Julius Sumner Miller: Lesson 36 - Vibrating Rods and Plates

A - A metal rod is held in the finger and thumg near one end - the rod in a vertical line. With a hammer we strike one end of the rod. As the position of the holding changes the pitch emitted by the bar changes. We are exciting the bar compress!onaIly - longitudinally. What it emits is governed by many things: its stuff - its geometry -where it is held. For certain positions of "damping" the bar has enormous acoustic life and very very high Q. By the Q of a system we mean - in a general way - how much energy it returns for the energy we give it. At certain positions this bar gives rise to fantastically high-pitched emission.

B - We have an array of metal plates - of different stuff - of different shape. These are called CHLADNI PLATES. They are fixed at the center to a pin rigidly bound to the plate. We bow the plate with a violin bow. It sounds. It is vibrating. But HOW? It is too fast to see. So we sprinkle it with sand or with sugar and low and behold - a WONDROUS thing arises: The sand takes up beautiful patterns - governed by how we bow the plate - where we bow the plate and so on. The beautiful figures resulting are called CHLADNI FIGURES.

C - It is rather well known that glasses and goblets can be stroked -
excited - bu rubbin a wet finger along the edge. If now we arrange a set of these - say eight - tuned with different levels of water - a delightful music can be played. And here again we have relaxation oscillations - the finger grips the edge of the glass - I ets go - and this action is repeated.

D - So too a steel drum can be shaped with a hammer - giving different shaped segments - of different size - of different thickness. The music which these emit is indeed delightful.

E - A tuning fork - say 512 vps - one octave above middle C - is struck. You hear 512 vps. Now if this fork is moved toward a distant hearer and then away from the hearer a change in pitch is noted. This is called the Doppler Effect. It plays a role in both light and in sound since it is a consequence of more waves or less waves reaching the observer per unit time. You detect this when you hear a Police siren! I