Chapter 20

Sound

1. ORIGIN OF SOUND

•The frequency of a sound wave is the same as the frequency of the source of the sound wave.

Demo - Oscillator and speaker Demo - Oscillator and speaker

The human hearing range is about

20 Hz - 20,000 Hz.

infrasonic.

Frequencies below 20 Hz are

Frequencies above 20,000 Hz are

ultrasonic.

2. NATURE OF SOUND IN AIR

Video - Tuning ForkVideo - Tuning Fork

Demo - Strobed Tuning ForkDemo - Strobed Tuning Fork

Sound in air is longitudinal with

Compressions (Condensations)

and

Rarefactions

Demo - Vortex BoxDemo - Vortex Box

3. MEDIA THAT TRANSMIT SOUND

• Air is most common but is a poor conductor.

• Solids and liquids are good conductors.

• A medium is required.

• Video - Vacuum PumpVideo - Vacuum Pump

(plus next 3 videos)(plus next 3 videos)

4. SPEED OF SOUND IN AIR

•Much slower than light

• Video - LumberjackVideo - Lumberjack

• Video - Thunder and Lightning Video - Thunder and Lightning

• Video - ExplosionVideo - Explosion

•Depends on wind, temperature, humidity

•Does not depend on frequency

At 0o C (dry air)

v = 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. REFLECTION OF SOUND

• Occurs anytime waves change media.

Example: Echoes

Example:

• Reverberation -

•What is not reflected is transmitted

and absorbed.

Examples: San Francisco Symphony Hall

Back of lecture room

•The study of sound properties is

acoustics.

6. REFRACTION OF SOUND

When different parts of a wave front move

at different speeds, the wave front will

bend.

This bending is known as refraction.

It occurs when different parts of a wave

front are traveling in different media.

Warm Air

Cool Air

Other Examples

Thunder and lightning

Sometimes distant lightning is not heard well.

Other times it is.

Submarines and SonarRefraction due to thermal gradients can

“hide” submarines.

Ultrasound in medicineUltrasound echo and dolphins

7. ENERGY IN SOUND WAVES

• Energy in sound is weak when compared to the energy in light.

• The human ear is a remarkable detector.

10 million people speaking at the same time produce approximately enough energy to light one flashlight.

High frequencies of sound in air

more easily lose their energies to

thermal energy than do low

frequencies.

That is why low frequencies can be

heard farther away.

8. FORCED VIBRATIONS

• Demo - Tuning Fork Touching a TableDemo - Tuning Fork Touching a Table

• Sound is intensified because of the larger surface area that can vibrate the air.

• The surface is forced to vibrate at the frequency of the tuning fork. (It is not a resonance phenomenon.)

• Examples: Musical sounding boards

9. NATURAL FREQUENCY• Demo - Drop Different Sounding Demo - Drop Different Sounding

ObjectsObjects

• Objects have natural frequencies at which they

vibrate.

• The natural frequency depends on elasticity

and shape.

• Demo - Gravity ChimesDemo - Gravity Chimes

• Demo – Boomwhackers – Boomwhackers

Yankee doodleYankee doodle

(arranged by Anthony T.)

Yankee doodleYankee doodle

(arranged by Anthony T.)

10. RESONANCE• Resonance occurs when

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

• Result - increased amplitude

Examples: Swinging Marching on a bridge (rout step or break step)

Video - Matched Tuning ForksVideo - Matched Tuning Forks

Video - Tuning Fork on GuitarVideo - Tuning Fork on Guitar

Video - Breaking GlassVideo - Breaking Glass

Demo - Matched Tuning ForksDemo - Matched Tuning Forks

Demo - Singing GlassDemo - Singing Glass

Glass armonica by Ben Franklin 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 BridgeVideo - Tacoma Narrows Bridge

Note:

11.INTERFERENCE

• Defined in Chapter 19

• Demo - Oscillator & Two SpeakersDemo - Oscillator & Two Speakers

• Slide - Oscillator & Two Speakers

• Examples:

Dead spots in theaters and music halls

Anti-noise technology

12. Beats

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

• It is an interference effect.

• Beats/s = difference in frequencies

• Musical instruments are tuned using beats.

• Dolphins use beats and Doppler effect.

Demo - Vibrating RodsDemo - Vibrating Rods

Demo - Two Combs on OverheadDemo - Two Combs on Overhead

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

SpeakersSpeakers

Demo - GuitarDemo - Guitar

Constructive Interference

Destructive Interference

Consider two waves of slightly different frequencies traveling together.

RADIO BROADCASTS

AM - 535 kHz to 1605 kHz

FM - 88 MHz to 108 MHz

Chapter 20 Review Questions

Frequencies of sound that are too high for the human ear to hear are called

(a) faster than the speed of sound(b) supersonic(c) infrasonic(d) ultrasonic(e) subsonic

(d) ultrasonic

For the same temperature for air, does sound travel faster in humid Galveston or in dry El Paso?

(a) Galveston(b) El Paso(c) same speed in either city

(a) Galveston

The bending of sound through air of uneven temperature is called

(a) reflection(b) refraction(c) interference(d) reverberation(e) resonance

(b) refraction

Lightning is seen, then ten seconds later thunder is heard. Approximately, how far away in feet is the thunder cloud?

(a) 10,500(b) 5280(c) 1050(d) 5250(e) 105

(a) 10,500

A 250 Hz tuning fork and a 260 Hz tuning fork are vibrating near each other. How many beats per second are heard?

(a) 255(b) 510(c) 10(d) 250(e) 260

(c) 10

(a) the lowest frequency had a node in the middle

(b) the vibration was longitudinal(c) there were antinodes at the ends(d) the lowest frequency had a wavelength

(in the rod) equal to the length of the rod

(d) the lowest frequency had a wavelength (in the rod) equal to the length of the rod

Which of the following isn’t true concerning the metal rod that I vibrated in class by stroking it along its length?