There are several different types of reeds used in wind instruments, probably the most common being the beating reed, as used in clarinets, saxophones, etc. These reeds are usually made from a thin piece of cane, or synthetic equivalent, fixed at one end over an opening in the flat surface of a mouthpiece. As the name suggests, the reed is slightly wider than this opening and when the player applies pressure, the reed beats against the mouthpiece and sets into motion a column of air whose pitch is determined by open fingerholes, or some similar mechanism. These are often called single reeds, in contrast with the double reeds of instruments such as the oboe, where the reed is in two parts which beat against each other. (To be accurate, rather than literally beating against the mouthpiece, or against the other half of the reed, beating reeds actually beat against a cushion of air on the surface of the mouthpiece or between the two halves of a double reed, but let's not complicate matters too much here.)
In contrast a free reed is a small strip of material (most commonly of metal, but in some cases made of plastic or vegetable matter such as bamboo) also fixed at one end, but which is set in or over a slot that is fractionally wider than the reed itself. As a result, when pressure (or suction) is applied, the reed swings freely though the slot to set up a vibrating column of air which gives voice to the instrument.
The free reed can be further divided into two distinct types. The older type, as used in traditional Asian free reed instruments, can be described as idioglottal - that is to say that the tongue of the reed is cut from the surrounding reedplate.
The tongue of the reed can be either rectangular or triangular, usually (though not always) lies flush with the reedplate and usually (though not always) responds to both positive and negative pressure (blowing and drawing). This type of free reed needs to be connected to an appropriate resonator in order to sound and the pitch produced is dependent upon both the natural pitch of the reed and the size and shape of the resonating chamber.
The more recent type of free reed as used in Western instruments such as the harmonica, accordion, concertina, etc., can be described as heteroglottal - that is to say the reed is attached to a separately made reedplate, either with one reedplate per note, or with one reedplate having multiple slots and corresponding reeds.
(Many toy instruments use reeds and reedplates made from a single piece of injection molded plastic. Although these could in a sense be considered idioglottal, as the reeds and reedplate are generally molded from a single piece of plastic, because of how they function it make more sense to think of them as heteroglottal free reeds.) This type of free reed is usually rectangular (or very slightly tapering) and is almost always set slightly above its slot, so that it normally responds only to one direction of airflow, separate reeds being required for blowing and drawing. The pitch of the note produced by this type of free reed is usually very close to the natural pitch of the reed and any resonating chamber attached to it normally tends to have only a very small effect on the pitch, although it can substantially modify the timbre of the sound produced.
In both cases, the sound heard is that of the column of air set into motion by the reed, rather than the sound of the reed itself, the method of tone generation being similar in many respects to that of a siren. (In certain electro-acoustic instruments, such as the Hohner Pianet, metallic reeds similar to those described in the previous paragraph are plucked by various methods and the sound of the reeds themselves is fed to an amplifier. However, these are best considered to be something akin to lamellophones, rather than free reed instruments.)
A Brief History of Mouth Blown Free Reed Instruments
What Is A Free Reed?
Origins Of The Free Reed
Eastern Free Reed Instruments
A Selective Discography Of Asian Free Reed Instruments
Western Free Reed Instruments
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