Sound Sound Pitch Pitch LoudnessLoudnessBeatsBeatsDoppler EffectDoppler Effect

PhysicsMs. Shaver

SoundSoundSound is a longitudinal mechanical

wave aka a pressure or compression wave

Compressions(High Pressure)Rarefactions(Low Pressure)

SoundSound Tiny hairs inside the inner ear (cochlea) translate air pressure into electrical impulses that can be read by the brain

Graphing SoundGraphing Sound

Speed of SoundSpeed of SoundDepends on the medium.

The more elastic the medium the faster sound will travel through it. 

Speed in metals>speed in water>speed in air

Sound can’t travel through vacuum.

Speed of Sound in AirSpeed of Sound in Air

v = 331 + 0.6 T ( in meters/sec) T is the temperature in 0C. In higher humidity, sound will travel

faster.

For 15°C, v = 331 + 0.6*15 = 340 m/s

Use 340 m/s as an average speed of sound in air.

Faster than the speed of Faster than the speed of SoundSoundSupersonic – motion faster than

speed of soundSonic boom – caused by an

object breaking the sound barrier (supersonic planes, bullets)

Plane reaches speed above 770 mph – Explosion of sound waves caused by air crashing behind the planeThe air can condense or sweep up vapor from engine – causing this picture

Pitch:Pitch: the frequency of the frequency of a sound wave.a sound wave.

Musical notes have a given pitch.

The note C has a frequency of 327Hz.

When two notes differ by a ratio of 2:1 they are one octave apart.

What would be the next higher C? (Ans:654Hz)

Pitch – frequency of sound Pitch – frequency of sound wavewaveHumans can hear frequencies

between 20 Hz and 20,000 HzInfrasonic frequencies: Below 20

Hz◦used by elephants and submarines

to communicate over long distancesUltrasonic frequencies

◦Dog whistles

Loudness = AmplitudeLoudness = AmplitudeLoudness is measured in decibels (dB)A +10 dB change we hear as twice as

loudA -10 dB change we hear as half as

loud

Decibels – Loudness Decibels – Loudness measuremeasureEXAMPLE: If a sound is 20 dB loud, answer how many dB these would be:

1)A sound twice as loud:2)A sound half as loud:3)A sound three times as loud:

30 dB

10 dB

35 dB

Decibels – Loudness Decibels – Loudness measuremeasure

Examples of Sound Intensity Examples of Sound Intensity LevelsLevels

jet plane taking off 140 dBair raid siren 125 dBthreshold of pain 120 dBloud rock music 115 dBear damage starts 85

dBbusy traffic 70 dBnormal conversation 60 dBquiet library 40 dBsoft whisper 20 dBthreshold of human hearing 0 dB

Timbre – tone qualityTimbre – tone qualityWhat makes a particular musical

sound different from another, even when they have the same pitch and loudness

EX: difference between a guitar and a piano playing the same note at the same loudness

Sounds can be described in terms of “coloration,” e.g. bright, dark, warm, harsh, etc.

Timbre – tone qualityTimbre – tone qualityColor of the note is due to the

presence of different harmonics. The “oo” has mostly low

harmonics, while the “ee” has mostly high harmonics

Harmonics - ReviewHarmonics - Review

v = λ f - The velocity of the wave stays the same, even if considering different harmonics….it’s still the same wave!!!

Harmonics - ReviewHarmonics - Review

Properties of SoundProperties of SoundReflection (Echo)RefractionInterferenceDiffraction

EchoesEchoesEchoes are sounds that are

reflected back by a hard boundary.

EchoesEchoesEcholocation – used by many

animals like bats to see through dark water or at night

Sonar – sound through water Radar – light waves through air Sonograms – sound waves

through the human body

BeatsBeatsInterference effectWhen two waves of close frequencies

interact causing alternating constructive and destructive interference

BeatsBeatsThe number of beats = difference

of two frequencies (absolute value)

EX: f1 = 345 Hz; f2 = 342 Hz◦Number of beats = 345 – 342 = 3◦There will be three beats per second

http://www.school-for-champions.com/science/sound_beat_frequencies.htm#.U2gpwletJW8

Bow (Shock) Waves Bow (Shock) Waves

When the speed of a moving sound source is greater than the speed of the wave, a pressure ridge builds similar to the wave created by the bow of a ship.

physlet animation

Sonic BoomSonic Boom

When the pressure ridge of a bow wave of a jet passes over an observer on the ground, the observer experiences a sonic boom.

Doppler EffectDoppler Effect

The change in a wave's perceived frequency due to the motion of either the sound source or the observer. 

It is applicable to any type of wave.

Austrian physicist Christian Doppler (1803-1853).train sound clip

Simulationsphyslet animationhttp://www.walter-fendt.de/ph14e/

dopplereff.htm

The Doppler EffectThe Doppler Effect

http://www.physicsclassroom.com/Class/sound/u11l3b.cfm

The Doppler EffectThe Doppler EffectIn front of the source the sound waves

are compressed (shorter wavelength λ) and this raises the frequency (pitch)

Behind the source the sound waves are stretched (longer wavelength λ) and this drops the frequency (pitch)

Anything moving at the same speed as the source will experience no change in frequency