Speakers A and B emit sound
waves of λ = 1 m which interfere
constructively at a donkey
located far away (say, 200 m).
What happens to the sound
intensity if speaker A steps
back 2.5 m?
(1) intensity increases
(2) intensity stays the same
(3) intensity goes to zero
(4) impossible to tell
PHYS 1022: Chap. 20, Pg 2
1
New Topic
PHYS 1022: Chap. 20, Pg 3
PHYS 1022: Chap. 20, Pg 4
2
New Topic
PHYS 1022: Chap. 21, Pg 5
A popular car stereo has four speakers, each rated at 60 . In
answering the following questions, assume that the speakers
produce sound at their maximum power.
Find the intensity of the sound waves produced by one 60- speaker
at a distance of 1.0 m in W/m2, in dB/m2
PHYS 1022: Chap. 21, Pg 6
3
New Topic
PHYS 1022: Chap. 21, Pg 7
PHYS 1022: Chap. 21, Pg 8
4
PHYS 1022: Chap. 21, Pg 9
New Topic
PHYS 1022: Chap. 21, Pg 10
5
PHYS 1022: Chap. 21, Pg 11
PHYS 1022: Chap. 21, Pg 12
6
PHYS 1022: Chap. 21, Pg 13
PHYS 1022: Chap. 21, Pg 14
7
PHYS 1022: Chap. 21, Pg 15
PHYS 1022: Chap. 21, Pg 16
8
PHYS 1022: Chap. 21, Pg 17
New Topic
PHYS 1022: Chap. 21, Pg 18
9
PHYS 1022: Chap. 21, Pg 19
  Occurs when an oscillating system is “driven” (by
adding energy) at a “natural frequency” of the system
  Examples
  pushing a kid on a swing
  “singing” water glasses
  musical instruments
  your ears
PHYS 1022: Chap. 21, Pg 20
10
PHYS 1022: Chap. 21, Pg 21
PHYS 1022: Chap. 21, Pg 22
11
The Cochlea is a coiled tube, stiffest at
the base and less so at its apex
• A pure tone will excite the cilia at one
spot
• Complex tones, all along
• The longer the wavelength (lower
frequency, the further the penetration)
• Loud noises will permanently kill the cilia
cells
PHYS 1022: Chap. 21, Pg 23
PHYS 1022: Chap. 21, Pg 24
12
PHYS 1022: Chap. 21, Pg 25
t = 2 ms
t = 3 ms
t = 4 ms
t = 6 ms
t = 8 ms
PHYS 1022: Chap. 21, Pg 26
13
original freq. f
freq. 2/3 f
freq. 3/2 f
freq. 1/4 f
PHYS 1022: Chap. 21, Pg 27
PHYS 1022: Chap. 21, Pg 28
14
D(x,t) = sin (kx – ωt + φ0)
Is this
destructive or
constructive
interference?
x1
x2
Δx
φ1
φ2
Δφ
Point A
Point B
Point C
Point D
PHYS 1022: Chap. 21, Pg 29
D(x,t) = sin (kx – ωt + φ0)
PHYS 1022: Chap. 21, Pg 30
15
D(x,t) = sin (kx – ωt + φ0)
x1
X2
Δx
φ1
φ2
Δφ
Point A
1
3
2
0.5π
2.5π
2π
Point B
2
4
2
π
3π
2π
Point C
3
5
2
1.5π
3.5π
2π
Point D
4
6
2
2π
4π
2π
PHYS 1022: Chap. 21, Pg 31
  A sailboat is 600 m from shore and travelling in a straight line
parallel to the shore. It receives simultaneous radio signals of the
same frequency from antennas A and B, which are separated by
400 m.
  The signal at the ship has a maximum at point C (why?).
  The first minimum of the signal occurs at point D, directly
across from B.
  What is the wavelength of the radio waves?
PHYS 1022: Chap. 21, Pg 32
16