Wooden sticks, when dropped on the floor, sound a variety of tones. While the bars of a xylophone are varied in tone by changing their length, these “tone bars” are all of the same length and width, but have different thicknesses and different densities and elastic properties.
This apparatus consists of three wires, each bent to resemble transverse waves. The wires are mounted in a rectangular brass box that was placed in front of the lens of a projector. The top two wires are identical, but positioned so that their shadows appear to move in opposite directions as a crank on the side of the box is turned. As they do this, the crests and troughs of the waves alternately lineup and overlap. The corresponding “interference” of the two waves is seen in the changing shape of the third wire.
This is a set of 16 Helmholtz resonators. Made from sections of brass that were spun on a lathe, they are wonderfully light and easy to hold. Helmholtz designed them to demonstrate his theory that all vowel and musical sounds are composed of combinations of simple, pure notes (Helmholtz’s “Theory of Timbre”). He correctly observed that musical sounds, particularly the higher tones, are often perceived as a single mass of sound.
In the second quarter of the 19th century, the French scientist Felix Savart invented this apparatus to demonstrate resonance. It consists of a “bell” (or brass bowl) and a moveable wooden resonator. In the demonstration the bell was activated by being either bowed or struck. As the bell rang, its’ loudness could be increased or diminished by moving the resonator closer or further away. When the sound of the bell became barely audible an effective demonstration was to quickly move the resonator right next to it. The increase in loudness – the 'resonant effect' – was striking.
This set of resonance bars, each with its’ own resonator, can be used in an interesting demonstration. First, because the bars are physically identical, they both have the same resonant frequency. And that sound is strongly amplified by the wooden resonators on which the bars are mounted. In the demonstration, the two instruments are placed some distance apart and the first bar is struck sharply to make a tone. Because the two bars are identical, the second bar will respond to the sound of the first by making the same tone.
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