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Jason Nong

Mutation scaling

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Apologies if this is a very basic question, but what is the idea behind principal-scaled mutations being part of the plenum and wider-scaled mutations not blending with a principal chorus? I occasionally find it fun to experiment adding a nazard and tierce to 8', 4' and 2' principals, like a cornet with an 8' principal instead of a bourdon.

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The question is not at all 'basic' because it raises some subtle issues relating to the psycho-acoustics of aural perception by the ear and brain. The acoustic power of a principal-toned stop is spread over a number of harmonics, typically 15 or so, whereas that of a more fluty tone (as produced by a wider pipe of the same pitch) is concentrated in a smaller frequency bandwidth represented by fewer harmonics. This means that, for equal subjective loudness, significantly more power is forced into the fundamental frequency of a flute than a principal. In turn this means that the frequency receptors in the cochlea and the subsequent neural processing circuits in the brain are handling higher power levels at this frequency for fluty tones. This has several consequences. One is that these high powers result in greater masking of tones at other frequencies so that we perceive them less easily, and another is that the nonlinearity of the ear and brain generates stronger sum and difference tones which we can detect, even though they are not present in the original signal arriving at the ear. The net result can be a loss of what is often called 'transparency' or a loss of clarity in the sound as perceived subjectively if one combines higher-pitched flutes with lower-pitched principals. It results in a degraded ability to follow the individual melodic lines of a polyphonic composition such as fugue, or to detect which notes make up a chord in homophonic music such as a hymn tune.

 

Of course, there are exceptions to the generalisation sketched above, and basically the dictum is "if it sounds OK, it is OK". There are also other subjective issues involved, about which other forum members will be able to respond better than I.

 

CEP

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