Characteristics of Sound
What Produces Sound??
If a tree falls in a forest and no one is there to hear
it, will there be sound?
What produces sound?
• Sound waves originate from a vibrating source
• Sound is also known as a “pressure wave”
• This is because vibrations cause changes in
pressure near the source
Vibrating Sources
• http://www.youtube.com/watch?
v=iYpDwhpILkQ
• http://www.youtube.com/watch?
v=3zoTKXXNQIU&feature=fvw
To Do
• Prove that sound is a pressure wave.....
– In groups of three, light a candle and use the
pressure generated by your voice to see if you can
blow it out
– Determine the characteristics of sound
(frequency/amplitude) that are the most
successful
Sound travels in longitudinal waves
Has areas of high pressure (compressions),
normal pressure, and low pressure (rarefactions)
Bell Jar Experiment
• http://www.youtube.com/watch?v=ce7AMJdq
0Gw
Characteristics of Sound
• Bell jar experiment tells us that sound cannot
travel through a vacuum.
• Sound requires a medium....which type of
wave is this?
• Sound is capable of travelling through
different types of solids, liquids, and gases.
• In which medium would sound travel the
fastest?
To Do
• Fill up one balloon with air and the other
balloon with carbon dioxide (use baking soda
+ vinegar to produce carbon dioxide)
• Investigate the differences in sound between
the two mediums
– Hold the balloon between your ear and some
source of low-volume sound (i.e. Ticking watch).
Compare the sound to the air filled balloon
– Experiment with various sources of sound &
record results
Speed of Sound
• At normal atmospheric pressure and 0
degrees C, the speed of sound is 332 m/s
• If air pressure stays the same and temperature
increases, speed of sound increases by 0.6 m/s
per degree C.
Speed of sound in air = (332 + 0.6 T)m/s
** T = temperature in degrees Celsius
Practice Questions
1) What is the speed of sound at 20 degrees C?
2) What is the speed of sound at – 20 degrees
C?
3) What is the speed of sound in air at 35
degrees C?
4) How much time is required for sound to
travel 1.4 km through air if the temperature
is 30 degrees C?
Physics – Speed of Sound Answer Key
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V = (332 + 0.6 (20))m/s
V = 344 m/s
V = (332 + 0.6 (-20)) m/s
V = 320 m/s
V = (332 + 0.6 (35)) m/s
V = 353 m/s
V = (332 + 0.6 (30))m/s
V = 350 m/s
1400 m x 1second/350 m = 4s
Frequency of Sound
Avg. Human can respond to frequencies
between 20 and 20 000 Hz.
< 20 Hz are called infrasonic
>20 000 Hz are called ultrasonic
Dogs  15 – 50 000
Bat  1000 – 120 000
Cats  60 – 65 000
Moth  3000 – 150 000
Frequency of Commonly Heard Sounds
Male speaking voice – 120 Hz
Female speaking voice – 250 Hz
Highest piano note – 4186.01 Hz
Stereo system – 15 – 30, 000 Hz
Dog – 452 – 1080 Hz
Cat – 780 – 1520 Hz
Bat – 10 000 to 120 000 Hz
Silent dog whistle – 20 000 to 24 000 Hz
Frequency vs. Wavelength