Resonance: Forced Vibrations
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Video Lecture 33 of 46
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### Lecture Description

If a sound of a certain frequency moves through a medium - say the air -and the pulses fall upon a member FREE to vibrate - and that member has the natural frequency of the wave coming upon it - that member will "pick up" the pulses and vibrate on its own. This is resonance.

A - Air columns have certain lengths. They therefore have certain special resonant responses. If now a sounding fork is addressed to such an air column the pipe will resonate. Clearly - closed pipes will have one responsive length - open pipes another - but these lengths can be readily calculated. We show such resonance response with card¬board tubes - one sliding within another to provide various lengths. Thus it is that organs have pipes of many lengths and different di¬ameters - and some are closed - some are open.

B - We have a monochord - ONE lone string mounted on a resonating box. We fix the string at a certain length - under a certain tension. The string is a wire so fat - so thick We now excite a tuning fork and present the stem of the fork to the bridge where the string passes over. On the string is a bit of paper so we can SEE what happens. The string is put into vibration by the fork and the amplitude is enough to bounce off the paper rider. A pretty thing to see. Now the important aspect of all this is this: That the string responds to ONE fork only.. .although it MAY respond to another of multiple frequency.

C - We excite a tuning fork. We put its stem on top of the table. The table vibrates. But what we hear sounds not like the fork. Why? Consider A played on a piano and A played on a fiddle. Do they not sound unlike? They are of the same frequency but they have different QUALITY.

D - The human vocal cavity called the mouth is a most remarkable thing. If explored for resonance with forks of different frequencies we find that - in general - female voice boxes have higher frequency response than male.

E - This business of RESONANCE has interesting historical aspects. In New England there are wooden bridges still standing from the pioneer days which have signs reading: "No trotting of horses on this bridge". And soldiers crossing a bridge break their step.
And in the Old Testament we read - Joshua 6:20 - "The priests blew their trumpets and walls fell down flat". Which is very good Physics! And so I ask: Why did the ancients not fell walls of the enemy cities by having their trumpeters blow on their trumpets?

F - Cables carrying electric current cross-country often are put into oscillation by stout winds. The cables are demonstrating resonant response to wind pulses. To minimize this action the cables are loaded at strategic points with "dampers" - which you can see if you look carefully at the high-voltage transmission lines.

And finally - the human ear responds just like this. The ear-drum takes up the oscillations which fall upon it. It goes into vibration - into oscillation. And imagine the acute response to every slight change in pitch. This energy is transmitted to the bopes of the inner ear and thence to the brain — so in the last analysis we hear with our brains.

### 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.