Vote for the best TAi-clicker-1

A. Samrat Dutta is very good

B. Samrat Dutta is ok/needs improvement

C. Ashley Carlton is very good

D. Ashley Carlton needs improvement

i-clicker-2

A. Maggie Baldwin is very good

B. Maggie Baldwin is ok/needs improvement

C. Calli Nguyen is very good

D. Calli Nguyen is ok/needs improvement

i-clicker-3

A. Jack Owen is very good

B. Jack Owen is ok/needs improvement

C. Zach Vance is very good

D. Zach Vance is ok/needs improvement

i-clicker-4

A. Brad Goetz is very good

B. Brad Goetz is ok/needs improvement

Chapter 9.2 Announcements:

- Remember: Homework 9.1 is due Thursday, March. 25, in class

Homework 9.2: due Thursday, April 1, in class (Calli Nguyen)

Exercises: 9, 10, 11, 12, 14, 15, 16, 23, 24, 31, 32

Problems: 1, 2, 3, 4

We’ll now cover only parts of each chapter (let me know if you want me to cover something that is not on the list and that interests you):

- 5.1 Balloons

- 7.1 Woodstoves

- 9.1 Clocks, harmonic oscillation

- 9.2 Musical Instruments, waves

- 10.3 Flashlights

- 11. Household Magnets & Electric Motor

- 11.2 Electric Power Distribution

- 15.1. Optics, cameras, lenses

- 16.1 Nuclear Weapons

Heads up: Midterm 2 is coming up on April 13

Chapter 9.2Musical Instruments (waves)

- waves in a room/stadium - waves in a pipe- a speaker (creating sound)- ear- tuning fork- waves on a string- wave modes (harmonics)

- longitudinal waves- transverse waves- traveling & standing waves- waves on a string- waves in an air column- ear and hearing- wave length- frequency/pitch- sound

Demos and Objects Concepts

Traveling transverse wavesCrest/bump travels

Transverse waves:

Transverse waves:

The particles of the disturbed medium move perpendicular to the wave motion

particle

wave

Longitudinal waves:

Longitudinal waves:

The particles of the disturbed medium move parallel to the wave motion

Traveling, longitudinal wavescompression travels

Examples of waves (i-clicker-1):

Which wave is a longitudinal wave?

A. “Bump” traveling down a string:___________________

B. Sound waves:____________________

C. La ola in a stadium (getting up/sitting down):_____________

D. Water wave: ____________________________

Basic Variables of Wave Motion

Terminology to describe waves

- Crest: “Highest point” of a wave

- Wavelength : Distance from one crest to the next crest.

- Wavelength : Distance between two identical points on a wave.

- Period T: Time between the arrival of two adjacent waves.

- Frequency f: 1/T, number of crest that pass a given point per unit time

Sound

- is a wave (sound wave)

- Rarefied and compressed regions

- Longitudinal wave

- air molecules move back and forth

Sound WavesSound waves are longitudinal waves.

They consist of compressed and rarified regions of gas (medium)

We can hear (audible) frequencies from about 20 Hz (low) to 15,000 Hz (high).

Infrasonic “sound” waves: below ~ 20 Hz

Ultrasonic sound waves: above ~ 15,000 Hz

The speed of sound in air: c ~ 343 m/s ~ 740 mi/hr ~ 0.2 mi/sec. (dry air, 68F)

i-clicker-2

A. 1 mile

B. 2 miles

C. 3 miles

D. 4 miles

E. 5 miles

It is a dark and stormy night.

Lightning strikes in distance.

You see the lighting, then, after ten seconds you hear the thunder.

How far away did the lighting strike?

Sound waves, hearing and the ear

http://www.innerbody.com/anim/ear.html

Notes and their fundamental frequency

Octaves: frequency doubles for each tone

Creating standing waves:

When two waves are traveling back and forth, under the right conditions (right frequency), we can create standing waves.

Standing waves have stationary nodes and antinodes

Examples we’ll talk about: - Standing waves on a string. - Standing waves in a pipe (open and closed).

L

m

T

L2

1f1

3f1

2f1

4f1

5f1

6f1

frequencyString Harmonics

L … Length of string; T … Tension m … mass of string

m

T

L2

1f1

L … Length of string

T … Tension (not period T)

m … mass of string

Standing waves have stationary nodes and anti-nodes

Strings as Harmonic Oscillators

• A string is a harmonic oscillator– Its mass gives it inertia– Its tension gives it a restoring force– It has a stable equilibrium– Restoring forces are proportional to displacement

• Stiffness of restoring forces determined by– String’s curvature– String’s tension

Fundamental Vibration

• String vibrates as a single arc, up and down– velocity antinode occurs at center of string

• This is the fundamental vibrational mode

• Pitch (frequency of vibration) is– proportional to– inversely proportional to string length– inversely proportional to

tension

massm

T

L2

1f1

How can a violin player play a lower note:

A.Increasing the tension in the string.B.Playing a string with less mass (thinner string).C.Shortening the string.D.A & CE.None of the above.

i-clicker-3

Overtone Vibrations

• In addition, string can vibrate as– two half-strings– three third-strings– etc.

• These are higher-order vibrational modes

• These modes have higher pitches – overtones

Harmonics in a String

• In a string, the overtone pitches are– two times the fundamental frequency (octave)– three times the fundamental frequency– etc.

• These integer multiples are called harmonics

• Bowing or plucking a string tends to excite a mixture of fundamental and harmonic vibrations, giving character to the sound

notes

E5

A4

D4

G3

i-clicker-4:Why do all musical instruments have a body (wood body, metal shell, etc)?

A.They look prettierB.They are easier to holdC.They act as resonators (amplify sound)D.They act as dampers (reduce sound)E.No good reason

Music and Resonance:Primary and secondary oscillators

String Instruments Wind Instruments

Air column

body Mouthpiecestrings

Connecting primary (strings) and secondary (body) oscillators

Producing Sound

• Thin objects don’t project sound well– Air flows around objects– Compression and rarefaction is minimal

• Surfaces project sound much better– Air can’t flow around surfaces easily– Compression and rarefaction is substantial

• Many instruments use surfaces for sound

Violin Harmonics

Viola Harmonics

Compare to Chladni plate demo

Why are some violins so expensive (Stradivarius : $ 1.5 M)?

Computer Tomography scan of a Nicolo Amati Violin (1654)

i-clicker-5:

A. Old stuff is always expensive.B. They are made of expensive materials.C. In fashion and music, you pay for the label.D. The secondary oscillator mixes a rich sound of harmonics.E. The primary oscillator produces unusual frequencies.

Woodwinds

Reed

Primary Resonators: Wind Instruments

Fixed Edge

Flute Brass

Lips

Open pipe:

Fund. Frequency L

cf

21

c… speed of sound

c = 343 m/s in airL2

c1f1

L2

c2f2

L2

c3f3

Half-closed pipe:

L4

c3f2

L4

c5f3

L

cf

411

Fundamental frequency:

You play an open organ pipe with a length of 1m. What is the fundamental frequency?

A.1 HzB.86 HzC.172 HzD.343 HzE.686 Hz

Now you close the pipe at one end. What will the frequency be then?

A.1 HzB.86 HzC.172 HzD.343 HzE.686 Hz

i-clicker-6; 7:

Air as a Harmonic Oscillator

• A column of air is a harmonic oscillator– Its mass gives it inertia– Pressure gives it a restoring force– It has a stable equilibrium– Restoring forces are proportional to displacement

• Stiffness of restoring forces determined by– pressure– pressure gradient

Fundamental Vibration

• Air column vibrates as a single object– Pressure antinode occurs at center of open column– Velocity antinode occurs at ends of open column

• Pitch (frequency of vibration) is– inversely proportional to column length– inversely proportional to air density

• A closed pipe vibrates as half an open column– pressure antinode occurs at sealed end– Velocity node occurs at the sealed end– frequency is half that of an open pipe

Harmonic Vibrations

• In addition, column of air can vibrate as– two half-columns

– three third-columns

– four fourth-columns

• These higher-order modes are the harmonics• Pitches are integer multiples of the fundamental• Blowing across column tends to excite a mixture

of fundamental and harmonic vibrations