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Two strings, one thick and the other thin, areconnected to form one long string. A wavetravels along the string and passes the pointwhere the two strings are connected. Whichof the following changes at that point:
A. FrequencyB. PeriodC. Wavelength
Sound waves
Longitudinal Speed of sound
Sound waves
Velocity of sound in air at 20o is 343 m/s
Helium is lighter than air
Velocity of sound in
helium at 20o is 972 m/s
Hzm
smvf 343
1
/343
Hzm
smvf 972
1
/972
Sound tend to travel faster in liquids and solids (the molecules are closer together).
The velocity of sound in airvin air = 343 m/s
The velocity of sound in liquidvin water = 1490 m/s
The velocity of sound in a solidvin Aluminum = 5100m/s
Cool experiment – make a milkshake!
Why do we not hear collisions and explosions from outer space?
In a vacuum there are not any molecules
–Without molecules we cannot have a pressure wave
–Sound cannot travel through a vacuum.
Sound in a vacuum Demo
JOHN 14:27
Peace I leave with you, my peace I give to you: not as the world gives, give I to you. Let not your heart be troubled, neither let it be afraid.
Intensity of sound
Intensity depends on __________ and the _____________ from the source.
24 r
Power
Area
PowerI
PowerDistance
Spherical Waves / Plane Waves
Decibel Scale
Zero decibels refers to the threshold of hearing
log (10x) = x log 10 = x
10logx=x
log(ab)=log a + log b
log(a/b)=log a - log b
Decibel Scale
Zero decibels refers to the threshold of hearing
At a rock concert a meter placed at 2 m in front of a loudspeaker measures 130 dB. What is the power output of
the speaker?
At a rock concert a meter placed at 2 m in front of a loudspeaker measures 130 dB. How far away do I need to stand to be at a safer level of 90 dB?
If you increase the intensity by a factor of 1000, add ____ to the
decibel level.a) 3 dBb) 10 dBc) 20 dBd) 30 dBe) 60 dB
If you increase the intensity by a factor of 2, add ____ to the
decibel level.a) 3 dBb) 10 dBc) 20 dBd) 30 dBe) 60 dB
Can you detect a 3dB difference (50% loss)
http://www.phys.unsw.edu.au/jw/hearing.html
Other Decibel Scales
Weighting dBm dBV dB SPL dBFV
Q15-2
Consider music from a radio. If I turn up the volumeso that the sound level is 20 dB louder, by what factorhas the intensity increased?
(a) 10(b) 20(c) 100
Q15-3
When I sit one meter from a radio, the sound level is80 dB. How far from the radio should I sit to reducethe sound level to 60 dB?
(a) 3 m(b) 10 m(c) 30 m(d) 100 m(e) 300 m
