear: are where nerve endings receive

and send the signal to the brain The brain interprets those signals as sound

Resonance: waves of the same frequency combine (constructive interference)

amplifies the sound Resonance can also cause to vibrate Every object has a natural frequency, if a

sound wave with the same frequency hits it, it will cause the object to vibrate

Seismic waves

Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

http://science.howstuffworks.com/nature/natural-disasters/earthquake4.htm