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Lecture 19

Lecture 19. November 3, 2004. SCHEDULE REMAINING. SCHEDULE REMAINING. Exam is only three weeks away!. 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)

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Lecture 19

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  1. Lecture 19 November 3, 2004

  2. SCHEDULE REMAINING

  3. SCHEDULE REMAINING Exam is only three weeks away!

  4. 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.

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

  6. Resonance in the Basilar Membrane(Computed)

  7. Simplified Version Resonance !!

  8. The Hair Cells

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

  10. 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.

  11. 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.

  12. 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!

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

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

  15. 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???

  16. Sound Spreads Out

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

  18. Or, sorta like this …

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

  20. Add ‘em up! Zip!

  21. DEMO String 'em up!

  22. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 In Phase --- Add up nicely

  23. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 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

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

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

  26. Two Waves … different frequencies f1 f2

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

  28. next topic = diffraction and room acoustics

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