This document discusses the properties and behavior of sound waves. It explains that sound waves are longitudinal waves that propagate through a medium by transmitting pressure variations. The speed at which sound travels depends on the properties of the medium, with denser and less compressible materials allowing for faster transmission. The document also describes how sound waves can be reflected, refracted, undergo interference and superposition, and how their amplitude and frequency determine the loudness and pitch we perceive.

1. World
Communicates
Sound
waves
Preliminary
Physics
8.2.2
By
S.
Choi

2. Sound
wave
(a)
• Sound
waves
are
produced
by
vibra;ng
objects
–
they
are
the
result
of
vibra;ons
or
oscilla;ons
of
par;cles
in
a
medium.
• Sound
energy
propagates
in
3-­‐D
as
alterna/ng
higher
and
lower
pressure
varia;ons
(compressions
and
rarefac:ons).
• The
pressure
varia;ons
propagate
through
a
medium
away
from
the
source
of
the
vibra;ons.
• Sounds
waves
travel
at
different
speeds
depending
on
the
medium
and
temperatures.
The
speed
of
sound
in
dry
air
is
340m/s.
They
can
be
reflected
(echo)
and
refracted.
• Sound
travels
faster
in
liquids
and
non-­‐porous
solids
than
it
does
in
air.

3. Ac;vi;es
with
tuning
forks
1. Listen
to
the
sound
when
a
vibra;ng
tuning
fork
is
touched
on
a
solid
wooden
surface.
2. Touch
a
vibra;ng
tuning
fork
against
the
nose
or
the
ear
lobe.
3. Listen
to
the
sound
produced
as
the
tuning
fork
is
rotated
close
to
the
ear
about
its
long
axis.
4. Dip
the
vibra;ng
tuning
fork
into
water.
The
vibra;ng
prongs
of
the
tuning
fork
cause
the
molecules
in
the
surrounding
air
to
vibrate
–
sound
propaga;ng
as
a
longitudinal
wave
mo;on
in
3-­‐D
away
from
the
source

4. Longitudinal
waves
represented
as
transverse
wave
(b)
• When
a
sound
travels
through
a
tube
fiOed
with
pressure
gauges,
the
pressure
gauges
will
record
varia;ons
in
pressure.
• Compressions
(C)
are
regions
of
higher
pressure
and
rarefac;ons
(R)
are
regions
of
lower
pressure.
• The
graph
shows
the
pressure
at
all
points
along
the
tube
at
one
instant
of
/me.
R C R C
Compression

5. Longitudinal
waves
represented
as
transverse
wave
(b)
• A
single
pressure
gauge
recording
the
pressure
at
a
fixed
point
in
the
tube
over
a
period
of
;me
would
show
regular
pressure
varia/ons.
• As
the
wave
propagates
from
leS
to
right
along
the
tube,
the
pressure
alternates
from
low
to
high
with
a
frequency
equal
to
that
of
the
sound
wave.
• The
graph
shows
the
pressure
varia;ons
at
a
fixed
point
as
the
wave
moves
along
the
tube.
R C R C

6. Longitudinal
waves
represented
as
transverse
wave
(b)

7. Volume
&
pitch
(c)
• Loudness
of
a
sound
is
caused
by
the
amplitude
of
the
sound
wave
–
higher
the
amplitude,
louder
the
sound
• Tone
or
the
pitch
of
a
sound
is
caused
by
the
frequency.
Higher
the
frequency,
higher
the
tone/
pitch
• Humans
have
the
hearing
range
from
20
Hz
to
20
kHz
(approximately).
The
range
shrinks
during
life.

8. Ultrasound
&
infrasound
Sound
waves
with
a
frequency
too
high
for
the
human
ear
are
called
ultrasound
Sound
waves
with
a
frequency
too
low
for
the
human
ear
to
hear
are
called
infrasound.
Animals
such
as
dogs,
bats,
birds
and
insects
can
hear
ultrasound.
Animals
such
as
whales,
elephants
and
hippopotamus
use
infrasound
to
communicate
over
distances.

9. Echo
(d)
• When
a
sound
wave
meets
a
boundary
between
two
media,
some
of
the
energy
of
the
wave
will
be
reflected
at
the
boundary.
• Knowing
the
speed
of
sound
in
a
par;cular
medium,
the
distance
to
a
reflec;ng
surface
can
be
determined.
This
principle
is
applied
in
SONAR
used
by
ships
to
locate
the
posi;on
of
objec;ons
under
water.
• Bats
use
a
sophis;cated
sonar
system
to
navigate
and
to
catch
prey.

10. The
speed
of
sound
in
different
media
Material
Density
(g/cm)
Speed
(m/s)
Copper
8.90
6420
Steel
7.86
5940
Beryllium
1.93
12890
Aluminium
2.58
6420
Water
1.00
1496
Ethanol
0.79
1207
Air
0.00139
331.45
Helium
0.000178
965
Fat
0.95
1450
Muscle
1.07
1580
Skull
bone
1.91
4080

11. Superposi;on
of
waves
(e)
• The
addi/on
of
waves
is
called
superposi:on.
Sounds
waves
from
separate
sources
can
be
added.
• If
two
out-­‐of-­‐phase
waves
interfere,
the
resultant
sound
wave
will
have
a
smaller
amplitude
than
either
of
the
original
waves
• If
two
in-­‐phase
waves
interfere,
the
resultant
sound
wave
will
have
a
greater
amplitude
than
either
of
the
original
waves.
Construc:ve
two
waves
are
in-­‐phase
(half-­‐cycle)
Destruc:ve
interference
occurs
when
interference
occurs
when
two
waves
are
out-­‐of-­‐phase
by
180°
(half-­‐
cycle).
If
the
amplitudes
of
both
waves
are
equal,
then
annulment
of
waves
will
occur.

12. Graphical
superposi;on
of
waves
(d)
No;ce
any
paOerns?

13. Beats
• When
two
sources
of
sound
of
the
same
amplitude
but
slightly
different
frequency
are
heard
together,
there
will
be
a
rhythmic
change
to
the
volume
of
the
sound.
• When
the
two
sound
waves
are
in
phase,
the
amplitude
of
the
resul;ng
sound
wave
is
the
sum
of
the
amplitudes
of
the
two
waves,
and
results
in
a
loud
sound.
• As
the
waves
driS
out
of
phase,
the
resultant
amplitude
will
become
smaller,
eventually
reaching
zero
before
increasing
again
as
the
waves
driS
back
into
phase.
• The
term
‘beats’
is
used
to
describe
the
varia;on
in
the
loudness
of
the
sound.