Sound Waves
Chpt 14.
What is sound?
• Sound waves are in all fluids
– Fluids are both gases and liquids
• People can hear from 20 Hz- 20,000Hz
• The process of hearing is an intricate
procedure.
The Amazing Ear
• There are sounds outside the limits of human
hearing
• Infrasonic: sounds with frequencies lower than 20 Hz
– Ex: Earthquakes, wind, weather, elephants, cattle
• Ultrasonic: sounds with frequencies higher than
20,000 Hz.
– Dogs <45,000 Hz
– Cats <70,000 Hz
– Bats <100,000 Hz
Test Your Range!
• http://www.jetcityorange.com/dog-whistle/
Speed of Sound in Different
Materials
Material
m/s
Air (0°ͦ)
331
Air (20°)
343
Helium (0°)
972
Water (25°)
1493
Seawater (25°)
1533
Copper (25°)
3560
Iron (25°)
5130
• Find the wavelength in air at 20° C of an 18 Hz
sound wave, which is one of the lowest
frequencies that is detectable by the human
ear.
• What is the wavelength of an 18 Hz sound
wave in seawater at 25°C?
• Find the frequency of a sound wave moving
through iron at 25°C with a wavelength of
1.25m.
• A 2280 Hz sound wave has a wavelength of
0.655m in an unknown medium. Identify the
medium
• Think of a string attached to a clamp on one
end and the other moves up and down, like an
oscillation.
• If the string is vibrated at the right frequency a
standing wave appears
– A standing wave looks like it’s not moving
Standing Wave Patterns
•The area where the
wave is “tall” are
areas of constructive
interference
•The areas where it
looks like a “point”
are areas of
deconstructive
interference
• When complete destructive
What Does a Standing Wave Look
Like?
• The Tacoma Narrows Bridge was a bridge that
Open Air Columns
• Many musical instruments are made up of air
column in metal tubes.
• Extra long tubes are bent back on top of each
other
• Flutes and organs are examples
• If each end of the pipe is open at both ends,
each end is an antinode.
Natural Frequencies
• Natural frequencies are know as harmonics of
the instruments
• There are standing wave patterns
• If both ends of the tube are open it is a openpipe resonator
– Ex: Trumpet,
Speed of Sound
• The speed of sound can change based on the
medium and the temperature it is propagating
through.
v = (331 + 0.6Tc)
V = velocity (m/s)
T = the air temperature in degrees Celsius
• Ex: On a cool October afternoon (air temp =
15 degrees) your seat on the centerfield
stands 113m from first base, you watch a play
at the World Series. You see the runner’s foot
tag the base, half a second later you hear a
baseman’s glove hit the base. Is he safe or
out?
• Sound is Intense!
• Intensity: the rate of energy transfer for a
given time over a given area.
I = P/A = P/ (4ΠR2)
• The intensity of a point source of sound is
inversely proportional to the square of the
distance from the source
• With this in mind:
• Sound intensity is perceived as loudness.
• Threshold of Hearing (I0): a sound wave at
1KHz with an intensity of 10-12 W/m2
• Threshold of Pain (Ip): a sound wave with an
intensity of 1 W/m2. At this point the sound is
uncomfortably loud and may be painful to the
ear
• Decibel (0.1 bel) named after A. Graham Bell
• Sound Intensity or decibel level (β):
β = 10 log I
I0
where I0 = 10-12 W/m2
• What unit have you typically heard of for
sound?
• See pg 488 Example 14.4
Logarithmic Sound Intensity Levels
Sound Levels and Damage Exposure
• You can damage
hearing by
excessive noise
• Turn to page 491
and look at table
14.2 for exposure
time and
intensities
• Ex: A 10 W speaker emits sound at full power.
What is the intensity 5.5m away?
• Find the relative intensity of the sound.
• If a rock band is playing at 92 dB, find the
intensity of the sound.
• Ex: What is the distance from a lightning strike
in 20C weather of you hear thunder 3.5 s after
the strike?
• Hmwk!
• Pg 508 #1, 10-13, 16, 20, 29, 30, 32, 35,36,
37b, 38