UNIT 5: VIBRATIONS, WAVES & SOUND

Waves and Oscillations

Pendulum swinging back and forth shows how oscillations can create waves.

WAVES Energy transfer: by doing work, by heat, or by

waves! Wave: a disturbance (vibration) that travels

mechanical waves require a material medium (solid, liquid, or gas) – particles vibrate in simple harmonic motion (water, sound, earthquake waves)

2 types – transverse and longitudinal electromagnetic waves travel through a material

or a vacuum – vibrating electric and magnetic fields (radio, microwave, infrared, visible light, ultraviolet, x-ray, gamma rays)

WAVES Transverse

waves: vibrations are perpendicular to wave direction

WAVES Longitudinal

waves: vibrations parallel to wave direction

rarefaction

WAVES Frequency, f: number of waves each second,

unit: Hertz (Hz) 1 Hz = 1 wave/sec Period, T: time for one wave to pass, unit: s

f=1/T Wavelength, : distance between identical

points on two waves, unit: m

WAVES Amplitude, A: maximum displacement

from equilibrium, unit: m

Wave speed, v: speed of the wave, not the particles, unit: m/s v=f use difference in wave speeds to find distance

ex: lightning & thunder

WAVE INTERACTIONS Reflectio

n: waves "bounce back" at boundary

WAVE INTERACTIONS Law of Reflection: i = r i: incidence, r:

reflection

WAVE INTERACTIONS Refraction: wave path bends as wave

crosses boundary. Note that speed & wavelength change as wave moves into new

medium, but frequency remains constant.

Refraction Examples 1. Dish filled with water 2. Light through

glass

Refraction Rules When a wave goes from fast to

slow mediums the wave will bend toward the normal.

When a wave goes from slow to fast mediums the wave will bend away from the normal.

Vi Sin r = Vr Sin i

WAVE INTERACTIONS Diffractio

n wave spreads out or “bends” beyond edge of barrier

WAVE INTERACTIONS Diffraction

greatest when is greater than or equal to the size of opening or object

SOUND INTERACTIONS Resonance (sympathetic vibration)

objects have natural vibrating frequency

sending waves to an object at at its natural frequency will make it vibrate

pushing a child on a swing using microwaves to heat up water

SOUND WAVES Source

: a vibrating object (vocal cord, string, reed, etc.)

SOUND INTERACTIONS

WAVE INTERACTIONS Interference:

waves pass through each other without changing each other, but their displacements add together

WAVE INTERACTIONS constructive interference: combined

wave displacement is greater than individual waves

WAVE INTERACTIONS destructive interference: combined

wave displacement is less than individual waves

WAVE INTERACTIONS Standing

Waves: interference of two identical waves goingopposite directions makes waves appear to vibrate in place

WAVE INTERACTIONS Standing Waves:

nodes: no displacement

antinodes: maximum displacement

Harmonic number is how many crests are trapped

SOUND WAVES Pitch: musical tone or note – frequency of a

wave sonic spectrum:

C major scale C D E F G A B C

frequency (Hz)

264

297

330

352

396

440

495

528

musical scale: specific proportional frequencies

MUSICAL INSTRUMENTS =2L/n

L: length of string, and n is 1,2,3…

f=v/v: wave speed in string

v=√TL/mT: tension, m: mass of string

MUSICAL INSTRUMENTS Stringed Instruments

quality: mixture of fundamental and harmonics (makes different instruments sound different)

sound boards & boxes: more air surface contact - amplifiers

MUSICAL INSTRUMENTS

MUSICAL INSTRUMENTS Wind Instruments

pitch = frequency of vibration of column of air

f = v/v: sound speed in air = 340 m/s : wavelength, depends on length of air column

MUSICAL INSTRUMENTS open-end tube: each end of tube is

antinode = 2L/n L: length of tube and n is

1,2,3… Examples: flutes, saxophones, some

organ pipes

MUSICAL INSTRUMENTS closed-end tube: closed end of

tube is node =4L/n L: length of tube and n is

1,3,5 Examples: clarinets, some pipe

organs

PHYSICS

UNIT 5: VIBRATIONS, WAVES & SOUND

SOUND INTERACTIONS The

Doppler Effect: apparent change in frequency due to motion of source or listener

SOUND INTERACTIONS Wave speed stays constant, remember

wave speed depends on medium. Frequency changes and wavelength

changes When source or observer moves toward

each other wavelength decreases and frequency increases.

When source or observer moves away from each other wavelength increases and frequency decreases

SOUND INTERACTIONS Radar: uses Doppler Effect in radio

waves reflected off an object to determine its speed (speed traps, locating enemy aircraft)

Red shift (decreased frequency) and Blue shift (increased frequency) of light tells astronomers whether a star or galaxy is moving toward or away from Earth.

SOUND INTERACTIONS

SOUND INTERACTIONS The Doppler Effect

sound barrier: “pile-up” of sound waves (pressure) in front of object traveling Mach 1

sonic boom: cone-shaped pressure pulse following an object traveling at supersonic speeds (bow wave, also called water wake, following a speedboat)

SOUND INTERACTIONS

PHYSICS

UNIT 5: VIBRATIONS, WAVES & SOUND

QUIZ 5.4 The speed of sound in earth is 3500 m/s. An

earthquake wave, frequency 5 Hz, travels from its source to a distant mountain range and returns in 3.4 minutes.

(a) How far away is the mountain range? (b) What is the wavelength of the earthquake

wave? (c) If the mountain range was moving away

at 0.50 m/s. what would be the frequency of the reflected wave?

357,000 m

700 m

5.00 Hz

UNIT 5 REVIEW

f = 1/T v = f i = r

visinr =

vrsini

node dist = /2

loop height = 4A

v = 331 + 0.6T

I = P/4r2

= 10log(I/

I0)

I0 = 1×

10-12 W/m2

open pipe = 2L

closed pipe = 4L

x = vt

km

2Ts gL

2Tp

B

v

E

vmTL

v

S

Lsh vv

vvff

S

Lsh vv

vvff