Sound Waves: Sources of Sound & Pitch and Frequency 
Sound Waves: Sources of Sound & Pitch and Frequency
by Prof. Miller
Video Lecture 31 of 46
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Date Added: February 5, 2015

Lecture Description

To have a SOUND we must have a vibrating system. There must be a thing moving. For the energy to be felt elsewhere there must be an intermediate stuff - a medium for travel. The vibrating body has a certain mechanical frequency. The note emitted we call the pitch.

A - We have two metal bars fixed to a resonating chamber. We strike one. It emits a sound. It has a certain vibrational frequency. The note we Hear is A. The bar is vibrating at 440 vibrations per second. Thus it is that the frequency governs the pitch.

B - We strike a tuning fork. It vibrates at 256 vps. The note it emits we call Middle

C - We take in hand different metal plates. When they are flexed they vibrate at different rates. The sound they emit is governed by this.

D - I talk hard and firm against my arm. I feel the pressure changes. Thus the evidence that a sound is the advance of a compressional wave. The sound that comes forth from my mouth is governed by many things: The -frequency of my vocal cords - the amplitude of their vibration - how I hold my tongue - how I hold my lips -whether I have teeth or not!

E - We have a shaft to which is fixed an array of slotted disks. The slots number 4 - 8 - 16 - 32. If this shaft is rotated in a motor and we hold a flexible card or metal strip against the disks we get an array of musical sounds of frequencies f - 2f - 4f - 8f.

F - A disk with holes in it is rotated on a motor. A stream of air is directed through the holes. The more holes the higher the pitch. AND - where the holes are symmetrically spaced we get MUSIC -where they are unsymmetric we get NOISE. We distinguish these by saying that musical notes can be represented by simple mathematics. Noises can not be so simply described. The question is: Is boogie-woogie music? And what is Bach or Mozart? And how about Rock and Roll?

G - An ordinary stick like a yard-stick or a meter stick is held by the hand on the table top with some hanging over. We flex that part hanging over. It vibrates. We hear a sound. Thus in a very simple way we can demonstrate vibrational rates and the pitch which emerges.

H - We do a classic experiment first explored by Galileo. The finger nail is moved over the milled edge of a silver coin. A sound comes forth. We tear some cloth: some sound emerges. We file a board: some sound comes forth. We flex a deck of cards. A sound comes out. We flex some metal plates. They SOUND. But how about the lead plate? No sound! Yes there is. But the frequency is too low - the pitch too low - for the human ear to detect it. Human hearing has a range roughly from 16 vps to 16000 vps - more or less.

I - A tuning fork is a metal bar which is bent. The region near the stem is a node.

J - The Classical Knotched Stick: This is a toy of very great complexity. We impose a vibration on the edge of the prismatic rod. A propeller is driven in one direction. We impose the vibration on another edge. The propeller goes the other way. This is a demonstration of compounded harmonic motions.

And finally we ask again: what happens to the instruments in an orchestra when things get hot? Their frequencies change - their pitches must change. And a good conductor hears this and if he is of a serious mind - as most are - the musicians had better adjust for this!

Course Index

  1. The Idea of the Center of Gravity
  2. Newton's First Law of Motion: Inertia
  3. Newton's Second Law of Motion: The Elevator Problem
  4. Newton's Third Law of Motion: Momentum
  5. Energy and Momentum
  6. Concerning Falling Bodies & Projectiles
  7. The Simple Pendulum and Other Oscillating Things
  8. Adventures with Bernoulli: Bernoulli's Principle
  9. Soap Bubbles and Soap Films
  10. Atmospheric Pressure
  11. Centrifugal Force and Other Strange Matters
  12. The Strange Behavior of Rolling Things
  13. Archimedes' Principle
  14. Pascal's Principle: The Properties of Liquids
  15. Levers, Inclines Planes, Geared-wheels and Other Machines
  16. The Ideas of Heat and Temperature
  17. Thermometric Properties and Processes
  18. How to Produce Heat Energy
  19. Thermal Expansion of Stuff: Solids
  20. Thermal Expansion of Stuff: Gases & Liquids
  21. The Strange Thermal Behavior of Ice and Water
  22. Heat Energy Transfer by Conduction
  23. Heat Energy Transfer by Convection
  24. Heat Energy Transfer by Radiation
  25. Evaporation, Boiling, Freezing: A Dramatic Adventure
  26. Miscellaneous Adventures in Heat
  27. The Drama in Real Cold Stuff: Liquid Nitrogen
  28. The Physics of Toys: Mechanical
  29. The Physics of Toys: Acoustic and Thermal
  30. Waves: Kinds of Properties
  31. Sound Waves: Sources of Sound & Pitch and Frequency
  32. Vibrating Bars and Strings: The Phenomenon of Beats
  33. Resonance: Forced Vibrations
  34. Sounding Pipes
  35. Vibrating Rods and Plates
  36. Miscellaneous Adventures in Sound
  37. Electrostatic Phenomena: Foundations of Electricity
  38. Electrostatic Toys, Part 1
  39. Electrostatic Toys, Part 2
  40. Adventures with Electric Charges
  41. Adventures in Magnetism
  42. Ways to "Produce" Electricity
  43. Properties and Effects of Electric Currents
  44. Adventures in Electromagnetism
  45. Further Adventures in Electromagnetism
  46. Miscellaneous and Wondrous Things in E&M

Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.


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