Transcript Sound - Nutley Public Schools

Sound
• Acoustics is the study of sound.
• All sounds are waves produced by
vibrating objects - tuning forks, vocal
chords, reeds, lips, columns of air,
strings, cricket legs
• Demo – tuning forks - water
Sound
Sound Waves
• Sound is a longitudinal wave with
compressions and rarefactions.
Sound Waves
page 272
Sound Waves
• Notice that the air molecules move in a
direction parallel to the direction of the
wave.
• Demo – Slinky on the floor.
Speed of Sound
• The speed of propagation
Medium
Air
Speed of sound
343 m/s
Steel
5100 m/s
Water
1370 m/s
Vacuum
0 m/s
Speed vs Temp
• As temperature increases, the speed
of sound increases.
0.6 m/s per oC
Speed of Sound
vs Speed of Light
Sound = 343 m/s
Light = 300, 000, 000 m/s
Notice that light travels much faster.
Lightning & Thunder
• Light reaches you
in an extremely
short period of
time.
• Sound reaches you
at a much slower
rate. It takes about
5 seconds to travel
1 mile.
The delayed sound reaching
your ear after the light.
• Other examples include – starters pistol,
chopping wood
• You see the event(light), count the number
of seconds until the sound arrives.
5 seconds = 1 mile
10 seconds = 2 miles
Figure 14-36
Problem 14-59
Path that sound travels in your ear
Sequence of vibrations from the source
• To the ear drum
• To the bones in the middle ear
• To the oval window in the middle ear
• To the fluid in the inner ear
• To the hairs in the cochlea in the inner
ear
• To the nerves which go to the brain on
the auditory nerve.
Pitch
• The pitch is determined
by the frequency of the
sound. Units are Hz or
vibrations per sec.
• Humans hear 20 to
20,000 Hz
Loudness of Sound
• How loud a sound seems is determined
by the wave’s amplitude. This is
proportional to its energy.
• We use a decibel scale to measure
loudness.
Sound
• Loud noises can damage your hearing. This
usually lowers your upper limit. The tiny
hairs in the inner ear may fall out.
Loudness
Sound
Decibels
Hearing
threshold
Rustle of leaves
0
Conversation
Rock Concert
Pain Threshold
Jet Engine
10
60
110
120
130
Reflection of Sound
• Reflection of sound
is called an echo.
• Sound waves
reflect off of hard
smooth surfaces.
• Curtains and rugs
results in most of
the sound being
absorbed
Johaan Christian Doppler
1803-1853
Doppler effect
A change in frequency (pitch) of
sound due to the motion of the
source or the receiver
Doppler Effect
Doppler: Source
Doppler: Observer
Approaching, the frequency is higher because the wavefronts are closer
together in time. Departing, the frequency is lower.
Sound - resonance
• Sound is produced by vibrating systems.
• All systems have one or more natural
frequencies.
• A natural frequency is the frequency at
which a system tends to vibrate in the
absence of any driving or damping
force.
Sound - resonance
• If a system is exposed to a vibration that
matches its natural frequency, it will
vibrate with an increased amplitude.
• This results in the amplification
(increase in amplitude). of that
frequency
• This phenomenon is called resonance.
Sound - resonance
• When resonance occurs in systems
standing waves are formed.
Sound - resonance
•In order for standing waves to form in a
closed pipe (closed at one end), the
length of the pipe L must be an odd
multiple of one fourth of the wavelength.
•The necessary condition is that there is
a node at the closed end, and an antinode
at the open end.
Sound - resonance
n
L
n  1,3,5, 7...
4
4L

n  1,3,5, 7...
n
Standing Waves in a Pipe That Is Open at One End
(Closed Pipe)
Sound - resonance
The air column lengths at which resonance
for a given frequency occurs,
increase in steps of

2
.
Sound - resonance
•In order for standing waves to form in
an open pipe (open at both ends), the
length of the pipe L must be a whole
number multiple of one half of the
wavelength.
•The necessary condition is that there are
antinodes at both ends.
Sound - resonance
n
L
n  1, 2,3...
2
2L

n  1, 2,3...
n
Sound - resonance
The air column lengths at which resonance
for a given frequency occurs,
increase in steps of

2
.
Figure 14-29
Standing Waves in a Pipe That Is Open at Both Ends
Sound - resonance
•Because the speed of sound in air is
constant, we can only vary pipe length or
frequency to obtain conditions needed
for resonance.
Sound - resonance
Example:
A tuning fork with a frequency of 392 Hz is found to
cause resonance in an air column spaced by 44.3 cm.
the air temperature is 27oC. Find the velocity of
sound in air at that temperature.
Sound - resonance
Example :
We know that resonance lengths are spaced by
half wavelengths.
Therefore   2  44.3cm  88.6cm  .886m
m
v  f    392 Hz .886m   347.3
s
Terminology – specifically for vibrating air columns.(pipes)
Fundamental frequency – (first harmonic) the frequency of the longest standing sound wave that can
form in a pipe.
Second harmonic – two times the frequency of the longest
standing sound wave that can form in a pipe.
Third harmonic – three times the frequency of the longest
standing sound wave that can form in a pipe.
Sound - resonance
Beats –
Beats occur when two waves of
slightly different frequencies
are superimposed. A pulsating
variation in loudness is heard.
Sound - resonance
Waves on a string –
the necessary condition for
standing waves on a string, is
that a node exist at either end.
Sound - resonance
As a consequence the wavelength of
the fundamental frequency1 is
1  2L
where L  the length of the string
v
The fundamental frequency( first harmonic) is f1 
2L
where v is the velocity of waves traveling on the string
Sound - resonance
Subsequent allowable frequencies f n
are whole number multiples of the
fundamental frequency
f n  nf1 n  1, 2,3,...
Sound - resonance
Subsequent allowable wavelengths n
are
n
1
2L
 =
n  1, 2,3,...
n
n
Figure 14-24a
Harmonics
Figure 14-24b
Harmonics
Figure 14-24c
Harmonics
Sound - resonance
Example: One of the harmonics
on a string 1.3 m long has a
frequency of 15.6 Hz. The next
higher harmonic has a frequency
of 23.4Hz. Find (a) the
fundamental frequency, and (b)
the speed of the waves on this
string.
Sound - resonance
f n 1  f n   n  1 f1  nf1  f1
 23.4 Hz  15.6 Hz  7.80 Hz
v
 v  f1  2 L  
f1 
2L
m
  7.8 Hz  2 1.30m   20.28
s