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 speakerDemo - Oscillator and speaker

Range of Human Hearing

20 Hz - 20,000 Hz.

infrasonic.Frequencies below 20 Hz are

Frequencies above 20,000 Hz are ultrasonic.

2. NATURE OF SOUND IN AIRTuning Fork

Strobed Tuning Fork

Sound in air is longitudinal with

Compressions (Condensations)

and

Rarefactions

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 Pump

4. SPEED OF SOUND

Much slower than light– Light speed is so fast that we consider it infinite

Thunder and LightningThunder and Lightning– Five second delay per mile

Depends on wind, temperature, humidity Does not depend on frequency

– Orchestra thanks you!

At 0o C (dry air)

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

increases with humidityincreases with temperatureincreases with density

How v varies:

SPEED OF SOUND

Calculator

5. REFLECTION OF SOUND

Occurs anytime waves change media.

Example: Echoes

prolonging of sound by reflection

Example: Rumble of distant thunder

(several reflections from different distances)

Reverberation -

What is not reflected is transmitted and absorbed.

The study of sound properties is acoustics.

Walt Disney Concert Hall

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 (sound travels faster here)

Cool Air (sound travels slower here)

Other Examples

Thunder and lightningSometimes distant lightning is not heard well.

Other times it is.

Submarines and sonar

Refraction due to thermal gradients can “hide” submarines.

Ultrasound in medicine

Ultrasound 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 sound frequencies in air lose their

energies (converted to thermal energy)

more easily than do low frequencies.

That is why low frequencies can be heard farther away.

ENERGY and FREQUENCY

9. NATURAL FREQUENCY

Demo - Drop Different Sounding Objects

Objects have natural frequencies at which

they vibrate.

The natural frequency depends on elasticity

and shape.

8. FORCED VIBRATIONS

Demo - 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.

Examples: Musical sounding boards

10. RESONANCE Resonance occurs when we force a

vibration at the natural frequency of the object.

Result - increased amplitude

Examples:SwingingMarching on a bridge

Demo - Matched Tuning Forks

Demo - Singing Glass

Video – Breaking Glass

Demo - Gravity Chimes

Demo – Boomwhackers

Yankee Doodle

(arranged by Anthony T.)

Can-can

Submultiples of the naturalfrequency also produces resonance.

Demo - 256 and 512 Hz Tuning ForksTacoma Narrows Bridge

Demo - Vibrating RodDemo - Vibrating Plate

Note:

11. INTERFERENCE

Defined in Chapter 19

– Superposition of waves occupying the same place at the

same time

Demo - Oscillator & Two SpeakersDemo - Oscillator & Two Speakers

Examples:

Dead spots in theaters and music halls

Anti-noise technology

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 - GuitarDemo - Guitar

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