SOUNDTiffany Rhodes

Physics

Topics Covered

• Source • Wave• Medium• Frequency• Amplitude• Speed• Mach Number

If a tree falls in the forest and nobody is around to hear it, does it make a sound?

•

I. EVERY SOUND IS CREATED BY A VIBRATION

• Guitar string vibrates• Reed of clarinet vibrates• Vocal cords vibrate• Table top vibrates when hit with a

hammer• Brain translates vibration into sound

II. SOUND IS A LONGITUDINAL WAVE

• Compressions and rarefactions

• AIR DENSITY CHANGES AS SOUND ENERGY TRAVELS THROUGH

• Only energy is transported!!!!

III. SOUND NEEDS A MEDIUM

• Sound can travel through air, liquids, and solids

• Sound travels faster thru solids!

• Sound can NOT travel thru a vacuum

Can sound travel through space?

Why?

IV. Frequency of Sound = PITCH

• LOW PITCH = LOW FREQUENCY

• HIGH PITCH = HIGH FREQUENCY

•1 Hertz = 1 vibration/second

V. HUMAN HEARING• Range 20 – 20,000hz

• Most sensitive at 200 to 4,000 Hz.

• Normal voice range is about 500 Hz to 2000 hz Low frequencies are vowels and bass. High frequencies are consonants

• Lose hearing as we age, especially at the high frequency end.

• Girls usually hear higher frequencies better than boys

Range of frequencies

FREQUENCIES

• Below 20 HZ = INFRASONIC

• Above 20,000 HZ = ULTRASONIC

• We can NOT hear either spectrum

Range of frequencies

VI. INTENSITY DEPENDS ON AMPLITUDE

• Intensity can be measured by an oscilloscope

• More Intense = Bigger amplitude

• Less intense = smaller amplitude

• Loudness is perceived intensity. Subjective

http://www.youtube.com/watch?v=ydxoTPwFwjs

Intensity• Unit of Intensity = Decibels (dB)

• Named after Alexander Graham Bell

• Zero Decibels = threshold of hearing

• Increase to 10 decibels (intensity increased by a factor of 10)

• Thus it is 10 times more intense

• 60 dB is 100 times more intense than 40 dB http://www.youtube.com/watch?v=RyIphO4Ypoo http://www.youtube.com/watch?v=oPWucNgN8TQ

Source of Sound Level in Decibels

Hearing threshold 0

Normal Breathing 10

Close Whisper 20

Library 40

Normal Speech 60

Busy Street Traffic 70

Average Factory 90

Old Subway train 100

Loud rock Music 115

Threshold of Pain 120

Jet Engine at 30 m 140

VI. SPEED OF SOUND

• Medium Affects Speed

• 2 Properties of a medium affect speed–1. Elastic properties–2. Inertial property

Temperature of medium also affects it.–Generally, Hotter = faster Cooler = slower

–See Hand out for more information

Speed of sound in Various Mediums

• Carbon Dioxide 258

• Air 332• Cork 500• Alcohol 1241• Fresh water 1500• Marble 3810• Steel 5050• Aluminum 5104• Earth 7000-13000

Which medium would sound travel fastest

through?

•Air•Aluminum•Ocean water•Helium

SPEED EQUATION

What is the Speed of Sound in air at 30 C

and –30 C?

At 30 Celsius:

V= (332 + 0.6 * 30)V = 350 m/s

At –30 Celsius:

V = (332 + 0.6 * -30)V = 314 m/s

Mach Number

• What do you already know about Mach Number?

Related to air planes??

Related to sound??

Mach Number• Relates speed of object with speed of sound• Usually used with aircrafts since they are our

fastest means of travel

• Mach Number = Speed of Object/ Speed of sound

• Speed of object and sound have to have same units. They cancel out…

• Mach Number has No units!

What do you know about an airplane that travels at mach 1?

• It is traveling at the same speed of sound

What do you know about an airplane that travels at mach 2?

It is traveling double the speed of sound

II. Wave Equation

• Sound is a wave so therefore the wave equation still applies.

• Speed of the wave = wavelength * frequency

• V = * f

Questions

• Mach Number problems?

• Wave Equation problems?

Warm Up

A rocket is traveling at 8643.6 km/hr.

If the air temperature is 20ºC, what is the rocket’s Mach

Number?

Answer• Speed of object = 8643.6 km/h

• Speed of sound = 332 + 0.6* Temp

= 332 + 0.6 (20)

= 344 m/s

Convert to km/h

344m 1 km 3600sec

1 sec 1000m 1h

= 1238.4 km/h

Mach # = Speed of Object/Speed of sound

= 8643.6/ 1238.4 = 7