Exam III
Physics 101: Lecture 22
Sound

Today’s lecture will cover Textbook Chapter 12
Physics 101: Lecture 22, Pg 1
Speed of Sound
for pulse on string: v = sqrt(F / m)
 For fluids:
v = sqrt(B/r)
 Recall
Medium
Speed (m/s)
Air
343
Helium
972
Water
1500
Steel
5600
Physics 101: Lecture 22, Pg 2
Helium ACTs
A sound wave having frequency f0, speed v0 and wavelength l0, is
traveling through air when in encounters a large helium-filled balloon.
Inside the balloon the frequency of the wave is f1, its speed is v1, and
its wavelength is l1
Compare the speed of the sound wave inside and outside the balloon
A. v1 < v0
B. v1 = v0
C. v1 > v0
Compare the frequency of the sound wave inside and outside the
balloon
A. f1 < f0
B. f1 = f0
C. f1 > f0
Compare the wavelength of the sound wave inside and outside the
balloon
A. l1 < l0
B. l1 = l0
C. l1 > l0
Physics 101: Lecture 22, Pg 3
Intensity and Loudness
 Intensity
if the power per unit area.
I = P / A
Units Watts/m2
 For
Sound Waves
I = p02 / (2 r v)
Proportional to p02 note Energy goes as A2
 Loudness
(Decibels)
Loudness perception is logarithmic
Threshold for hearing I0 = 10-12 W/m2
b = (10 dB) log10 ( I / I0)
b2 – b1 = (10 dB) log10(I2/I1)
Physics 101: Lecture 22, Pg 4
Decibels ACT
 If
1 person can shout with loudness 50
dB. How loud will it be when 100 people
shout?
1) 52 dB
2) 70 dB
3) 150 dB
Physics 101: Lecture 22, Pg 5
Intensity Preflight
Suppose you are standing a distance D away from a speaker that is
radiating sound in a spherically uniform way. You walk away from
the speaker until the loudness of the sound is reduced by a factor of
two. About how far from the speaker are you now? (neglect any
reflections from the ground)
1. 10D
2. 4D
3. 3D
4. 2D
Physics 101: Lecture 22, Pg 6
Intensity ACT

Recall Intensity = P/A. If you are standing 6 meters
from a speaker, and you walk towards it until you
are 3 meters away, by what factor has the intensity
of the sound increased?
1) 2
2) 4
3) 8
Physics 101: Lecture 22, Pg 7
Standing Waves in Pipes
Open at both ends:
Pressure Node at end
l = 2 L / n n=1,2,3..
Open at one end:
Pressure AntiNode at
open end :l = 4 L / n
n odd
Physics 101: Lecture 22, Pg 8
Organ Pipe Example
A 0.9 m organ pipe (open at both ends) is
measured to have it’s first harmonic at a
frequency of 382 Hz. What is the speed
of sound in the pipe?
L=0.9 m
Physics 101: Lecture 22, Pg 9
Resonance ACT
 What
happens to the fundamental frequency
of a pipe, if the air (v=300 m/s) is replaced
by helium (v=900 m/s)?
1) Increases
2) Same
3) Decreases
Physics 101: Lecture 22, Pg 10
Preflight 5
 As
a police car passes you with its siren
on, the frequency of the sound you hear
from its siren
Doppler Example Audio
Doppler Example Visual
1) Increases
2) Decreases
3) Same
Physics 101: Lecture 22, Pg 11
Doppler Effect

When source is moving toward you:
 Distance between waves decreases
 Frequency increases

fo
When source is moving away from you:
 Distance between waves increases
 Frequency decreases

When moving toward source:
 Velocity of waves increases
 Frequency increases

fs
=
vs 

1 

v 

When moving away from source:
 Velocity of waves decreases
 Frequency decreases
fo = f s
vo 

f o = f s 1 

v 

v  vo 
v  vs 
Physics 101: Lecture 22, Pg 12
Doppler ACT
A: You are driving along the highway at 65 mph, and behind you a
police car, also traveling at 65 mph, has its siren turned on.
B: You and the police car have both pulled over to the side of the
road, but the siren is still turned on.
In which case does the frequency of the siren seem higher to you?
f
f’
1. Case A
v
2. Case B
3. same
vs
vo
Physics 101: Lecture 22, Pg 13
Interference and Superposition
Constructive interference
Destructive interference
Physics 101: Lecture 22, Pg 14
Superposition & Interference

Consider two harmonic waves A and B meeting at x=0.
Same amplitudes, but 2 = 1.15 x 1.

The displacement versus time for each is shown below:
A(1t)
B(2t)
C(t) = A(t) + B(t)
DESTRUCTIVE
INTERFERENCE
CONSTRUCTIVE
INTERFERENCE
Physics 101: Lecture 22, Pg 15
Summary
 Speed
of sound v = sqrt(B/r)
 Intensity
B = (10 dB) log10 ( I / I0)
 Standing Waves
fn = n v/(2L)
fn = n v/(4L)
 Doppler
Open at both ends n=1,2,3…
Open at one end n=1,3,5…
Effect fo = fs (v-vo) / (v-vs)
Physics 101: Lecture 22, Pg 16