SOUND

Longitudinal Wave• Travels through some medium• Cannot travel through a vacuum• How does vibrating drum produce sound?• Skin moving up presses air molecules close together and creates high pressure – compression

• Skin moving down spreads molecules out causing low pressure - rarefaction

Speed of Sound

The elasticity of steel is greater than air, so we would expect the velocity of sound in steel to be ________ than air

A) Less than

B) Greater than

C) The same as

D) Not enough information

Some SpeedsAir (20°) 343 m/s

Air (0°) 331 m/s

Helium 1005 m/s

Hydrogen 1300 m/s

Water 1440 m/s

Seawater 1560 m/s

Iron or Steel 5000 m/s

Speed In Air• Speed in air also depends on temperature• v = 331 + (.60) T

What is the speed of sound in air at 45° C?

Example

• A general rule of finding the distance of a lightning strike is that there is “one mile for every 5 seconds before the thunder is heard”. Is this true at about 20°C? 1 mile = 1610m

Characteristics of Sound• Loudness – intensity of sound wave• Loudness is determined by the amplitude of the wave

Pitch• Tone of the sound• Depends on the frequency• Higher frequency, higher pitch

low high

A sound with a frequency of 150 Hz is ___________ in pitch of a 300 Hz sound.

Range of Sound• Audible range – for humans, we can hear tones between

20 Hz and 20000 Hz• Infrasonic – tones lower than 20 Hz• Ultrasonic – tones higher than 20000 Hz

A sound of 35000 Hz is __________

Graphing Sound• Longitudinal waves can be hard to show graphically• Can be represented by plotting pressure vs. time

Intensity • Loudness• Energy transported by a wave per unit time across an

area• Proportional to square of amplitude

Intensity = Power/Area = W/m2

Human Hearing• Can detect 10-12 W/m2 to 1 W/m2 • Threshold of Hearing – minimum intensity we can hear• Threshold of Pain – above this sound is painful

True or FalsePeople can hear sounds of 10-14 W/m2

Log Scale• Since intensity increase by factor of 10, log scale can be

used• Unit of scale is called the decibel (dB) – symbol

= 10 log ( I/ I0)

I – intensity of the sound

I0 – threshold of hearing

For Comparison gentle breeze 10 dB soft whisper 20 dB quiet library 40 dB conversation 60 dB machine shop 100 dB rock band 110 dB threshold of pain 120 dB jet plane 140 dB rocket launch 180 dB

Example• What are the decibels of sounds with intensities of 5.0 x

10-6 W/m2 ?

What is the intensity level for a sound of 20 dB?

A) 1 x 10-8

B) 1 x 10-9

C) 1 x 10-10

D) 1 x 10-11