Sound
Students will learn about sound as a
wave.
Sound
• Sound is a mechanical wave that requires a
material medium for transmission. The
energy disturbance is propagated by the
molecules of air, water, or solids.
• Sound is a longitudinal wave exhibiting simple
harmonic motion parallel to the direction of
motion. “Condensations” are like crests;
“rarefactions” are like troughs.
No Sound in Space
• Because of an absence of air there is no sound
in space.
Sound
• Loudness is transmitted via the amplitude.
louder sound waves have greater
amplitude.
Pitch is transmitted via frequency.
higher pitch sounds have higher frequency
lower pitch sounds have lower frequency
Speed of Sound through materials
• Material
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–
–
–
–
–
–
–
–
–
Air (room temp)
Air (ice-cold)
Helium
Hydrogen
Fresh water
Sea water
Iron
Glass
Aluminum
hardwood
• Speed (m/s)
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–
–
–
–
–
–
–
–
–
343
331
1,005
1,300
1,440
1,560
5,000
4,500
5,100
4,000
Distance from a lightning strike
• “one mile for every five seconds after lightning
strikes until the thunder is heard.”
• “one thousand feet for every second after
lightning strikes until the thunder is heard.”
• The speed of sound is 340 m/s so to travel
1,000 m it takes about 3 seconds. One mile is
1.6 kilometers so the time to travel one mile is
1.6 x 3 = 5 seconds.
Distance by echo
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•
•
•
•
Source generates a pulse of sound (“yells”)
Sound travels across space
Sound reflects back as “echo”
Time measured is (pulse plus echo)
Use ½ of the (pulse plus echo) time times
speed to calculate distance.
Example
• If you make a pulse of sound (“yell”), and it
reflects back as an echo with a total time of 2
seconds. Calculate the distance.
• ½ of 2 seconds is 1 second
• Speed of sound in air is 340 m/s so 1 second
times 340 m/s is 340 meters
Sonar
• Ships use “sonar” to measure undersea
distances.
• Ships put out pulses.
• The pulse reflect off objects and return as
echoes.
• Ships look for the bottom of the sea,
shipwrecks, schools of fish, pods of whales.
Ultrasound
• Diagnostic testing uses ultrasound in the
range of 20 kiloHertz to 100 KiloHertz
• High frequency sound means small
wavelengths.
• Diagnostic testing for pregnancy, tumors,
kidney stones, gall stones, pockets of fluid,
brain, heart, liver, and kidneys.
Doppler Effect (J. C. Doppler, 1842)
• Applies to all waves
• If there is motion between the source and the
observer.
• If the distance between the source and the
observer decreases, the “perceived”
frequency increases (higher pitch).
• If the distance between the source and the
observer increases, the “perceived” frequency
decreases (lower pitch).
Group Activity Questions
• 1. Police car siren emits at a frequency of
1600 Hertz. What frequency would you hear
at rest and the car is at rest? What frequency
would you hear if the car is moving 25 m/s
toward you? What frequency would you hear
if the car is moving away from you?
Group Activity Questions
• 2. A hiker determines the length of a lake by
listening for the echo of her shout reflected by
a cliff at the far end of the lake. She hears the
echo 1.5 seconds after shouting. Estimate the
length of the lake.
• 3. A sailor strikes the side of his ship below
the water line. He hears the echo of the
sound reflected from the ocean floor 2.0
seconds later. How deep is the ocean floor?