Sound WavesSound Waves

Physics Chapter 13 Section 1Physics Chapter 13 Section 1

I. Production of sound wavesI. Production of sound waves Produced by an object vibrating Produced by an object vibrating

-ex. Tuning fork-ex. Tuning fork Prongs move back and forth pushing the air Prongs move back and forth pushing the air

molecules together or spreading them apartmolecules together or spreading them apart

Compression Compression medium molecules move medium molecules move togethertogether

higher density and pressurehigher density and pressure Rarefaction Rarefaction medium molecules spread apart medium molecules spread apart

lower density and pressurelower density and pressure

Online Tuning ForksOnline Tuning Forks

As the tuning fork vibrates it sends a series of As the tuning fork vibrates it sends a series of compressions and rarefactions that expand out compressions and rarefactions that expand out in all directions.in all directions.

*like pond ripples*like pond ripples

*air molecules vibrate*air molecules vibrate Sound waves are longitudinal waves (particles Sound waves are longitudinal waves (particles

move parallel to wave direction)move parallel to wave direction)

CompresionalCompresional waves video waves video

II. Characteristics of sound wavesII. Characteristics of sound waves

Frequency – number of cycles per unit of timeFrequency – number of cycles per unit of time audible sound waves audible sound waves (humans can hear)(humans can hear)

frequency range 20 Hz to 20,000 Hzfrequency range 20 Hz to 20,000 Hz frequencies < 20 Hz frequencies < 20 Hz infrasonic (very long infrasonic (very long

wavelength) sound waves.wavelength) sound waves. frequencies > 20,000 Hz frequencies > 20,000 Hz ultrasonic (short ultrasonic (short

wavelength) sound waves.wavelength) sound waves.

audible depends on our ability to detect audible depends on our ability to detect factors: age, ear damage due to excessive loud factors: age, ear damage due to excessive loud

noisesnoises

as frequency increases, wavelength decreasesas frequency increases, wavelength decreases frequency determines pitchfrequency determines pitch pitch:pitch: how high or low we perceive a sound to how high or low we perceive a sound to

bebe depending on the frequency of a sound wave. depending on the frequency of a sound wave. low pitch = low frequencylow pitch = low frequency high pitch = high frequencyhigh pitch = high frequency * a perceived measurement** a perceived measurement*

ultrasonic waves produce images of objectsultrasonic waves produce images of objects

short wavelengthshort wavelength

provides medical usesprovides medical uses

- images of internal body structures- images of internal body structures

- waves reflect off small objects of varying- waves reflect off small objects of varying

densitiesdensities

ex. Ultrasounds of fetusesex. Ultrasounds of fetuses Ultrasonic pulses emitted and received as they Ultrasonic pulses emitted and received as they

reflect off of fetal tissuesreflect off of fetal tissues

Ultrasound PicturesUltrasound Pictures ex. Echolocation of dolphins and batsex. Echolocation of dolphins and bats

Speed of sound depends on the mediumSpeed of sound depends on the medium

sound waves can pass through solids,sound waves can pass through solids,

liquids, and/or gases (mechanical waves)liquids, and/or gases (mechanical waves)

Depends on how quickly particles of the Depends on how quickly particles of the medium can transfer the vibrationmedium can transfer the vibration

More dense (particles are closer together) = More dense (particles are closer together) = faster transfer of energyfaster transfer of energy

solids generally conduct sound the fastestsolids generally conduct sound the fastest

gases generally conduct sound the slowestgases generally conduct sound the slowest

Also depends on the temperature of the Also depends on the temperature of the mediummedium

higher temperature = particles of the medium higher temperature = particles of the medium colliding more frequentlycolliding more frequently

faster sound wavesfaster sound waves

Each medium has its own set of values for Each medium has its own set of values for speed of sound (table 13-1 page 482)speed of sound (table 13-1 page 482)

In normal air (25In normal air (25ooC) the speed of soundC) the speed of sound

equals 346 m/sequals 346 m/s

Sound waves spread out (propagate) in 3 Sound waves spread out (propagate) in 3 dimensions in approximately spherical patternsdimensions in approximately spherical patterns

Represented on paperRepresented on paper

in 2-D as concentric in 2-D as concentric

circlescircles

Distance between each adjacent wave front = Distance between each adjacent wave front =

1 wavelength (1 wavelength ()) Rays indicate direction of travel of the wave Rays indicate direction of travel of the wave

frontsfronts Plane wave – a segment of a wave front very Plane wave – a segment of a wave front very

far from the source that appears to be a straight far from the source that appears to be a straight line and parallel to the adjacent wave frontsline and parallel to the adjacent wave fronts

Doppler Effect – frequency shift that is the Doppler Effect – frequency shift that is the result of relative motion between the source of result of relative motion between the source of waves and an observerwaves and an observer

(page 485 figure 13-6)(page 485 figure 13-6)

Doppler effectDoppler effect

linklink

Example: #1Example: #1 A car (train, ambulance) moving A car (train, ambulance) moving toward or away from an observer while blowing the toward or away from an observer while blowing the horn or sirenhorn or siren

Pitch appears to change, but the frequency is not Pitch appears to change, but the frequency is not changingchanging

Frequency of the source remains constantFrequency of the source remains constant

Wave fronts will reach the observer in front of the Wave fronts will reach the observer in front of the moving source more often (with highermoving source more often (with higher

frequency) due to source moving toward thefrequency) due to source moving toward the

observerobserver

Wave fronts “pile up” on each otherWave fronts “pile up” on each other

Pitch of sound gets higherPitch of sound gets higher

Example #2 Example #2 Source moving away from Source moving away from observerobserver

Wave fronts will reach observer behind aWave fronts will reach observer behind a

moving source less often (lower frequency)moving source less often (lower frequency)

due to source moving away from observerdue to source moving away from observer

Wave fronts “spread out”Wave fronts “spread out”

Pitch sounds lowerPitch sounds lower

** Speed of the sound waves does not change** Speed of the sound waves does not change Same will happen with a stationary source and a Same will happen with a stationary source and a

moving observermoving observer Doppler effect occurs whenever there is relative Doppler effect occurs whenever there is relative

motion between an observer and a source of wave motion between an observer and a source of wave frontsfronts

** Most common to sound waves, but happens with ** Most common to sound waves, but happens with all waves all waves

- - Moving faster than the speed of soundMoving faster than the speed of sound