Tremolo harmonica

Tremolo harmonicas are a type of harmonica, distinct by having two reeds per note. In a tremolo harmonica the two reeds are tuned slightly off a reference pitch, one slightly sharp and the other slightly flat. This gives a unique wavering or warbling sound created by the two reeds being not exactly in tune with each other and difference in their subsequent waveforms acting against one another. The degree of beating can be varied depending on the desired effect. Instruments where the beating is faster due to the reeds being farther apart from the reference pitch are called "wet", whereas those where the beating is slower and less noticeable due to the reeds being more closely in tune are called "dry".

"Tremolo" is most often defined as a periodic change of volume (sometimes incorrectly defined as a change in pitch, strictly vibrato). Most orchestral instruments achieve this effect by repeated playing of a single note. The tremolo harmonica achieves the effect by employing the physical phenomenon called beats. Each note is played simultaneously by two reeds, fractionally out of tune with each other. The resulting sound is of constant pitch but the volume oscillates between loud and quiet. This effect is fairly common amongst Western free-reed instruments and is found in accordions, harmoniums and reed organs under various names (celeste, vox jubilante, etc.), but it is also used, for example, on the piano, where each of the three strings is tuned very slightly sharper or flatter, giving a richer sound to the instrument.

Most tremolo harmonicas are built upon what is termed the "Wiener system", named after the city of Vienna (Wien in Austria) where they first emerged. In this design the two beating reeds are distributed one on each reed-plate (top and bottom) and these share a common chamber. In practice, however, it is common for each individual reed to have its own air chamber. Unlike the standard ten-hole harmonicas (built on the "Richter system", e.g. Hohner's Marine Band) the blow and draw reeds do not share a common chamber, but are separated off from one another. This allows the player to isolate each reed. While normally the player simply plays both the tremolo reeds at once, it is possible to achieve a wide variety of bends and other effects through selecting certain reeds and chambers and not others. Similarly, it is possible to play without the tremolo effect by only choosing the top or bottom chambers and blocking off the others with the lips. In practice, though, these are primarily used for effects and mostly the instrument is played as if the two beating reeds shared a single chamber.

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Wikipedia