Herman von Helmholtz (1821-1894) gave his name to the arrangement of two parallel coils of wire which is used to produce a region of uniform magnetic field, and to the acoustic resonators which were used in the nineteenth century for Fourier analysis.
The Helmholtz resonator consists of a known volume (of any shape) with rigid walls and a small hole in one side. In the two examples above, the hole is on the lower side of the volume, and cannot be seen. An outside variation in air pressure causes the plug of air in the hole to oscillate in and out, producing adiabatic compressions and rarefactions of the enclosed air. The system is similar to a spring-mass system, with the enclosed volume of air acting as the spring, and the plug of air acting as the mass. Note that the resonator does not utilize standing waves, as do most other acoustic resonators.
In order to hear the resonance, the nipple of the resonator is held to the ear. The resonators show sharp resonances, with Q values as high as 40, making it possible to detect individual frequency components of complex acoustic wave shapes.
The resonators from the College of Wooster were purchased from Max Kohl of Chemnitz about 1900, and can be tuned by pulling the telescoping cylindrical segments downward. Marks on the inner cylinder give the notes at which the system will resonate.
|Helmholtz resonators by Rudolph Koenig of Paris, looking very much like the Kohl resonators on this page, are quite common. This large, unmarked resonator set in the Garland Collection of Classical Physics Apparatus at Vanderbilt University, was ordered in 1873 from Koenig and arrived in Nashville in 1875.|
| The set of Helmholtz resonators at the right
was sold by Ritchie of Boston for $25.50 toward the end of the nineteenth
century. The 1881 Ritchie catalogue notes that it "gives approximately the
cost at which we can import them to order from Mr. Koenig... We have made
arrangements with Mr. Koenig by which we can furnish his instruments at his
prices, adding only the actual expenses of importations."
This set of resonators is in regular use at the Physics
Department at the University of Texas, which opened in 1890.
The 1928 catalogue of Max Kohl of Chemnitz, Germany lists these pieces of apparatus as "Tone Variators, after William Stern, the tone being varied steadily and uniformly." The reference is to Zeitschrift für Psychologie und Physiologie der Sinnesorgane, vol 30, pg 422, 1902. The catalogue notes that "the apparatus is suitable for demonstration purposes and for tuning, for psychological investigations and practical investigations by otologists [audiologists]."
The rotating dials control a mechanism that lifts the bottom of the resonators, thus raising the frequency. The dials are marked off in frequencies and in musical tones. At the top of the apparatus is a pressure gauge.
REFERENCE: Thomas B. Greenslade, Jr., “Experiments with Helmholtz Resonators”, Phys. Teach., 34, 228-230 (1996)
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