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A fipple is a constricted mouthpiece common to many end-blown flutes, such as the tin whistle and the recorder. These instruments are known variously as fipple flutes, duct flutes, or tubular-ducted flutes.
How it works 
In the accompanying illustration of the head of a recorder, the wooden fipple plug (A), with a "ducted flue" windway above it in the mouthpiece of the instrument, compresses the player's breath, so that it travels along the duct (B), called the "windway". Exiting from the windway, the breath is directed against a hard, bladed edge (C), called the "labium lip" or windcutter, producing a Bernoulli effect or siphon. The air flowing over the voicing mouth creates a flow-controlled valve, or "air reed."  Interaction between the air reed and the air column in the body of the instrument excites standing waves in the air column, which determines the pitch of the sound. This oscillation results in the "whistle sound" in ducted flue instruments. See wind instrument and flue pipe. A distinct tone color, determined by the dimensions of the instrument and the voicing mouth, is then slightly modified by the player's technique or embouchure. In instruments such as the recorder, the player can vary the pitch of the resulting musical note by opening or closing finger holes along the bore of the instrument, thus changing the effective length.
The windway consists of the "wind canal" or "flue", the upper portion of the voicing/mouth as carved into the headjoint itself, and the ducted flue windway, as carved onto the top surface of the fipple block. The space created between the ducted flue windway and the labium ramp edge is referred to as the "mouth" or "voicing".
The size of the mouth (length, width and depth) is usually in proportion to the instrument's bore, depending on the model of instrument and specifically which original instrument is being copied (in the cases of recorders). Many mass-produced factory instruments feature a voicing of a rectangular or flat lozenge cross section. Such a flat and rectangular voicing however, produces a less-than-sweet tone and offers far less dynamic flexibility (pitch bending) than a flute embouchure. The recorder voicing was designed to limit pitch bending for a more stable tone with variations in breath pressure. Typically, a shallow ramp instrument, such as a tabor pipe, will allow faster register changes, pitch bending and "flutey" tone, while an instrument with a deeper ramp will limit fast register changes, pitch bending and produce a more "reedy" tone.
Some modern recorder makers now produce curved labium lip voicings to add harmonic tone color. If the air stream strikes a curved "D" shaped lip, there will be slight turbulence created at the voicing mouth. This translates to extra sympathetic harmonics or "tone color".
The chamfer/rounding at the end of the windway that opens on the mouth/voicing is responsible for the quality of articulation of the ducted flue instrument. It consists of one or both of the windway exit lips being rounded. This can be seen by looking through the labium (window) at the place where the windway opens out on the mouth/window. These rounded edges affect the responsiveness (tonguings) produced by the player. This enables the rhythmic and dynamic language of the instrument to be "spoken". Articulations such as "Ta", "Da", "Ra", "Ta-ka" and "Da-ga" and "Diddle" will be very clearly differentiated in a good instrument played by a good player. An inferior instrument lacking these modified rounded edges on the windway exit will greatly limit the dynamics of tone or create "dead spots" in the music. The lack of this feature will degrade the performance of a ducted flue instrument, regardless of the effort made by the player to correct tone, or his or her level of skill.
Because of the fixed position of the windway with respect to the labium, fipple instruments can make a musical sound without the kind of embouchure required with (for example) the flute. However, it is not true that no embouchure is required to make a beautiful-sounding tone.
Embouchure on fipple flutes is centered around the idea of focusing the air inside the instrument's windway and bore alike, following the shape of the bore. Thus, a bore with a wide "bell" at the bottom of the instrument (as with Renaissance recorders) responds best to holding the throat wide open, to direct the airflow in a wide current so as to resonate the entire length and width of the bore. A bore which tapers down to a narrow "bell" (such as in Baroque-modeled recorders and school instruments) sounds best when the lips are used to focus the air to a tighter stream, to focus the air to the narrower "bell" at the bottom of the instrument. The idea is to always resonate the full length and width of the bore with the airflow, focusing the air accordingly. At all times, closing the lips around the "beak" of the recorder or fipple flute will help to focus the air down the narrow windway. This is very important to tone production on any fipple flute.
While a tight seal between the lips and the "beak" of the recorder focuses the tone, a tight facial musculature will also produce a raspy sound (with recorders, specifically). The combination of a clean seal with the lips around the beak, with the relaxing of the cheeks and face muscles, while allowing the cheeks to puff out in response to the flow of air, will be ingredients in the greater recipe of factors which produce a focused, ringy tone. This "greater recipe of factors" includes not only embouchure, but posture, articulation, breathing and fingering technique alike. Care should be taken not to block the windway with the teeth, which filters and scatters the airflow, producing a less-than-focused sound with a fuzzy edge, so to speak.
L.E. McCullough notes that the oldest surviving whistles date from the 12th century, but that, "Players of the feadan are also mentioned in the description of the King of Ireland's court found in Early Irish law dating from the 7th and 8th centuries A.D."
The Tusculum whistle is a 14-cm whistle with six finger holes, made of brass or bronze, found with pottery dating to the 14th and 15th centuries; it is currently in the collection of the Museum of Scotland.
One of the earliest surviving recorders was discovered in a castle moat in Dordrecht, the Netherlands in 1940, and has been dated to the 14th century. It is largely intact, though not playable. A second more or less intact 14th century recorder was found in a latrine in northern Germany (in Göttingen): other 14th-century examples survive from Esslingen (Germany) and Tartu (Estonia). There is a fragment of a possible 14th-15th-century bone recorder in Rhodes (Greece); and there is an intact 15th-century example from Elblag (Poland).
Instruments that use a fipple 
Fipples are used in the following musical instruments:
- Diple (or dvojnice)
- Flageolet (forerunner of the tin whistle)
- Ocarinas, and among them the Gemshorn
- Organ pipe
- Pipe (as with tabor)
- Slide whistle (aka swanee or swannee whistle, piston flute, jazz flute)
- Tin whistle (or penny whistle)
See also 
- Wind instrument for additional information on sound production
- L.E. McCullough (1976). Historical Notes on the Tinwhistle The Complete Irish Tin Whistle Tutor, Oak Publications. ISBN 0-8256-0340-4.
- Nigel Gatherer. The Scottish Whistle.
- Hunt, Edgar "The Recorder and Its Music" 1962 John Gardner Printers, Ltd Hawthorne Rd, Liverpool, England
- VanHauwe, Walter "The Modern Recorder Player" 1984 Schott, ISBN 0-901938-96-3