A duct or fipple is a constricted bevel that is carved into the mouth piece, that is common to end blown flutes, such as the tin whistle, recorder, Catalan flabiol etc. These instruments are known as fipple or duct flutes or tubular-ducted flutes and are indicated by the code 421.2 in the Hornbostel–Sachs classification.
How the fipple / duct works: In such a construction the top [or head] of the recorder. A musician blows into the top of the recorder where the mouth piece lays. The head or top of the mouth piece which the fipple or duct is attached too. During performance the stream of air is operated by the “labium lip” producing a Bernoulli effect or siphon.
The air flowing over the voicing mouth creates a flow-controlled valve. 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 wind way consists of the wind canal or flue, the upper portion of the voicing or mouth as carved into the head joint itself, and the ducted flue wind way, as carved onto the top surface of the fipple block. The space created between the ducted flue wind-way and the labium edge is referred to as the mouth or voicing.