Vibrating Bars and Strings: The Phenomenon of Beats
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Video Lecture 32 of 46
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Date Added: February 5, 2015

### Lecture Description

A - We look again more circumspectly at the two bars mounted on a reson¬ating chambers. They are fixed at two very special points. When sounded each alone they sound ALIKE - IDENTICAL. But they are not! One is 440 and the other 441. Hence we hear ONE beat per second. The "beat frequency "is the difference in the natural frequencies.

B - We explore this matter with two mounted tuning forks. They ARE identical. But now we load one with a rubber band - thus in¬creasing its inertia and lowering its pitch.
We now hear beats.

C - A long steel bar is held at a certain place along its length.. .at a point quite like the point where the A-bar is fixed to its resonating chamber. We strike the bar transversely. It vibrates with two nodes.
We ask: How far along the length of the bar is a node? The answer might well appear to be 1/4 the length - that is 0.25 L. But this is not quite right. It is 0.224 L — and this is a very difficult exercise for students of Physics. Look into this some time.

E - Two tuning forks mounted on their resonating chambers are highly responsive to each other. If they are identical we can strike ONE -damp it - that is - stop it - and low and behold - the other is heard.

F - A set of tuned sticks cut to precise lengths - of hard wood - clear -grained - can give rise to the major diatonic scale if dropped in proper sequence on a hard floor.

G - We now look at vibrating strings: A magnetically-driven steel sliver has a string fixed to it. We hold the remote end of the string. With a certain length and a certain tension in the string we can show the fundamental of the string - the first overtone - the second - and so on — called by some the harmonics. What the string does is governed by its length - its tension - its linear density - that is -how fat it is.

And so we see and hear the beautiful things that sounding devices give us. Of all our human faculties and senses that of hearing is a truly remarkable one and it is a pitiful thing for those who cannot hear.

### 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.