What is What is sound ? sound ?
Feb 21, 2016
What is sound ?What is sound ?
Mystery music of the dayMystery music of the day
You have 20 seconds to shout author and title
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Answer:Answer:
PHILIP GLASSPHILIP GLASS, Rubric (from Glassworks, 1976)
Robert BoyleRobert Boyle
16601660
sound needs a medium to sound needs a medium to propagatepropagate
sound is air in sound is air in motion, but it is motion, but it is notnot wind wind
It’s an oscillationIt’s an oscillation
Sound is a compression wave of matter Sound is a compression wave of matter (usually air)(usually air)
Energy propagates. Matter does not.Energy propagates. Matter does not.
molecules in air
• are tiny, ~0.0000000003 m= 3 10-10 m across
• are fast moving, ~450 m/s ~ 1600 km/h ~ 1000 miles/h ~fast airplane
• are close to each other, ~ 0.000000003= 3 10-9 m = 10 molecule sizes
• fly about 0.0000001 m = 10-7m between collisions
• exert pressure
PressurePressure
force on the wall
Higher the temperature, density higher the forceHigher the temperature, density higher the force
air denser air (more
molecules)
hotter air (faster
molecules)
Pressure (P) is the force (F) per unit of area (A):
F = P AF = P A
The pressure on the air around us is enormous:
10 tons/m2 or 15 pounds/inch2
It doesn’t crush us because the pressure is also inside us pushing out
We can now understand how sound propagatesWe can now understand how sound propagates
compressed aircompressed air
Sound speed in gases is given by :Sound speed in gases is given by :
Pv
at constant entropy (for scientists only)
TCm
(for everybody else)
temperaturetemperature molecule massmolecule mass
• heavier molecules are harder to push around, heavier molecules are harder to push around, sounds propagates slowlysounds propagates slowly
• hotter air, faster molecules, faster soundhotter air, faster molecules, faster sound
Our ears can detect tiny variations in air pressure:Our ears can detect tiny variations in air pressure:
• normal air pressure 100.000 Pa
• pressure variation for the just audible sound = 0.00002 Pa
• pressure variation on the pain threshold = 20 Pa
Sound exists also on liquids and solidsSound exists also on liquids and solids
• Air (normal temperature) v = 344 m/s = 770 miles/hour or “5 seconds, one mile”
• Water v =1400 m/s
• Steel v = 5100 m/s
Sound properties Sound properties (shared by all waves)(shared by all waves)
• reflection
• refraction
• diffraction
• interference
• “inverse square law”
• …
Properties of waves, sound includeProperties of waves, sound include
Reflection
hard wall
same angle
Reflection on an angleReflection on an angle
If instead of a hard wall we have something more If instead of a hard wall we have something more flexible (but not elastic), much of the sound flexible (but not elastic), much of the sound
energy will dissipateenergy will dissipate
acoustic insulator
Anechoic chamberAnechoic chamber
Someone at the red point can hear a conversation on the blue point better than anybody else in the room
The same is said to occur in some gothic cathedrals
Band shell Band shell
Band shell Band shell
We hear a lot of reflected sound all the We hear a lot of reflected sound all the time. It usually arrives very close to the time. It usually arrives very close to the direct sound and we don’t notice it as a direct sound and we don’t notice it as a distinct sound. Still, it contributes to the distinct sound. Still, it contributes to the
quality of the sound.quality of the sound.
Compare how an organ sounds in a• cathedral• outdoorsCompare how your voice sounds in a• bathroom• living room
We will discuss all this with more detail later…
RefractionRefraction
faster medium
slower medium
Initial and final directions are different.Initial and final directions are different.
The sound made a curve !The sound made a curve !
As an animation now
At night
During the day
hotter air
colder air
Acoustic lensAcoustic lens
It’s a good time now to read section 2.2 and It’s a good time now to read section 2.2 and 2.3 of Berg & Stork (except 2.3 of Berg & Stork (except interferenceinterference))
““Inverse square law”: Inverse square law”: how sound intensity how sound intensity decreases with distancedecreases with distance
Intensity ~ 1/rIntensity ~ 1/r22