Scale in pipe-making is used to refer to two related things; the absolute diameter of a given pipe in a rank, or the ratio of diameter to length of a pipe.
Scale as a ratio has an important effect on the sound of the pipe; for flue pipes, thinner pipes (‘narrower” scale) produce a more stringy sound (more overtones) than fatter pipes (“wider” scale) which produce a broader or more flute-like sound (fewer overtones). Compare the proportions of violin and flute pipes to see this difference. Scale is not the only determinant of tone quality, and the design and proportions of the mouth, the pipe material, the wind pressure and the voicing technique also play significant roles.
Scale as an absolute measure is used as a practical design specification for a stop, and is normally defined as the diameter of a pipe at 8’C. An organbuilder may specify the scale of the pipe at various points in the rank (each ‘C’ pipe for instance) in order to achieve specific tonal effects. Commonly, the diameter of an organ pipe will halve every seventeen notes; scaling however is a highly sensitive topic and an intrinsic part of tonal design, providing the character of a specific builder’s instruments.