Julius Sumner Miller: Lesson 32 - Sound Waves - Sources of Sound - Pitch and Frequency

Описание: 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!