Transcript Ch 19 SOUND - Physics & Astronomy | SFASU

Ch 20 SOUND
• Sound is a compression wave
in an elastic medium. These can
include solids, liquids and gases
or a plasma.
Human hearing can detect sound
waves with frequencies between 20
Hz and 20,000Hz (20kHz).
Frequencies below 20Hz are
called infrasonic. They are
inaudible to human hearing.
(We can’t hear them!)
• Frequencies above 20,000Hz are
called ultrasonic. They are also
inaudible to human hearing.
(Again, we can’t hear them, but
some animal can!)
Media That Transmit Sound
•Any elastic material can transmit
sound.
•Steel is a very good conductor of sound.
•Water is not as good a conductor as
steel, but is better than air.
•Air is a poor conductor of sound
Speed of Sound
• Sound waves will have different
speeds in different media. The speed
also varies with temperature.
•Sound travels 15 times faster in steel
than in air, and it travels 4 times
faster in water than in air.
•Sound travels at about 330 m/s in air
at 0 oC and 340 m/s in air at 20 oC.
Reflection of Sound
•Echo - The reflection of sound.
•E.G. Singing in the shower.
•Sound reflects off of a smooth
surface at the same angle at which it is
incident.
•(i.e. The angle of reflection equals the
angle of incidence.)
Refraction
• Refraction is the bending of the sound
wave. Sound waves bend when parts of
the wave front travel at different
speeds.
•This occurs when there are:
–Uneven Winds.
–Uneven Temperatures.
•The speed of sound is slightly
faster in warm air than in cool air.
Energy in Sound Waves
• Sound generally transmits only a small
amount of energy.
•(10,000,000 people talking at the
same time = energy needed to
light a common flashlight).
•Sound energy dissipates to thermal
energy (heat) while sound travels
through air.
• High frequency sound dissipates to
thermal energy more rapidly than low
frequency sound.
•Hence, low frequency sound has a
greater range in air than high frequency
sound. (Fog horns typically produce low
frequency sound, but have a long range)
Forced Vibrations
• The setting up of vibrations in an
object by a vibrating force.
Examples:
1) Sounding board of a musical
instrument.
2) The box of a music box.
Natural Frequency
A frequency at which an elastic
object naturally tends to vibrate if
it is disturbed and the disturbing
force is removed.
The natural frequency of an
object depends on such factors as
the elasticity and shape of the
object.
• When the frequency of a forced
vibration (driving frequency)
matches the object’s natural
frequency, a dramatic increase in
amplitude occurs.
This is known as resonance.
Example: Pushing someone on a swing.
Resonance
• The response of a body when a
forcing frequency matches its
natural frequency.
Sound Interference
• Sound waves can interfere with each other
just like transverse waves interfere.
Transverse Wave Interference
+
Longitudinal (Sound) wave interference
+
Beats
• When two tones of slightly different
frequencies are sounded together, a
fluctuation of the loudness of the combined
sounds is heard.
•Alternating loud and faint sound can be
heard. This periodic variation in loudness
is called beats.
Beats
f1
f2
fbeat= |f1 - f2|
Standing Waves on a String
STANDING WAVES ON A STRING
SUPERPOSITION OF WAVES
Parts of a Standing Wave on a String
Fundamental (1st harmonic)
NODES
2nd harmonic (1st overtone)
Antinodes
3rd harmonic (2nd overtone)
Fundamental (1st harmonic)
L = /2
2nd harmonic (1st overtone)
L=
Tuning fork
L1= /4
Standing Waves in an air column –
open tube at one end
Fundamental (first harmonic)
L2= 3/4
Third harmonic- open at one end
End of Chapter 19
Simulations