Chapter 20 Sound

Slide - A tree in the forest.

1. ORIGIN OF SOUND  The frequency of a sound wave is the same as the frequency of the source of the sound wave. frequency of the source of the sound wave. Demo - Oscillator and speaker VideoVideo – You Tube - Hearing Test Video

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 Fork Demo - Strobed Tuning Fork Sound in air is longitudinal with Compressions (Condensations) andRarefactions Demo - 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 Pump

4.SPEED OF SOUND IN AIR  Much slower than light  Video – Lumberjack Thunder and Lightning Thunder and Lightning Explosion Explosion  Depends on wind, temperature, humidity  Slide - Wind and Snoopy  Does not depend on frequency  Video – Sound Delay

At 0 o 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. Slide - Sounds and Sleeping Dog

Warm Air Cool Air

Other Examples Thunder and lightning Sometimes 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 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 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.)  Demo – Call Mobile Phone on Table Examples: Musical sounding boards

9.NATURAL FREQUENCY  Demo - Drop Different Sounding Objects  Objects have natural frequencies at which they vibrate.  The natural frequency depends on elasticity and shape.  Demo - Gravity Chimes  Demo – Boomwhackers Demo

Yankee Doodle (arranged by Anthony T.)

Beethoven’s Ode to joy (arranged by Anthony T.)

Christmas Boom Whackers Can BoomWhackers Christmas Boom Whackers Can BoomWhackers

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 Forks Tuning Fork on Guitar Breaking Glass Demo - Matched Tuning Forks Demo - Singing Glass Glass armonica Glass armonica by Ben Franklin Glass armonica

Demo - Vibrating Rod Demo - Vibrating Plate Demo - Fog Horn Submultiples of the natural frequency also produce resonance. Demo & 512 Hz Tuning Forks Video - Tacoma Narrows Bridge (url) (url) Note:

11.INTERFERENCE  Defined in Chapter 19  Demo - Oscillator & Two Speakers  Slide - Oscillator & Two Speakers Slide

 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.

Slide - Beats

Constructive Interference Destructive Interference Consider two waves of slightly different frequencies traveling together. D CC D D C

URLURL - Beats URL Demo - Vibrating Rods Demo - Two Oscillators & Two Speakers Demo - Guitar

RADIO BROADCASTS Slide - Radio Broadcasts AM kHz to 1605 kHz FM -88 MHz to 108 MHz