Parish Organ

How to read the specifications for Letourneau Opus 98

Letourneau Opus 98 has four divisions: three manual divisions played by the hands, and the pedal division played by the feet. Each part of the specification refers to the stops played on those keyboards respectively.

The name on the stop refers mainly to function or musical color. Some organ stops are nonimitative and produce foundational organ tone, e.g. Diapason, Principal, Octave. Others resemble wind or string instruments, e.g., Chimney Flute, Harmonic Flute, Gamba, Trumpet, Oboe, Trombone.

The numbers following the stop names refer to pitch as a function of the length of the longest pipe in the stop. Eight foot pitch is unison pitch. It agrees with the pitches of the corresponding keys on the piano, for instance. The science of acoustics works on the simple principle that halving or doubling the length of a vibrating body changes its pitch by an octave. Therefore, the shorter and longer designations after stops indicate the pitch at which they sound. The combination of these with unison stops produces the complex ensemble sound of the instrument.

Stops with a Roman numeral following the name, or mixtures, correspond to the upper harmonics of the overtone series and have multiple pipes per note tuned to these overtones.

The second number after the stop name, in the rightmost column, refers to the number of pipes in the entire compass of the stop, or the rank. Ordinarily, a complete manual stop would have 61 pipes, corresponding to the 61 keys of a manual keyboard. Where fewer are indicated, the stop may be an extension of another stop at another pitch (extensions often have 12 pipes to finish a bottom or top octave), or it may be incomplete for some particular reason. Occasionally, stops will play in more than one location in the instrument and these are indicated by the name of the home division in the final column.

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