What is sound ?

What is sound ?What is sound ?

Mystery music of the dayMystery music of the day

You have 20 seconds to shout author and title

Worth 1 (one) brownie point

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