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Part of Koenig-style apparatus for the analysis of sound, deploying a set of Helmholtz resonators
Image 1: Part of Koenig-style apparatus for the analysis of sound, deploying a set of Helmholtz resonators (Wh.3429).

This late 19th-century apparatus (image 1) is part of a sound analysis device. It imitates a design by Rudolph Koenig for what is considered the first spectrum analyzer. It is displayed alongside a flame manometer and a rotating mirror (image 3), which would have formed crucial parts of this analysis machine.

Description

The object in image 1 consists of ten Helmholtz resonators arranged in a wooden frame, and was acquired from the Wheatstone Laboratory, King's College London, in the 1980s. The resonators were made by Rudolph Koenig in the late 19th century, but the maker of the frame is not certain. Most probably this apparatus was assembled at King's as a more affordable version of Koenig's apparatus for the analysis of sound, using a standard set of resonators.

Background

Drawing of Koenig's apparatus for the analysis of sound
Image 2: Drawing of Koenig's apparatus for the analysis of sound, from his 1865 catalogue. Image from Wikimedia Commons.

Musical sounds and the vowels of human speech have their particular sound quality, or timbre, in virtue of the specific overtone frequencies present at various intensities. That is, a musical note at concert A 440Hz, for example, will contain that frequency as well as a series of harmonic partials at integer multiples of 440 (e.g., 880Hz, 1320Hz, etc) at varying intensities. It is the particular spectrum of overtone intensities that gives musical notes their particular timbre. Using resonators held against his ear, Herman von Helmholtz was able to detect the presence of these overtones and thus perform an analysis of the sound. This practice, however, left an unwelcome element of subjectivity in acoustic experimentation. With his analysis device, Koenig removed this flaw by designing a way to make the specific composition of sounds visible as well as audible.

Operation

Flame manometer and rotating mirror
Image 3: Flame manometer on stand and rotating mirror for observing the flame vibrations (Wh.6271).

In Koenig's design (image 3) each resonator has one end open to the air and the other end attached to a rubber pipe. That pipe is connected to a 'flame manometer' capsule, consisting of two hemispheres separated by a thin rubber diaphragm. One hemisphere is connected to the resonator via the pipe, and through the other hemisphere flows a flammable gas, which burns at the end of a narrow jet pipe at the top. The pressure of the gas in the capsule is dependent on the pressure of the air in the other hemisphere, which in turn is dependent on the vibrations of air within the resonator. Thus, during resonance, the gas flame vibrates in sympathy rendering visible the presence of the given frequency in the sound under investigation. An experimenter would then sing or make a sound in front of the open resonators and observe the vibrations of the flames in a rotating mirror, acting like a stroboscope, which made it easier to see the vibrations (image 4). Flame manometers were also used with the "Trombone de Koenig", as well as for many other acoustical experiments.

Torben Rees

Torben Rees, 'Koenig's apparatus for the analysis of sound: the first spectrum analyzer', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2009

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