Whisper vault

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A whisper vault or whisper gallery is colloquially a place where people can talk, communicate or hear each other over an unusually large distance through physical effects.

description

Various physical effects can be the cause:

Sound transmission with two parabolic mirrors
  • Speaker and listener are in the focal points of aligned, parabolic "sound mirrors" (see picture). The small reflective surface with a diameter of 2 m is sufficient to transmit soft spoken words over a distance of approx. 40 m. The sound cannot be heard outside the two focal points. In principle, this structure is an acoustic radio link that z. B. is used in science centers to demonstrate the function of whisper vaults in elliptical rooms.
  • The speaker and the listener are in the focal axes of a room with an elliptical floor plan or, even better, in the focal points of an ellipsoid of revolution . In these cases, all sound waves emanating from the speaker are reflected by the building walls in such a way that they reunite in the second focus, at the listener. In the past, this effect had a special meaning in the cathedral of Agrigento in Sicily: inside it has the shape of an ellipsoid of revolution, and a confessional was positioned there in one of the focal points. As a result, the confession was clearly audible to other people at another point in the room, which led to some discord.
  • The speaker and the listener are in a dome at mirrored positions with respect to their center. The hemispherical dome is a special case of an ellipsoid in which both focal points unite in the center. A speaker standing in the center therefore hears his own voice echoing at an unusual volume. When the speaker moves away from the center of the dome, the sound waves are reflected from the walls to a point equidistant from the center on the opposite side of the dome. In principle, this makes it possible for anyone standing in such a dome to have a conversation with a partner on the other side, and this is also possible for many different conversations at the same time. However, the effect weakens with increasing distance from the center of the dome, because then increasingly there are phase differences in the converging sound fronts. Incidentally, this phenomenon previously had negative effects in the Statuary Hall in Washington, since MPs from various parties were undesirably “whispering partners” in this room.
Wilhelma's whispering gallery with a sign and an experimenting visitor
  • The speaker and listener are in an arched corridor or close to the wall of a dome. If someone speaks against the curved wall, the sound is gradually and gently collected through multiple reflections and directed in a direction parallel to the wall surface. The normal, three-dimensional sound propagation with the typical decrease in volume with the square of the distance is restricted by this effect. In an arched corridor, the additional sound reflection from the floor and ceiling creates a kind of “acoustic waveguide”. In this case, the greater the distance, the sound energy is not distributed over an increasing area of ​​the sound front, so that the wave weakens less and can continue to run accordingly. This mechanism does not make great demands on the homogeneity and smoothness of the curved wall and produces e.g. B. the whispering gallery in the walkway of the Wilhelma's Moorish Garden .
  • Speaker and listener are close to a smooth wall that is homogeneous on the inside. If the sound hits the wall at a certain angle, a so-called Rayleigh wave can arise. This type of wave was theoretically predicted by the later Lord Rayleigh for stress-free interfaces and experimentally demonstrated in the dome of St Paul's Cathedral in London. This type of acoustic wave moves directly at the interface between wall and air and can also arise on completely flat or convex surfaces, so that the resulting effect is not limited to a vault. The intensity of a Rayleigh wave decreases rapidly perpendicular to the wall as a function of the sound frequency, whereby a substantial part of the sound energy is concentrated within a wall distance that is smaller than the wavelength of the sound (e.g. 0.3 m for 1000 Hz or 3 m for 100 Hz in air). As a result, the comparatively high frequencies of whispering are led particularly close to the wall and carried correspondingly far, which is where the name "Whisper Gallery" originally comes from. The famous whispering gallery of St Paul's Cathedral enables whispered conversation through this effect along the entire gallery, which has a diameter of 32 m.

Examples of whispered vaults or galleries

Individual evidence

  1. ↑ Hörgarten in Oldenburg
  2. ↑ English-language page on the Whispering Gallery
  3. ↑ Jearl Walker: The Flying Circus of Physics. R. Oldenbourg Verlag, Munich 1996, ISBN 3-486-23726-8 , pp. 21-22.
  4. ↑ article in the New York Times about the whispering gallery of the Statuary Hall in Washington from 1892
  5. ↑ Description of a whispering gallery in the book "The Flying Circus of Physics"
  6. ↑ A short article on the whispering gallery of the Saint Pauls Cathedrale in the magazine "nature" from 1921
  7. ↑ Hörgarten in Oldenburg
  8. ^ Description of the whisper gallery on the Wilhelma page
  9. ↑ Whisper vault in Görlitz

literature

  • Emil Jochmann: Ground plan of experimental physics: for use in teaching at higher educational establishments and for self-study. Verlag der Springersche Buchhandlung - Max Winckelmann, Berlin 1874, p. 115. (books.google.de)
  • Christoph Metzger: Architecture and Response. Jovis Verlag, Berlin 2015, ISBN 978-3-86859-270-2 .
  • Jearl Walker: The Flying Circus of Physics. R. Oldenbourg Verlag, Munich 1996, ISBN 3-486-23726-8 .