Chapter 20Chapter 20

SoundSound

SlideSlide - A tree in the forest. - A tree in the forest.

1. ORIGIN OF SOUND1. ORIGIN OF SOUND

The frequency of a sound wave is The frequency of a sound wave is

the same as thethe same as the frequency of the frequency of the source of the sound wave.source of the sound wave.

Demo - Oscillator and speaker Demo - Oscillator and speaker Video – You Tube - Hearing Test – You Tube - Hearing Test

The human hearing range is aboutThe human hearing range is about

20 Hz - 20,000 Hz.

infrasonic.

Frequencies below 20 Hz are

Frequencies above 20,000 Hz areultrasonic.

2. NATURE OF SOUND IN AIR2. NATURE OF SOUND IN AIR

Video - Tuning ForkVideo - Tuning Fork

Demo - Strobed Tuning ForkDemo - Strobed Tuning Fork

Sound in air is longitudinal with Sound in air is longitudinal with

Compressions (Condensations)Compressions (Condensations)

andand

RarefactionsRarefactions

Demo - Vortex BoxDemo - Vortex Box

3. MEDIA THAT TRANSMIT SOUND3. MEDIA THAT TRANSMIT SOUND

Air is most common but is a poor Air is most common but is a poor

conductor.conductor.

Solids and liquids are good conductors.Solids and liquids are good conductors.

A medium is required.A medium is required.

Video - Vacuum PumpVideo - Vacuum Pump

4.4. SPEED OF SOUND IN AIRSPEED OF SOUND IN AIR Much slower than lightMuch slower than light Video – LumberjackVideo – Lumberjack

Thunder and LightningThunder and Lightning

ExplosionExplosion

Depends on wind, temperature, humidity

Slide - Wind and SnoopySlide - Wind and Snoopy Does not depend on frequency

Video – Sound DelayVideo – Sound Delay

At 0At 0o o C (dry air)C (dry air)

v = 1090 ft/sv = 1090 ft/s = 750 mi/hr = 330 m/s

increases with humidity

increases with

temperature

is faster in liquids and

solids

How v varies:

prolonging of sound by reflection

Rumble of distant thunder

(several reflections from

different distances)

5.5. REFLECTION OF SOUNDREFLECTION OF SOUND Occurs anytime waves change media. Occurs anytime waves change media.

Example: Echoes

Example:

Reverberation -

What is not reflected is transmitted What is not reflected is transmitted

and absorbed.and absorbed.

Examples: San Francisco Symphony Hall

Back of lecture room

The study of sound properties is acoustics.

6.6. REFRACTION OF SOUNDREFRACTION OF SOUND

When different parts of a wave front move at When different parts of a wave front move at

different speeds, the wave front will bend.different speeds, the wave front will bend.

This bending is known as refraction.This bending is known as refraction.

It occurs when different parts of a wave front are It occurs when different parts of a wave front are

traveling in different media.traveling in different media.

Slide - Sounds and Sleeping Dog Slide - Sounds and Sleeping Dog

Warm Air

Cool Air

Other ExamplesOther Examples

Thunder and lightningThunder and lightning

Sometimes distant lightning is not heard Sometimes distant lightning is not heard

well.well.

Other times it is.Other times it is.

Submarines and SonarRefraction due to thermal gradients can

“hide” submarines.

Ultrasound in medicineUltrasound echo and dolphins

7.7. ENERGY IN SOUND WAVESENERGY IN SOUND WAVES

Energy in sound is weak when compared Energy in sound is weak when compared

to the energy in light.to the energy in light.

The human ear is a remarkable detector.The human ear is a remarkable detector.

10 million people speaking at the same 10 million people speaking at the same

time produce approximately enough time produce approximately enough

energy to light one flashlight. energy to light one flashlight.

High frequencies of sound in air High frequencies of sound in air

more easily lose their energies to more easily lose their energies to

thermal energy than do low thermal energy than do low

frequencies.frequencies.That is why low frequencies can be heard farther away.

8.8. FORCED VIBRATIONSFORCED VIBRATIONS Demo - Tuning Fork Touching a TableDemo - Tuning Fork Touching a Table Sound is intensified because of the larger Sound is intensified because of the larger

surface area that can vibrate the air. surface area that can vibrate the air. The surface is forced to vibrate at the The surface is forced to vibrate at the

frequency of the tuning fork. (It is not a frequency of the tuning fork. (It is not a resonance phenomenon.)resonance phenomenon.)

Demo – Call Mobile Phone on TableDemo – Call Mobile Phone on Table

Examples: Musical sounding boards Examples: Musical sounding boards

9.9. NATURAL FREQUENCYNATURAL FREQUENCY Demo - Drop Different Sounding Demo - Drop Different Sounding

ObjectsObjects

Objects have natural frequencies at which they Objects have natural frequencies at which they

vibrate.vibrate.

The natural frequency depends on elasticity The natural frequency depends on elasticity

and shape.and shape.

Demo - Gravity ChimesDemo - Gravity Chimes

Demo – Boomwhackers Demo – Boomwhackers

Yankee Doodle

(arranged by Anthony T.)

Beethoven’s Ode to joy

Beethoven’s Ode to joy

(arranged by Anthony T.)

Christmas Boom WhackersChristmas Boom Whackers

Can Can BoomWhackersCan Can BoomWhackers

10.10. RESONANCE RESONANCE

Resonance occurs when successive Resonance occurs when successive impulses are applied to a vibrating impulses are applied to a vibrating object in time with its natural object in time with its natural frequency. frequency.

Result -Result - increased amplitudeExamples: Swinging

Marching on a bridge (rout step or break step)

Video - Matched Tuning ForksVideo - Matched Tuning Forks

Tuning Fork on GuitarTuning Fork on Guitar

Breaking GlassBreaking Glass

Demo - Matched Tuning ForksDemo - Matched Tuning Forks

Demo - Singing GlassDemo - Singing Glass

Glass armonica by Ben Franklin Glass armonica by Ben Franklin

Demo - Vibrating RodDemo - Vibrating Rod

Demo - Vibrating PlateDemo - Vibrating Plate

Demo - Fog HornDemo - Fog Horn

Submultiples of the naturalfrequency also produce resonance.

Demo - 256 & 512 Hz Tuning ForksDemo - 256 & 512 Hz Tuning Forks

Video - Tacoma Narrows Bridge (url)Video - Tacoma Narrows Bridge (url)

Note:

11.11.INTERFERENCEINTERFERENCE

Defined in Chapter 19Defined in Chapter 19

Demo - Oscillator & Two SpeakersDemo - Oscillator & Two Speakers

Slide - Oscillator & Two SpeakersSlide - Oscillator & Two Speakers

Examples:Examples:

Dead spots in theaters and music hallsDead spots in theaters and music halls

Anti-noise technologyAnti-noise technology

12. Beats12. Beats

Fluctuating loudness due to two tones of Fluctuating loudness due to two tones of slightly different frequencies that are slightly different frequencies that are sounded together (tremolo)sounded together (tremolo)

It is an interference effect.It is an interference effect. BeatsBeats//s = difference in frequenciess = difference in frequencies Musical instruments are tuned using Musical instruments are tuned using

beats.beats. Dolphins use beats and Doppler effect.Dolphins use beats and Doppler effect.

Slide - Beats Slide - Beats

Constructive Interference

Destructive Interference

Consider two waves of slightly different frequencies traveling together.

DC

URL - BeatsURL - Beats

Demo - Vibrating RodsDemo - Vibrating Rods

Demo - Two Oscillators & Two Demo - Two Oscillators & Two

SpeakersSpeakers

Demo - GuitarDemo - Guitar

RADIO BROADCASTSRADIO BROADCASTS

Slide - Radio BroadcastsSlide - Radio Broadcasts

AM -AM - 535 kHz to 1605 kHz

FM - 88 MHz to 108 MHz