2. Lecture 19 November 3, 2004

3. ITEMDATEWEIGHT (%) Exam #1Friday, 9/2415% Exam #2Friday, 10/2215% Exam #3Monday, 11/2215% OP QuestionsDaily25% Final ExamDec. 6th30% SCHEDULE REMAINING

4. ITEMDATEWEIGHT (%) Exam #1Friday, 9/2415% Exam #2Friday, 10/2215% Exam #3Monday, 11/2215% OP QuestionsDaily25% Final ExamDec. 6th30% SCHEDULE REMAINING

5. Last Time  We discussed the loudness of sounds Loudness is subjective … in our heads. Loudness is not LINEAR so we introduced the logarithmic DECIBEL scale (dB or db) We looked at the way our brains add loudness on the db scale.  We discussed the frequency dependence of our hearing.  We discussed how very loud sound can damage our ears.

6. The Cochlea Schematic Rubber Membrane Low Frequency High Frequency Frequency Info

7. Resonance in the Basilar Membrane (Computed)

9. Simplified Version Resonance !!

10. The Hair Cells

11. Damage from very LOUD noises. Extreme Acoustic Trauma Control, not exposed After Exposure Guinea Pig Stereocilia damage (120 dB sound)

12. The Overall Hearing Process  Sound is created at the source.  It travels through the air.  It is collected by various parts of the ear (semi-resonance).  The tympanic membrane moves with the pressure variations.  The inner ear filters/amplifies the sound.

13. Hearing Continued  The sound hits the membrane at the entrance to the cochlea.  The pressure on the basilar membrane causes it to mive up and down.  The resonant frequency of the membrane varies with position so that for each frequency only one place on the membrane is resonating.

14. Some more on hearing  There are hair cells along the basilar membrane which move with the membrane.  The motion of the hair cells creates an electrical (ionic) disturbance which is wired to the brain.  The disturbance is in the form of pulses.  The brain somehow relates the number of pulse firings per second to tone and..  Wallah … music!

15. The source/brain connection Sound Source dB ? ? Signal Source Issues Room Acoustics How the Brain Reacts to certain sounds

16. In what follows keep the following in mind: The wave spreads out.

17. Sounds do funny things  Experiment (1 signal from speaker)  Class: Hold your head still. Move it side to side by about a foot or so. What happens???

18. Sound Spreads Out

19. We get more than we bargained for. Different Distances

20. Or, sorta like this …

21. Bounce off wall so travels farther and more waves before getting to ear! Direct Bounce off of wall.

22. Add ‘em up!

23. DEMO

24. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 In Phase --- Add up nicely

25. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Out of Phase --- Don’t Add Up One Wave Out INTERFERENCE

26. Wave Interference  CONSTRUCTIVE The waves ADD together.  DESTRUCTIVE The waves are “out of phase” and cancel each other out.

27. Let the beat go on …..  Consider TWO sourced of sound. perhaps two speakers  Both are emitting sound.  The two frequencies are NOT the same.

28. Two Waves … different frequencies f1f2f1f2

29. Beat Frequency We will return to this when we discuss consonance.

30. next topic = diffraction and room acoustics