What is sound ?

Mystery music of the day You have 20 seconds to shout author and title Worth 1 (one) brownie point

Answer: PHILIP GLASS PHILIP GLASS, Rubric (from Glassworks, 1976)

Robert Boyle 1660

sound needs a medium to propagate

sound is air in motion, but it is not wind It’s an oscillation

Sound is a compression wave of matter (usually air) Energy propagates. Matter does not.

molecules in air are tiny, ~ m= m across are fast moving, ~450 m/s ~ 1600 km/h ~ 1000 miles/h ~fast airplane are close to each other, ~ = m = 10 molecule sizes fly about m = m between collisions exert pressure

Pressure force on the wall Higher 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 A The pressure on the air around us is enormous: 10 tons/m 2 or 15 pounds/inch 2 It doesn’t crush us because the pressure is also inside us pushing out

We can now understand how sound propagates compressed air

Sound speed in gases is given by : at constant entropy (for scientists only) (for everybody else) temperature molecule mass heavier molecules are harder to push around, sounds propagates slowly hotter air, faster molecules, faster sound hotter air, faster molecules, faster sound

Our ears can detect tiny variations in air pressure: normal air pressure Pa pressure variation for the just audible sound = Pa pressure variation on the pain threshold = 20 Pa

Sound 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 (shared by all waves) reflection refraction diffraction interference “inverse square law” …

Properties of waves, sound include Reflection hard wall

same angle Reflection on an angle

If instead of a hard wall we have something more flexible (but not elastic), much of the sound energy will dissipate acoustic insulator

Anechoic 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

We hear a lot of reflected sound all the time. It usually arrives very close to the direct sound and we don’t notice it as a distinct sound. Still, it contributes to the quality of the sound.

Compare how an organ sounds in a cathedral outdoors Compare how your voice sounds in a bathroom living room We will discuss all this with more detail later…

Refraction faster medium slower medium Initial and final directions are different. The sound made a curve !

As an animation now

At night

During the day hotter air colder air

Acoustic lens

It’s a good time now to read section 2.2 and 2.3 of Berg & Stork (except interference)

“Inverse square law”: how sound intensity decreases with distance Intensity ~ 1/r 2