What is sound ?

1. What is sound ?

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

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

4. Robert Boyle 1660

5. sound needs a medium to propagate

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

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

9. 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

10. Pressure force on the wall Higher the temperature, density higher the force

11. air denser air (more molecules) hotter air (faster molecules)

12. 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/m2 or 15 pounds/inch2 It doesn’t crush us because the pressure is also inside us pushing out

13. We can now understand how sound propagates compressed air

14. 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

15. 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

16. 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

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

18. Properties of waves, sound include Reflection hard wall

19. Reflection on an angle same angle

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

21. Anechoic chamber

22. 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

23. Band shell

24. Band shell

25. 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.

26. 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…

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

28. As an animation now

29. At night

30. During the day colder air hotter air

31. Acoustic lens

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

33. “Inverse square law”: how sound intensity decreases with distance Intensity ~ 1/r2