Sound as a wave

1
Sound as a wave
Interference of sound Diffraction of sound Other
wave properties of sound
1
2
3
2
One day some construction work is proceeding
outside your home.
How to reduce the noise?
?
?
?
What is the principle behind this/these method(s)?
3
Interference of sound
a Seeing sounds
Seeing sound with a CRO microphone.

• Microphone

• connects to CRO

• changes the vibration of sound to electrical
  vibrations shown on CRO screen

4
Interference of sound
Set the signal generator at 2 kHz.
Covering one ear, walk across in front of the
loudspeakers.
Detect any changes in the loudspeakers.
5
Interference of sound
Connect a microphone to a CRO and move it across
in front of the two loudspeakers.
CRO
metre rule
microphone
Detect any changes in the amplitude of the
waveform on the CRO.
6
Interference of sound
b Interference pattern of sound
In Expt. 10 h, alternate loud and soft sounds are
heard while walking across in front of the
loudspeakers.
The sounds from the loudspeakers reinforce each
other at some positions...
The sounds from the loudspeakers cancel each
other at some positions...
7
Interference of sound
b Interference pattern of sound
v f?
Wave formula
can also apply to sound waves.
8
Interference of sound
c Coherent sound sources for interference
Two loudspeakers are connected to a signal
generator.
The sounds emitted from the loudspeakers are in
phase and same frequency
They are two coherent sources.
9
Diffraction of sound
a Diffraction of sound
Sound can bend around corners (diffract) like
water waves and light waves
Examples
- hear others conversation behind a doorway
- hear a plane flying when staying indoor
10
Diffraction of sound
b Degree of diffraction
If the wavelengths of sound waves are comparable
to the size of the gap
a remarkable diffraction pattern appears.
11
Diffraction of sound
b Degree of diffraction
Since the width of the doorway is comparable to
the wavelength of most speech and music...
such sound can bend around the corner.
12
Other wave properties of sound

• Reflection (e.g. echo)

• Refraction

Waves are refracted and focused.
13
Other wave properties of sound

• Diffraction (e.g. hearing people talking around
  corner)

• Interference

14
Q1 Two loudspeakers are
Two loudspeakers are connected to a signal
generator.
signal generator
P
S
R
Q
R is in the midway of the loudspeakers.
15
Q1 Two loudspeakers are
If alternate loud and soft sounds are heard along
PQ,
which is INCORRECT?
A When the loudspeakers are put close together,
fewer soft sounds are heard along PQ.
B Alternate loud and soft sounds can also be
heard along RS.
C The loudness of the sound at S decreases when
one of the loudspeakers is disconnected.
16
Q2 Why can we hear the...
Why can we hear the conversation of people around
a corner?
Oh, my students like me!
Yes, she is very kind...
Have you met the new teacher Ms Chan?
17
Q2 Why can we hear the...
Since sound waves of most speech have _________
wavelengths, they ________ around corners.
long
bend
? we can hear people talking around the corner.
18
Example 6
How far apart should loudspeakers be separated?
To produce interference, a signal of frequency 2
kHz is fed into two loudspeakers.
How far apart should the two loudspeakers be
placed?
(they are separated by 3?)
Speed of sound in air 340 m s-1.
19
Example 6
How far apart should loudspeakers be separated?
From the wave equation v f?,
? 3? 0.51 m
The two loudspeakers should be placed about 0.5 m
apart.