Chapter 26

SOUND

All Sounds are produced by the vibrations of material objects.

Saxophone: vibrating reed

Voice: vibration of vocal cords

Piano or guitar: vibrating strings

Flute: fluttering column of air at mouthpiece.

Pitch:subjective impression about the

frequency of sound.

We normally hear between 20 and 20000 Hertz.

Infrasonic: sound waves below 20 hertz

Ultrasonic: sound waves above 20000 hertz

we can not hear ultra or infrasonic sound waves

Sound is a Longitudinal wave.

Sound is also a pressure wave.Because of the longitudinal

motion of the air particles, there are regions in the air where the

air particles are compressed together and other regions where the air particles are

spread apart.

Compression:a pulse of compressed air

(….when you quickly open a door it pushes the air that

direction.)

Rarefaction:a disturbance in air in which the

pressure is lowered.

(….when you shut a door it pulls the air toward the door.)

For all wave motion, it is not the medium that travels across the room, but the pulse that travels

Transmissions of Sound:Air is the slowest at transmitting

sound.Water is in the middle and

Solids are the best……

vsolids > vliquids > vgases

The Speed of Sound:in dry air at 0°C is about 330 meters per second or about 1200 kilometers per hour.

This is about 1 millionth the speed of light.

This is why you see lightening before you hear thunder.

An increase in the temperature of water vapor will increase the

speed of sound.For every degree increased sound will travel faster by

0.60 m/s.

Speed of sound through materials depends on the elasticity of the materials.

Elasticity:ability of a material to change

shape in response to an applied force, and then resume its initial shape once the distorting force

is removed.

Steel is very elastic and putty is inelastic.

In elastic materials the atoms are relatively close together and

respond quickly to each other’s motions.

Sound travels about 15X’s faster in steel than in air and 4X’s faster in

water than air.

The intensity of the sound is proportional to the square of the

amplitude of the sound.

It is measured by an oscilloscope.(page 394)

Loudness:is a physical sensation sensed

in the brain.

It is subjective but is related to sound intensity.

The unit of intensity for sound is the decibel (dB).

Decibel Ratings:It starts at 0 for the hearing

threshold. Then 10 dB which is 10X as intense as 0. 20 dB is not twice as intense as 10dB

but 10X as intense as 10dB or 100X as intense as 0dB.

SourceIntensity

Level

# Times Greater Than

TOH

Threshold of Hearing (TOH) 0 dB 100

Rustling Leaves 10 dB 101

Whisper 20 dB 102

Normal Conversation 60 dB 106

Busy Street Traffic 70 dB 107

Vacuum Cleaner 80 dB 108

Large Orchestra 98 dB 109.8

Walkman at Maximum Level 100 dB 1010

Front Rows of Rock Concert 110 dB 1011

Threshold of Pain 130 dB 1013

Military Jet Takeoff 140 dB 1014

Instant Perforation of Eardrum 160 dB 1016

Forced Vibration:Vibration of an object that is made to vibrate by another

vibrating object near by.

The sounding board in a musical instrument amplifies the sound through forced vibration.

A guitar would be barely audible without the sounding

board. All stringed instruments use sounding boards.

Natural frequency:Frequency at which an elastic

object once energized will vibrate.

Resonance:to resound, or sound again

radio stations tune into a certain frequency and resonate at that

frequency.

In 1831 an infantry troop marched across a bridge and

caused the bridge to collapse. They marched at the natural

frequency of the bridge causing it to resonate and fall.

Now military personnel “break step” when crossing a bridge.

Interference:affects the loudness of sounds

through constructive and destructive interference.

Beats:the periodic and repeating

variations in the loudness of sound.

Important note: the graphs on the following page represent sound as a

transverse wave. This is for a better understanding of the

concepts addressed. Remember ….. Sound is a

longitudinal wave.