A View from Dresden onto the History of Speech Communication
Part 4: Helmholtz Resonators
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 “On the sensations of tone as a physiological basis for the theory of music” in 1863. There he described his invention of a special type of resonator, which is now known as Helmholtz resonator. These resonators were devised as highly sensitive devices to identify the harmonic components (partial tones) of sounds and allowed significant advances in the acoustic analysis of vowel sounds and musical instruments.
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 to oscillate. The tiny amplitudes of this oscillation can be visually detected when fine grained sand is distributed over its surface. When the membrane is excited with its lowest resonance frequency, the sand accumulates at the rim of the membrane or along specific lines on its surface, when higher order modes are excited. With a set of membranes tuned to different frequencies, a rough spectral analysis can be conducted.
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” (Fig. 2a). 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, one of which is shown in Fig. 2a. This set contains 11 resonators, which are tuned to frequencies between 128 Hz and 768 Hz. A related kind of resonators by Schaefer (1902) is shown in Fig. 2b. 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 resonance of the vocal tract at the frequencies 175 Hz (note f), 494 Hz (note b’) and 988 Hz (note b’’), respectively. For each of the other investigated German vowels, he detected two resonances, which are given in Fig. 3 in terms of musical notes. The single resonances detected for /u/, /o/ and /a/ probably correspond to the clusters of the nearby first and second resonances of the corresponding vowels. Obviously, his method of analysis was not sensitive enough to separate the two individual resonances of each of the vowels.
To synthesize the vowels /u/, /o/, and /a/ with a single resonance, he simply connected a reed pipe to Helmholtz resonators tuned to the corresponding frequencies. For the vowels with two resonances, he selected a Helmholtz resonator for one of the resonances and attached a 6-10 cm long glass tube to the outer opening of the resonator to create the second resonance. These experiments showed that Helmholtz had surprising insight in the source-filter principle of speech production, which was fully elaborated by Gunnar Fant and others 100 years later.