Sound Waves

More Than Meets the Eye!!

What type of wave is this???

Transverse

How does this wave travel???

Perpendicular

What types of waves are Transverse???

Electromagnetic

Visible Light

Microwaves

X-Rays

An Example…….

What type of wave is this???

Longitudinal

Compressional

Parallel

How does this wave travel???

Sound!!

What type of wave is Longitudinal???

How sound works….

Waves are periodic, meaning they are disturbances of

energy which repeat.

1 Cycle

Successive identical parts of a wave is a cycle, or λ

The time it takes for one cycle is a period or T

Frequency is cycles per second.

f =1

T

A wavelength (λ) is a cycle.

λ is usually crest to crest.

A wavelength (λ) is a cycle.

But λ can be any two successive identical points on a wave.

Sound is vibrations which create compressions in the

air.

These compressions travel to our ear drums where we

experience sound.

High frequency waves have a high pitch.

Low frequency waves have a low pitch.

This is best observed in the Doppler Effect.

When wave source is moving waves are high frequency in

front of the object.

When wave source is moving waves are low frequency in

behind the object.

A wave on a string moves faster the more tension the

string has on it.

Just like compressions move through a slinky faster when

the coils are tighter..

Remember…..• v = λf or v = λν• The frequency of a sound wave determines the pitch of the sound.• High frequency = High pitch• Low frequency = Low pitch• The loudness of sound is determined by the amplitude. • More amplitude (thicker compression) means louder sound.

Sound moves faster through warmer air because the

molecules in warmer air are moving faster.

Medium Speed of Sound

Air at 0°C 331 m/s

Air at 20°C 343 m/s

Sound moves faster through more dense matter and slower

through less dense matter.

Substance State Speed of Sound

Oxygen at 20°C Gas 316 m/s

Freshwater (20°C) Liquid 1482 m/s

Copper Solid 5010 m/s

Glass (Pyrex) Solid 5640 m/s

Speed of sound in Various Media

Medium m/sAir (0°) 331Air (20°) 343Helium (0°) 972Water (25°) 1493Seawater (25°) 1533Copper (25°) 3560Iron (25°) 5130

Sound is measured in decibels (dB)

Common Sound Approximate Decibels

Threshold of hearing 0 dB

Whisper 20 dB

Normal Talking 65 dBCar without muffler 100 dB

Loud Music 120 dB

Hurts Ears 130 dB

Ultrasonic sounds are so high frequency we can’t

hear them

Bats can hear much higher frequency of sound, which they use as

sonar.

Sonograms use ultrasonic sound to make images of

developing fetuses.

Humans hear best at frequencies of 1,000 to

5,000 HZ.

Sonograms are done at up to 15,000,000 Hz !!!

Sound waves can be used to look at the heart as well.

Sound Waves and Music

1st Harmonic

2nd Harmonic

3rd Harmonic

4th Harmonic

5th Harmonic

6th Harmonic

Sound Waves and Music

• At n = 1, λ = ½

• At n = 2, λ = 1 or 2/2• At n = 3, λ = 2/3

• At n = 4, λ = 2/4 or 1/2

• At n = 5, λ = 2/5• L = length of string

Standing Waves are created in musical instruments.

We can look at compressions and rarefactions as crests and

troughs..

And apply this to musical instruments…..

In the first harmonic of an instrument closed at one end and open at the other ( a clarinet) the

harmonic is ¼ of the total wavelength.

Saxophone is closed end.

And apply this to musical instruments…..

Instruments with a closed end resonate at antinodes.

And apply this to musical instruments…..

4

λ

λ = 4L

4

λ

L

λ = 4/3 L

4

λ

L

3Node

λ = 4/5 L

4

λ

L

5Node

Closed-Tube HarmonicsResonates at odd number

quarter wavelengths

Flute is open pipe.

Open-Tube HarmonicsResonate an even number of quarter wavelengths.

Some wavelengths for open pipes.

• λ = 2L

• λ = L

• λ = 2/3 L

They are the same just at nodes or antinodes.

• λ = 2L

• λ = L

• λ = 2/3 L

Excellent Summary!!

The End