 # Mechanical Waves & Sound

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Mechanical Waves & Sound
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

Vibration: a repeating motion that follows a pattern
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

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

Up & Down, or Side to Side motion Draw a picture of a transverse wave
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
corresponds to the amount of energy carried by the wave wavelength crests amplitude nodes troughs

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
compression wavelength rarefaction Amount of compression corresponds to amount of energy  AMPLITUDE.

Draw a longitudinal wave Ex. of longitudinal waves
Sound

Up & down movement like a transverse
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

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) 1 second shorter wavelength = higher frequency = higher energy

v = wave λ × f D. Measuring Waves V:velocity (m/s) λ:wavelength (m)
Velocity ( v ) speed of a wave as it moves forward depends on wave type and medium V:velocity (m/s) λ:wavelength (m) ƒ: frequency (Hz) v = wave λ × f

D. Measuring Waves f λ v GIVEN: WORK: v = ? v = λ × f
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 WORK: v = λ × f v = (3.2 m)(0.60 Hz) v = 1.92 m/s λ v f

D. Measuring Waves f λ v GIVEN: WORK: λ = 417 m f = v ÷ λ
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 = ? WORK: f = v ÷ λ f = (5000 m/s) ÷ (417 m) f = 12 Hz 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.