Sound lab

Digital piano

Oscilloscope with speakers

Speaker into tuning box for resonance on scope

Speaker w/mic and tuning box on scope for beats (203-230)

Mic straight into speakers

Sounding board

Scope freq versus wavelength

Pitch versus freq (sin versus square wave)

Hydrogen fuel (fracture water) w/ resonance

CH 15 Sound

The nature of Sound

• Sound is a longitudinal wave

• A sound wave is created by a vibrating object and is transmitted through a medium

SoundWhen a diaphragm moves, it compresses the air molecules in front of it.

The compression causes the air pressure to rise.

Sound Waves• Sound travels in waves known as “Longitudinal” Waves.• The picture below shows a Longitudinal wave formed in air

molecules due to a sound.• The higher density areas are known as compressions.• The lower density areas are known as rarefactions. • When studying interference in sound waves, treat the

compressions as crests and the rarefactions as troughs.

Sound• Sound waves move through air by creating

variations in air pressure.• The air molecules collide, transmitting pressure

oscillations.

• Areas of high pressure are compression• Areas of low pressure are call Rarefactions

Sound travels in all directionsDistance between compressions is λ• Wave front can be created by connecting similar point on the waves

• Note the Energy of the sound is now spread over each “growing” wave front.

• The area of the wave front is πr2

• The Intensity (I) drops by the area of a circle (πr2)

Pressure Amplitude• Loudness depends on the pressure

amplitude of the wave

• Sound Intensity I (Watts W/m2)

P PowerA Arear Radius

PI

A 24

PI

r

Area of a sphere

Pressure Amplitude example

A bus generates sound intensity of 10μW/m2. A student barely hears the sound from pressure wave of 0.076W. How far away is he?

24

PI

r

4

Pr

I

2

6

0.076

(10 10 )(4)(3.14)wm

Wr

x

24.5r m

Pressure Amplitude• Sound Intensity I (Watts W)

• Sound Intensity level is measure in decibels (dB)

• Io=1x10-12 W/m2 Threshold of human hearing

1010log ( )I

1010log ( / )oI I

Intensity LevelThe threshold for human hearing is 1E-12 W/m2. The average whisper is 1E-10 W/m2. How many dB louder is the whisper?

1010log ( / )oI I 10 12

1010log (1 10 /1 10 )x x

1010log (100) 20dB

Intensity Level

For every 10x increase in Intensity, the Intensity Level increase 10dB.

Event W / m2 dBThreshold of hearing 1x10-12/ 1x10-12 0

Whisper 1x10-10 / 1x10-12 20

Classroom 1x10-7 / 1x10-12 50

Normal Conversation 1x10-6 / 1x10-12 60

City Street 1x10-5 / 1x10-12 70

Threshold of pain 1x10-0 / 1x10-12 120

IntensityTwo different stereo systems can produce a 45dB output and a 25dB output. What is the difference in their intensities?

1010log ( / )oI I 10 2 110log ( / )I I

10 2 145 25 10log ( / )dB dB I I 10 2 12 log ( / )dB I I

10 2 1log ( / )210 10 I I1

2

100I

I

Practice Problems

• WS 15.1 1-8

• WS 15.2– #’s1-6

Resonance A Phenomenon that occurs when the forced frequency on an object matches the object’s natural frequency, and a dramatic increase in amplitude results.

• Tacoma narrows bridge video

Beat Frequency

• Beats: A variation in amplitude (loudness) caused by the interference of two slightly different frequencies (tones).

• Ex: A 330Hz and a 332Hz tuning forks are struck simultaneously. What is the beat frequency?

2 1f f f

330 332f

2Beat Hz

Beat Frequency Example

A student has 2 tuning forks. One is 349Hz, the other is an unknown frequency. Together they create a beat of 3Hz.

What are the possible frequencies?

2 1f f f

23 349Hz f

2 352 346f Hz or Hz

23 349Hz f

2 349 3f Hz Hz

Doppler Effect• Doppler Effect: the change in wavelength

and frequency due to the motion of the source.

Police

Doppler Effect• Sound waves propagate out from the source in

all direction. • If the source isn’t moving, the wavelengths are

constant

Police

Sonic boom from real player library

Doppler Effect

• Source moving towards you:1

( )1

o ss

f fvv

Police

• Source moving away from you:1

( )1

o ss

f fvv

Doppler Effect Example

A train is approaching you at 31m/s and blows its whistle of 305hz.

a)What frequency do you hear?

1( )1 so s v

s

f f

31 /343 /

1305 ( )

1o m sm s

f Hz

335of Hz

Doppler Effect ExampleA train is approaching you at 31m/s and blows its whistle of 305hz.

b) What frequency does your friend hear if the train has past him and continues moving away?

1( )1

o ss

f fvv

1305 ( )

31 /1 343 /of Hz

m sm s

279.7of Hz

Expanding Galaxy

Practice Problems

Ws 15.2a

1-5

WS 15.3

1-4

Standing Waves

• Standing Wave:

• Node:

• Antinode:

• Record these vocabulary terms and define their meaning using your text.

Standing Waves

• Standing Wave: has stationary nodes and antinodes. It is the results of identical waves traveling in opposite direction.

• Node: The medium is not displaced as the waves pass through

• Antinode: The displacement caused by interfering waves is largest.

Standing Waves

• In order for a standing wave to exist, there must be an identical wave traveling in the opposite direction

• Standing wave demo

Standing Waves

Harmonics: Standing wave that consist of more than one pulse

Standing waves• Fundamental Frequency: The lowest frequency

that creates a standing wave in a given medium.

• Harmonics (overtones): frequencies with integer multiples of the fundamental frequency.

• These frequencies make up the harmonic series.

Harmonic Series in a string

1

2

2

2

3

2

Length L

1,2,3...2

vfn n n

L

Sound

• End Ch 15