Sound Sound Pitch Pitch LoudnessLoudnessSound Intensity LevelSound Intensity LevelDoppler EffectDoppler Effect

PhysicsMrs. Coyle

SoundSound

Sound is a longitudinal mechanical wave.

Compressions(High Pressure)Rarefactions(Low Pressure)

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.

Properties of SoundProperties of SoundReflection (Echo)RefractionInterferenceDiffraction

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)

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

Doppler EffectDoppler Effect Detected Frequency, (f Detected Frequency, (fDD))

ffDD = f = f (( vv±v±vo o ) )

v±v v±vss

f : frequency of source

v : speed of sound

vo : speed of observer (+)when observer moving towards source

vs : speed of source (-)when source is moving towards observer

Example 1Example 1 An ambulance is approaching a stationary observer.

The siren of the ambulance emits a frequency of 480Hz and the speed of the ambulance is 50km/h(=13.88m/s).

What frequency will the stationary observer hear when the ambulance is approaching? Assume T=20oC.

Answer: fd = 500Hz

Sound Intensity (I)Sound Intensity (I) I = (Power transmitted by the

Wave)/Area

Threshold of human hearing:

Io =1 x 10-12 Watts/m2

Threshold of Pain:IP = 1 Watts/m2

Sound (Intensity) LevelSound (Intensity) LevelA measure of our perception of

the loudness of the sound.

Unit: decibel(dB)

Sound (Intensity) Level, Sound (Intensity) Level, (decibel)(decibel)

log I , unit for is dB (decibel)

I0

The decibel compares the sound intensity (I), to Io (1 x 10-12 Watts/m2) ,

the threshold of human hearing.

Remember: log(10x )= x

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

Example 2 Example 2 What is the sound level (decibel) of a sound of 10-4 W/m2 intensity ?

Answer: 80dB

Example 3Example 3How many times louder is a quiet library sound (40 dB) compared to a soft whisper (20 dB)?

Hint 1: Property of logs: log(A/B) = logA –logB

Hint 2: Take the difference in dB and then find (I2 /I1)

Answer: (I2 /I1 )= 100/1