WAVES AND SOUND SPH3U

WAVES AND SOUND
SPH3U
WAVE THEORY
• Robert Hooke in 1665 proposed the idea of a wave theory used to predict
how waves behave.
• 20 years later, Christiaan Huygens (1629-1695) postulated: "Every point on a
wave-front can be considered as a point source of tiny secondary wavelets
that spread out in front of the wave at the same speed as the wave itself. The
surface envelope, tangent to all the wavelets, constitutes the new wavefront."
• This was a useful theory to predict the future position of waves.
WAVE THEORY
• The straight line travel of waves is called rectilinear propagation.
• This is clearly evident with light where sharp shadows are produced and we
can not see around corners!
• The source producing a wave supplies energy which is transmitted in the
form of a disturbance. (Sound, explosions, tidal waves)
TERMINOLOGY
• Wavelength: (λ) The distance between adjacent crests or troughs of a wave.
(Measured in metres)
λ
• Period: (T) The time it takes for a wave to travel one wavelength. The time for
one complete cycle. Measured in seconds.
• Frequency: (ƒ) The number of waves passing a fixed point every second or the
number of cycles per second. The inverse of period, measured in Hertz. (Hz, s-1)
The frequency of a wave remains constant as it is determined by the source.
• Ƒ = 1/T
UNIVERSAL WAVE EQUATION
• A wave travels a distance equal to its wavelength in a time equal to
its period.
d
v
t
is modified as
and is finalized as
v = ƒλ

v
T
WAVE TYPES
• Transverse Waves: A wave in which the wave’s components move
perpendicular to the direction of motion of the wave.
Wave direction of travel
Examples: Water waves, All EM waves (Light, etc) and waves in springs.
WAVE TYPES
• Longitudinal waves: A wave in which the wave’s components move parallel
to the direction of motion of the wave. The wave consists of compressions
and rarefactions. Examples: Sound waves, waves in springs
SOUND
• Sound is a longitudinal wave composed of compressions and
rarefactions.
• Compressions occur where the molecules are close together.
• The pressure of the air molecules oscillate around an average
value. (Mean air pressure)
• The air molecules are in constant random motion and collide to
transmit energy away from a sound source.
SOUND
• Sound moves at 332 m/s at 0oC and 1 atm.
• As temperature increases, so does molecular motion and therefore
the speed of sound.
• Sound waves are transmitted through some medium to a listener
(there is no sound in space).
332 0.6 /
=
+
[]


EXAMPLES
• Find the speed of sound at 16.0oC.
332 0.6 /
=
+
[]


v = 332 m/s + (0.6 m/s/oC)(16.0 oC)
= 342 m/s
EXAMPLES
• Thunder is heard 8.0 s after lightning is seen. If T = 30.0oC, how far
away was the lightning?
• Assume the observation of the light was instantaneous (as light is
much faster than sound).
332 0.6 /
=
+
[]


v = 332 m/s + (0.6 m/s/oC)(30.0 oC)
= 350 m/s
d = vt
d = 350 m/s(8.0 s) = 2.8 km
EXAMPLES
332 0.6 /
=
+
[]


• A biologist is dropped into an 80.0 m deep dry well for safekeeping. How long after dropping him will you hear the sound (if T
= 20.0oC) of his body hitting the ground at the bottom?
• Part 1: Find time to free fall to bottom (Kinematics formulae)
• Part 2: Find time for sound to travel back up to our ears. (d = vt)
• 4.04 s + 0.23 s = 4.27 s
CHALLENGE QUESTION
• You drop a rock into a well and 5.37 s later
you hear the splash. If T = 16.0oC, how
deep is the well?
CLASS/HOME WORK
• Sound worksheet
• Textbook work on sound speed, etc.