Today: Sound Waves, Interference, Beats, Pitch

Exam #1 is Thursday!!!In this room, BRING A PENCIL!!!

Essential that you practice the quizzes!

Next reading, quiz, homework assigned later in week (after exam)

Big eared Townsend Bat,Nevada Bureau of Land Management

Quiz # 2 Results

Last week’s puzzler:Wind and Doppler Effect

Does the wind affect the pitch of a factory whistle you hear on a windy day?

1. Yes2. No

Another perspective, may cause “aha!”:

What if both the whistle and the person were moving in the same direction? Is that any different than a wind blowing?

The compression / rarefaction waves are

what you hear…

Remember it’s the frequency that

determines the pitch

Last week’s puzzler:Wind and Doppler Effect

Does the wind affect the pitch of a factory whistle you hear on a windy day?

1. Yes2. No

Another perspective, may cause “aha!”:

What if both the whistle and the person were moving in the same direction? Is that any different than a wind blowing?....CLICKER Q!

1. Yes

2. No

Does the wind affect the pitch of a factory whistle you hear on a

windy day?

Author’s very good analogy, why wind does not affect pitch of sound

If the first guy is only putting down items once per second…

How could a faster belt increase the number of items per second the second guy can pick up?

Let’s explore Doppler effect for sound with demos and applets

http://www.falstad.com/mathphysics.html ripple

Nerf Doppler Demo

Very good animationhttp://physics-animations.com/Physics/English/waves.htm

Relative movement between source and receiver

DOES change the frequency and wavelength

Does NOT change the wave speed

Pay attention to things like frequency,

wavelength, and wave speed during applets

Is the frequency of a sound wave affected by bouncing off moving surface?

This effect is used by some bats in echolocation!

Can determine whether insect is approachingCan detect rapidly beating wings of insects

Big eared Townsend Bat,Nevada Bureau of Land Management

Is the frequency of a sound wave affected by bouncing off moving surface?

This effect is used in ultrasonography!

Doppler shift in sound frequency reveals blood velocity

User:Ekko on wikipedia (above) also:http://www.youtube.com/watch?v=L88hNMak4Go

Sound in air: Compression / Rarefaction Waves

Most of the sound you hear comes from traveling waves through the air.

Tricky: Compressions and Rarefactions travel in the same direction

Applet for visualizing traveling sound wave http://positron.ps.uci.edu/~dkirkby/music/html/demos/PlaneWave/Highlighted.html

(a) compression (b) rarefaction(c) wave propegation of a tuning fork as seen from aboveFrom http://www.pjwstk.edu.pl/~kmarasek/3-sound.ppt

What is going on when I strike this tuning fork?

A) They are carried by the wave to your ear.

B) They are dislocated only a little by the wave.

When a sound is emitted at the front of the lecture hall, what happens to the air

molecules in the sound wave?

A) They are carried by the wave to your ear.

B) They are dislocated only a little by the wave.

When a sound is emitted at the front of the lecture hall, what happens to the air

molecules in the sound wave?

Waves transmit energy not matter!

If air molecules are dislocated only a little, how are we able to hear the sound?

Pressure on our ear drums!

Speed of sound in air about 770 miles per hour

Speed does not depend on frequency or amplitude (usually)pressure or density

Speed does depend on temperature, humidity (mass of molecules)…wind!

Dry air, 68 degrees F: Speed about 770 mph or 340 m/s (Remember this!)(Speed of light in vacuum, ~670 million mph or 3 * 108 m/s…1 million

times faster)

Clicker Question—Speed of sound

Above is an amination of the time it takes light to reach the moon from earth (~ 3 seconds). How long would it take the sound from an incredible explosion to reach the moon?

A) SameB) 1,000 times longerC) 1,000,000 times longerD) Sound would never survive the trip

Clicker Question—Speed of sound

Above is an amination of the time it takes light to reach the moon from earth (~ 3 seconds). How long would it take the sound from an incredible explosion to reach the moon?

A) SameB) 1,000 times longerC) 1,000,000 times longerD) Sound would never survive the trip

Sound waves cannot travel through a vacuum!

So, you can’t have waves in space, right?

http://www.youtube.com/watch?v=3drcpE-0Inc

Clicker questionIn air, what is the wavelength of a 440 Hz sound wave?

A) 0.75 millimetersB) 0.75 metersC) 7.5 metersD) 75 meters

http://www.phys.unsw.edu.au/~jw/notes.html

440 Hz

Clicker questionIn air, what is the wavelength of a 440 Hz sound wave?

A) 0.75 millimetersB) 0.75 metersC) 7.5 metersD) 75 meters

First: try order of magnitude

http://www.phys.unsw.edu.au/~jw/notes.html

440 Hz

Wavelength =Wave speed

frequency

¾ meter = 340 m/s / 440 Hz

There can be constructive or destructive interference

Constructive Interference“In Phase”

Destructive Interference“180 degrees out of phase”

Remember the wave table demo with Ashley on Tuesday?This interference happens with sound waves as well.

Noise “cancellation” headphones use wave interference to reduce noise

Electronics in headphone detect incoming sound, and create new sound wave 180 degrees out of phase

Sound reflects very well…and interferes with itself

Remember the waves on the rope reflecting off wall?

Waves reflect when the form of matter changes (interfaces)

Much work goes into “good acoustics” fororchestra halls…preventing “dead spots” Let’s try this out! w/ speakers

and sound level meters

Sound waves of two different frequencies also interfere… “beats”

The “beat frequency” is the difference in the two source frequencies: f2 – f1

Demo w/ big speakers

http://www.ngsir.netfirms.com/englishhtm/Beats.htm

Clicker Question—Beats

Consider two different beat situations: (A) 440 Hz and 442 Hz sound wave (B) 225 Hz and a 220 Hz sound wave

Which one will be perceived as beating more quickly?

Clicker Question—Beats

Consider two different beat situations: (A) 440 Hz and 442 Hz sound wave (B) 225 Hz and a 220 Hz sound wave

(A) will beat twice per second, and (B) will beat 5 times per second. (But (A) will sound as higher pitch)

Frequency of wave influences pitch perception

http://www.phys.unsw.edu.au/~jw/notes.html

4186 Hz

440 Hz

261.6 Hz

27.5 Hz

“Psychoacoustics”

Shepard’s tones http://www.netalive.org/tinkering/shepard-effect/

Lissajous curves & beatshttp://www.youtube.com/watch?v=DXpntnHxNZQ

Parting question: Is pitch perception physics? Biology?

Exam #1 is Thursday!!!In this room, BRING A PENCIL!!!