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If you open an accordion, you will find that for every button there are two reeds, one which plays when in the pull direction of the bellows and one in the push. Sometimes these reeds are tuned to different notes/pitches for each bellows direction (diatonic accordions), sometimes to the same note/pitch (chromatic accordion).

A question I have always had and which I have explored in the past is whether it is physically possible to make a single reed that plays the same note/pitch in both bellows direction. (thus saving one extra reed for chromatic accordions)

From my somewhat brief research in the past it seems it is not physically possible to make such a reed in theory, though I was wondering if anyone has tried to do this in practice?

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The sheng has been mentioned. The problem with bidirectional reeds is that not only is it quite harder to use air flow for supplying energy but also to give a reliable reed start. The unidirectional free reeds in Western instruments start on comparatively little pressure and basically immediately. To further increase their response, valves are used on the opposing reeds (not in harmonicas, and not in piccolo pitched reeds) to minimize the air loss, further making for a response that's basically immediate.

Accordion music tends to be fast and leggiero. The response characteristic of the unidirectional free reeds plays into that.

  • Indeed - a perfectly symmetrical reed with a perfectly symmetrical air flow won't start vibrating at all, in theory. But there is always some small amount of imperfection in practice, of course. – user19146 Aug 13 '18 at 10:13
  • Thank you so much for your reply, this is quite interesting: in the past I looked at some articles/papers that talked about bidirectional free reeds (like that of the Sheng), but alas the research mentioned that for these types of reeds the design of the resonator is what largely determines the vibrating pitch of the reed (not the reed itself, which is troublesome to design). On the other hand for the "imperfect off-centered unidirectional reeds, the response is quick and the pitch is controlled primarily by its geometry." I guess we are stuck with unidirectional reeds after all... – Camilo Tejeiro Aug 14 '18 at 1:59
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It is possible, and there are some instruments that use them.

The reeds in accordions etc are unsymmetrical, i.e. they are mounted in the reed frame with a small offset from the central position. Such reeds only sound for one direction of air flow. Of course in the harmonica that is an essential part of the design of the instrument, since two reeds tuned to different pitches are connected to a single air supply, and only one reed sounds depending on the direction of the air flow.

However a symmetrically mounted free reed can sound for either direction of air flow. Some Asian instruments such as the sheng use this type of free reed.

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