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ISCApad #204 |
Tuesday, June 16, 2015 by Chris Wellekens |
A View from Dresden onto the History of Speech Communication Part 4: Helmholtz Resonators Complete article including figures available at: http://interspeech2015.org/conference/historical-review/ Hermann von Helmholtz (1821 –1894) was a German physician and physicist who made important contributions in many areas of science. One of these areas was acoustics, where he published the famous book Before the invention of Helmholtz resonators, strong partial tones in a sound wave were typically identified by very thin, elastic membranes that were spanned on circular rings similar to drums. Such a membrane has a certain resonance frequency (in fact multiple frequencies) that depends on its material, tension, and radius. If the sound field around the membrane contains energy at this frequency, the membrane is excited and starts It was also known that the sensitivity of this method could be improved when the membrane was spanned over the (removed) bottom of a bottle with an open neck end. The key idea of Helmholtz was to replace this bottle by a hollow sphere with an open neck at one 'end' and another small spiky opening at the opposite 'end'. The spiky opening had to be inserted into one ear canal. In this way, the eardrum was excited similarly to the membrane with the sand of the previous technique. However, due to the high sensitivity of the ear, partial tones could be detected much more easily. A further advantage of these resonators was that their resonance frequencies can be expressed analytically in terms of the volume of the sphere and the diameter and the length of the neck. Hence these resonators became important experimental tools for the subjective sound analysis in the late 19th century and the early 20th century. The HAPS at the TU Dresden contains three sets of Helmholtz resonators. The biggest of these sets contains 11 resonators, which are tuned to frequencies between 128 Hz and 768 Hz. The HAPS also contains a related kind of resonators that were invented by Schaefer (1902). These resonators are tubes with one open end and one closed end. The closed end also has a small spiky opening that has to be inserted into the ear canal. These resonators maximally respond to frequencies of which the wavelength is four times the length of the tube. Helmholtz used his resonators not only for sound analysis, but also for the synthesis of vowels. Therefore, he first had to analyze the resonances of the vocal tract for different vowels. He did this by means of a set of tuning forks, which he placed and excited directly in front of his open mouth when he silently articulated the different vowels. When the frequency of a tuning fork was close to a resonance of the vocal tract, the resulting sound became much louder than for the other frequencies. For each of the vowels /u/, /o/, and /a/, he was only able to detect a single To synthesize the vowels /u/, /o/, and /a/ with a single Peter Birkholz |
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