Historical Context
In physics the term “resonance” refers to the natural tendency of many objects to vibrate more vigorously at some frequencies than at others. The frequencies at which this occurs are called the object’s “resonance frequencies.” In acoustics great use has been made of a particular kind of resonance, called air resonance. This occurs when the air in a container is made to vibrate and produce a sound. An example of this is the tone produced when you blow across the top of an empty bottle. As the air in your breath hits the edge of the bottle’s opening it sets up pressure waves in the bottle that, in turn, make the air inside vibrate rapidly and in unison. This rapidly vibrating mass of air is what makes the sound. The shape and size of the container are what determine its’ frequency. Blowing harder or softer only affects how loud it is. In the 1850s, the German scientist Herman Helmholtz used this principle to create a powerful new scientific instrument – the acoustic “resonator”. It still involved a moving mass of air, but instead of producing a sound, this instrument was used to detect it. Helmholtz was able to design vessels that would only respond to a specific frequency of sound, and would greatly amplify that sound when it was present. Resonators could also extend the time that a tone was sounded, which in an age without microphones or speakers, was an important advancement.
Supporting Artifacts
Helmholtz Resonators
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Larger image Demonstration (41 sec.)Helmholtz Resonators
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. But with these resonators, even people with no musical training could easily pick out the simple, pure tones, even when they were faint and mixed with other sounds.
Each resonator was carefully tuned to respond to only a single frequency. For the person using it, the resonance would occur quite suddenly, with an unmistakable amplification of a particular sound. To use these resonators, the small end was inserted directly into the ear and sealed with a bit of warm wax. The other ear was also sealed with a wax plug. Once this was done, Helmholtz wrote: “most of the tones produced in the surrounding air will be considerably damped; but if the proper tone of the resonator is sounded, it brays into the ear most powerfully.”
Savart's Bell and Resonator
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.
Savart’s Bell and Resonator
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Larger image Demonstration (39 sec.)Resonance Bars
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Larger image Demonstration (38 sec.)Resonance Bars
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. If the first bar is damped, so that it no longer makes a sound, it will be noticed that the sound is now being produced (faintly) by the second bar. The second bar “resonates” with the sound of the first.
Introduction Video
- Watch the resonance video
