Upholstered pipe

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An upholstered pipe is a physical model for explaining acoustic vibrations. For example, when blowing into an open tube of a suitable size, the human lips (cushions) are set in motion - hence the name upholstered pipe . Musical instruments of this type are the only ones whose sound generator is a human organ. This principle is decisive for brass instruments , even if they are made of other materials, such as B. wood ( alphorn , didgeridoo , zinc ). The material used is irrelevant, as it does not perform any functionally significant natural oscillation.


Schematic sketch for the function of an upholstered pipe

A fundamental function of the upholstered pipe is the Bernoulli effect . The closed, sideways tense lips (=  cushion ). are opened by the generated lung air pressure. The pressure difference inside and outside the lip area is thus suddenly equalized. The air now accelerating between the lips creates a negative pressure that leads to the lip seal again (Bernoulli force). However, these are immediately blown up again by the constant blowing air pressure, etc. The swinging ( buzzing ) thus results from the combination of blowing pressure and lip tension, but not, as is sometimes assumed, from a constant-frequency stimulation of the lip muscles by nerve impulses.

The upholstered pipe as a model for the human voice

The upholstered pipe is considered a greatly simplified model for the human voice . The vocal folds form the pads. Extensive investigations with the help of a special upholstered pipe on blowing pressure and pitch have been carried out by Wethlo since 1913. A Wethlo's upholstered pipe was recently found and restored. Hoffmann was able to couple it with a pharynx model and thus generate articulated, voice-like sounds. Modern notions of the function of the voice continue to be based on a myoelastic-aerodynamic theory of sound generation from a function similar to a cushion pipe, although very complex multi-mass models are used instead of simple cushions. (See also article Glottis ).

The upholstered pipe as a model for brass instruments

Lip oscillation period

When playing brass instruments, the lips open and close according to the above principle. As with the human voice, the lips open and close according to the pitch played, for example when playing the c3 on the trumpet around 1000 times per second. This vertical oscillation is only part of the complex three-dimensional oscillation of the lips. Particularly in the case of lower tones, the inward and outward vibrations of the lips are also significantly involved in the generation of the sound.


Web links

  • phys.unsw.edu.au English page on the physics of the upholstered pipe (with animations)

Individual evidence

  1. F. Wethlo: Experiments with upholstered pipes . In: Passow-Schaefer's contributions to the entire physiology. 6 (3), 1913, pp. 268-280.
  2. ^ R. Hoffmann, D. Mehnert, R. Dietzel, U. Kordon: Acoustic Experiments with Wethlo's Larynx Model. In: Mária Gósy, Hans Grassegger (Hrsg.): To the memory of Wolfgang von Kempelen: (1724-1804). (= Graz Linguistic Studies. 62). Graz 2004, DNB 975222880 .
  3. The lip swing on trumpet and trombone. Slow motion insights into the mouthpiece