The stopped diapasons are closed at their ends with a lid. As a result, they sound an octave lower than open pipes of the same length.
The "closing" of an organ pipe works only with labial pipes/"flutes". Each sounding air column needs an opening at its end to communicate with the atmospheric air pressure, because every air vibration within a pipe consists of rapidly changing pressure waves in the air located there - without an exchange with the outside air nothing sounds, because inflated air (positive pressure) must be able to escape somewhere again. This happens with covered pipes on the labium (permanently open to the outside air) - the pressure wave runs as it were to the end of the (closed) pipe, from there back to the labium, where it can escape (which in turn creates negative pressure, and subsequently a swinging air "pendulum", which becomes audible as actual sound); it thus covers a path twice the length as within an open pipe, the result is a pitch an octave lower.
An 8' covered pipe thus produces the pitch of a 16' open pipe, so instead of C the C'. From the time this phenomenon was applied in organ building around the 15th century it allowed an enormous saving of space in the constructing pipes for deep sounds. As a rule, the lowest ranks consist entirely or partially of covered pipes. Stopped pipes are frequently used in any sorts of small and miniature organs of all kinds up to slot machines and cuckoo clocks.
A special case is the stop "Chimney flute", a stopped rank with an opening in the lid, occasionally only a small hole, but more often an open tube inserted into such a hole, which facilitates the tuning. Although this obviously opens the pipe again at the end, in this opening a separate air wave forms inside such a tube with a correspondingly narrow diameter, which separates the vibrating air within the pipe from the atmosphere. Thus, the chimney flute sounds as a stopped pipe, but has a special, characteristic tone rich in harmonics.
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