Chapter 15

The Nature of Sound

What is Sound???

Sound is a Longitudinal Wave traveling through matter.

Longitudinal Waves

Longitudinal Waves Matter vibrates in the same direction as the wave travels.

Longitudinal WavesCompression

Rarefaction

λ

Sound from a Tuning Fork

Speed of Sound

Sound is transmitted through matter.

The Velocity of Sound depends on the

matter that carries it. 

Sound travels at a velocity of 332m/s in air at 0C.

•Sound travels faster through warm air than through cold air.

•The velocity of sound increases about 0.6m/s for each degree in temperature.

•Sound travels much faster through liquids and solids than through gases.

•At 20C sound travels at 344m/s.

Comparing Media

Media Speed of Sound

Air at 0°C 331m/sAir at 20°C 343m/s

Water at 25°C 1493m/sSea Water at 25°C 1533m/s

Iron at 25°C 5130m/sRubber at 25°C 1550m/s

Human Hearing

Frequency of Sound 20 Hz to 20,000 Hz.

Sound above 20,000 Hz - Ultrasonic

Sound less than 20 Hz – Subsonic (Infrasonic)

Frequencyis

Pitch

Detection of Pressure Waves

Detection of Pressure Waves

EarDrum

Intensity and Loudness

Intensity of Sound Depends on the amplitude

of the wave.

LoudnessDescribes a person’s response

to sound intensity.

Loudness is measured in Decibels(dB)

For every 10dB change the sound doubles!!

70dB is twice 60dB

80dB is four times 60dB

Faintest Sound Heard 0dBWhisper 15dBRustling Leaves 20dB

Purring Cat 25dBAverage Home 50dBVacuum Cleaner 75dBNoisy Restaurant 80dBPower Mower 100dB

Chain Saw 115dB------Painful ------- 120dBJet Plane Taking Off 150dB

InterferenceConstructive Interference

Occurs when the compressionsand rarefactions of two or

more waves come together.

LouderSound

InterferenceDestructive Interference

Occurs when a compression of one wave arrives at the same time as a

rarefaction of another wave.

QuieterSound

InterferenceBeats

The result of compressions and rarefactions of two slightly

different frequencies reaching your ears together.

Beats

Beatsf1 = 512Hz

f2 = 514Hz  Beats = f1 - f2 

Beats = 2Hz (beats/s)

= 514Hz - 512Hz  

The Doppler Effect

The change in wave frequency caused by the motion of the sound source or the motion

of the observer.

The Doppler Effect

Shorter WavelengthHigher Frequency

The Doppler Effect

Longer WavelengthLower Frequency

Speed of Sound

Greater than the Speed of Sound

Resonance

A resonant frequency is a natural frequency of

vibration determined by the physical parameters of

the vibrating object.

Harmonics

Vibrations which occur at a particular

frequency is known as a harmonic.

First Harmonic

The lowest possible frequency at which a string could vibrate

to form a standing wave pattern is known as the

fundamental frequency or the first harmonic.

First Harmonic

Second Harmonic

Third Harmonic

Resonance in Air Columns

Closed Air Column

λ = 4L

L

λ = 4/3L λ = 4/5L

Resonance in Air Columns

Open Air Column

λ = 2L

L

λ = L λ = 2/3L

Example

A tuning fork is placed above an open-pipe resonator in which the

length can be changed. The loudest sound is heard at a length of 67cm and the next loudest was heard at 100.5cm. If the temperature of the

air is 20°C what is the frequency of the tuning fork?

Example

67cm

100.5cm

(100.5 - 67)= 33.5cm

33.5cm = ½λ

2•33.5cm = λ

67cm = λ

Example

λ = 67cm = 0.67m v@20°C = 343m/s

v = λff = v/λ

f = 512Hz

f = 343m/s 0.67m

Music to Your EarsA back and forth motion is set up in a

string, resulting in a regular vibration. The vibration is called a standing wave the location of the crests and troughs

are always in the same place.

In a wind instrument, holes are opened and closed, changing the length of the vibrating column of

air. This changes the size of the standing wave.

Noise

Sound with no regular pattern or definite pitch.

Tone Quality

The differences among sounds of the same pitch and loudness.

Music

Musical SoundsBased on a series of notes

called a musical scale.

The Sound Spectrum:Fundamental and Harmonics

Open Air Column

λ = 2L

L

λ = L λ = 2/3L

f1 = v/λ

f1 = v/2L

f2 = v/L

f2 = 2f1

f3 = v/2/3Lf3 = 3f1

Fundamental Frequency

First Overtone

SecondOvertone

Third

Overtone

262Hz

524Hz

786Hz

1048Hz

Closed Air Column

λ = 4L

L

λ = 4/3L λ = 4/5L

f1 = v/4L f2 = v/4/3L f2 = 3f1

f3 = v/4/5L

f3 = 5f1

Fundamental Frequency

First Overtone

SecondOvertone

Third

Overtone

256Hz

768Hz

1280Hz

1792Hz

Harmony Notes that sound pleasing together.

The ratio of the frequencies of tones that are in harmony are small whole numbers.

         Notes that are one octave apart. Middle C and C 524/262 = 2/1          Notes E and C 330/262 = 5/4

Dissonance and Consonance

•Dissonance combination of pitches that sound unpleasant.

•Consonance combination of pitches that sound pleasant.

Musical IntervalsOctave: Two notes that have a

ratio of 1:2.

Example: 440Hz

880Hz one octave higher.

220Hz one octave lower.

InterferenceConstructive Interference

Occurs when the compressionsand rarefactions of two or

more waves come together.

LouderSound

InterferenceDestructive Interference

Occurs when a compression of one wave arrives at the same time as a

rarefaction of another wave.

QuieterSound

InterferenceBeats

The result of compressions and rarefactions of two slightly

different frequencies reaching your ears together.

Beats

Beatsf1 = 512Hz

f2 = 514Hz  Beats = f1 - f2 

Beats = 2Hz (beats/s)

= 514Hz - 512Hz  

Homework #15-3

Practice Problem:10Section Review

Page: 367Due: 3/18/03

Homework #15-4

Study GuideDue: 3/19/03Test: 3/20/03