This is PHYS 1240 - Sound and Music

Lecture 5

Professor Patricia Rankin

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Clickers on

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hours

Graduate student Teaching Assistant

Tyler C Mcmaken - online 10-11am Friday

Undergraduate Learning Assistants

Madeline Karr Online 6-7pm MondayRishi Mayekar Online 11-12noon ThurMiles Warnke Online 3-4pm Wed

Professor

Patricia Rankin Online 2-3pm Wed

Physics 1240 Lecture 5

Today: Shock waves, Doppler Effect, Beats, Propagation

of Sound, Interference, Superposition

Next time: Characteristics of sound, Waves in

times/space, waves in 2d, 3d, Standing Waves

physicscourses.colorado.edu/phys1240

Canvas Site: assignments, administration, grades

Homework – HW2 Due Wed Jan 29th 5pm

Homelabs – Hlab2 Due Monday Feb 10th 5pm

Debrief – last class

Normal or natural modes

Q – Damping

number of cycles before amplitude

drops to 4.32% of starting value

Q – Resonance – peak frequency/bandwidth that excites

motion at least half peak amplitude

Estimating distance using sound – echoes, lighteningAfter you see lightning, start counting to 30 (30s). If you hear thunder before you reach 30, go indoors. Suspend activities for at least 30 minutes after the last clap of thunder.

𝜈 =𝜆

𝑇= 𝜆𝑓

Wavelength

PeriodFrequency

Tutorial

How can you improve piano key estimate?

What other info might help?

#of pianos sold in a year

#of piano tuners

#of violins

#of piano students

Socioeconomic status

Period/frequency

Dimensional Analysis

CLICKER 5.1

When sound travels from a source to a listener,

what do air molecules never do?

a) Travel from the source to the listener at 344 m/s

b) Move back and forth with the frequency of the sound wave

c) Bump into other air molecules

d) Move in the direction of wave propagation

e) Air molecules do all of the above

CLICKER 5.1A

When sound travels from a source to a listener,

what do air molecules never do?

a) Travel from the source to the listener at 344 m/s

b) Move back and forth with the frequency of the sound wave

c) Bump into other air molecules

d) Move in the direction of wave propagation

e) Air molecules do all of the above

I grab the left end of a LONG string and

wiggle it up and down. There is a purple

painted spot somewhere on the string.

What choice below best shows the motion

of the purple spot?

A) (up and down)

B) (pushed steadily to the right)

C) (left and right)

D) (no motion)

E) (circular path)

5-4

Clicker 5.2

I grab the left end of a LONG string and

wiggle it up and down. There is a purple

painted spot somewhere on the string.

What choice below best shows the motion

of the purple spot?

A) (up and down)

B) (pushed steadily to the right)

C) (left and right)

D) (no motion)

E) (circular path)

5-4

Clicker 5.2 A

A dust particle is located in front of a

speaker. The speaker moves back and forth at a

constant rate, creating a loud sound wave.

Which choice below shows the motion of the

dust particle?

speakerdust

5-3

A) (up and down)

B) (pushed steadily to the right)

C) (left and right)

D) (no motion)

E) (circular path)

Clicker 5.3

A dust particle is located in front of a

speaker. The speaker moves back and forth at a

constant rate, creating a loud sound wave.

Which choice below shows the motion of the

dust particle?

speakerdust

5-3

A) (up and down)

B) (pushed steadily to the right)

C) (left and right)

D) (no motion)

E) (circular path)

Clicker 5.3 C

Shock waves

• “Sonic boom” occurs when object travels faster than the

local speed of sound (waves can’t get out of the way fast

enough)

• Bullets

• Whip Crack

• Apatosaurus tail

Boat wakes

If you toss a pebble in a pond, little waves will form

in concentric circles and propagate away from the point

of impact. If a boat travels through the pond at 3 to 5 miles

per hour, little waves will propagate in the same way both

ahead of and behind the boat, and the boat will travel

through them.

if a boat travels faster than the waves can propagate

through water, then the waves "can't get out of the way" of

the boat fast enough, and they form a wake. A wake is a

larger single wave. It is formed out of all the little waves

that would have propagated ahead of the boat but could

not.

When an airplane travels through the air, it produces sound waves. If the plane

is traveling slower than the speed of sound (the speed of sound varies, but 700

mph is typical through air), then sound waves can propagate ahead of the

plane. If the plane breaks the sound barrier and flies faster than the speed of

sound, it produces a sonic boom when it flies past. The boom is the "wake" of

the plane's sound waves. All of the sound waves that would have normally

propagated ahead of the plane are combined together so at first you hear

nothing, and then you hear the boom they create.

Doppler Effect

• Doppler effect: the shift in frequency of a wave where

the source and the observer are moving relative to one

another (higher frequency if moving toward each other)

∆𝑣

𝑣sound≅ 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑎𝑙 change in 𝑓 =

𝑓1−𝑓0

𝑓0

∆𝑣 = source velocity – observer velocity

𝑓0=emitted frequency

𝑓1=received frequency

Clicker 5.4

A fire truck travels away from you at 67 mph (30 m/s). If the speed of sound

is 343 m/s, do you hear a lower- or higher-pitched note from the siren than

the driver? What’s the percent change in the frequency?

A) You hear 8.75% lower

B) You hear 8.75% higher

C) You hear 19.5% lower

D) You hear 19.5% higher

E) You hear the same pitch as the driver

Clicker 5.4 A

A fire truck travels away from you at 67 mph (30 m/s). If the speed of sound

is 343 m/s, do you hear a lower- or higher-pitched note from the siren than

the driver? What’s the percent change in the frequency?

A) You hear 8.75% lower

B) You hear 8.75% higher

C) You hear 19.5% lower

D) You hear 19.5% higher

E) You hear the same pitch as the driver

% change in 𝑓 ≅∆𝑣

𝑣sound=

30m/s

343m/s≅ 0.0875

What happens when sound

wave encounters an

obstacle?

Provides basis for acoustics

Sound Propagation – Possibilities

• Reflection (diffuse or spectral)

• Absorption (greater for softer surfaces)

• Refraction (when speed of sound changes)

• Diffraction (greater for larger wavelengths)

Acoustic Shadows

Regions where sound has failed to

propagate

Refraction

Light

Why straws look bent in water

Refraction:

bending due to a change in the

speed of sound (change in medium)

Diffraction

Allows sound to get around corners

Depends on wavelength of wave/size of gap

Diffraction – single slit (ripple

tank)

Diffraction Double Slit

peaktrough

Adding/Superimposing

• Waves just add

• Adding two waves can be constructive or destructive

Interference/Superposition

Constructive Interference

• For ∆𝐿 = 𝐿2 − 𝐿1 = difference between your distance from one source

and your distance from a second source:

• Constructive: ∆𝐿 = 𝑛λ (where 𝑛 is an integer)

Destructive Interference

• For ∆𝐿 = 𝐿2 − 𝐿1 = difference between your distance from one source

and your distance from a second source:

• Destructive: ∆𝐿 = (𝑛 + 1/2)λ2

(where 𝑛 is an integer)

Noise Cancellation

Clicker 5.5

If you are in a room with two speakers each producing sine waves with a

wavelength of 2 meters, where should you stand if you don’t want to hear any

sound?

A) 2 meters from one speaker and 2 meters from the other

B) 2 meters from one speaker and 4 meters from the other

C) 2 meters from one speaker and 3 meters from the other

D) 3 meters from one speaker and 5 meters from the other

E) 1 meter from one speaker and 0.5 meters from the other

Clicker 5.5 C

If you are in a room with two speakers each producing sine waves with a

wavelength of 2 meters, where should you stand if you don’t want to hear any

sound?

A) 2 meters from one speaker and 2 meters from the other

B) 2 meters from one speaker and 4 meters from the other

C) 2 meters from one speaker and 3 meters from the other

D) 3 meters from one speaker and 5 meters from the other

E) 1 meter from one speaker and 0.5 meters from the other

Can also add waves of different

frequencies