March 20th, 2000

A quote from Robert Jourdain's book Music, the Brain, and Ecstasy; How Music Captures Our Imagination, regarding dissonance.

We call sounds that go well together consonant; the rest are dissonant. Like descriptions of heaven and hell, science is not nearly so adept at explaining consonance as it is dissonance. Dissonance arises from three quite different sources, one based in neurology, one in acoustics, and one in music theory. Let's consider each in turn.

The neurological explanation of dissonance focuses on the inner ear, the cochlea. ...a loud pure-frequency sound stimulates a wide range of receptor cells along the cochlea's basilar membrane. The membrane is most deformed, and the receptors most activated, at the point along the membrane associated with the particular frequency. But receptors on either side also fire. This range of activation is called the critical band for the sound. It has been found that two frequencies form a dissonant interval when their critical bands overlap. By falling so close together along the cochlea, the two sounds upset each other's perception.

I find this very interesting. I never considered that there could actually be a physical reason why some sounds are dissonant. This makes it much harder for us as human to view a dissonance as a consonance. Back in the middle ages, an interval of a third was considered dissonant. Now, it's a consonance.

So, a large part of the reason why a minor 2nd or major 2nd sounds dissonant is because it activates the critical band. In theory, a major 7th should sound less dissonant than a minor 2nd because there's a greater range between the cycles per second of the root and the 7th.

More on dissonance: For lower frequencies, however, the cochlea crams a broader range of frequencies into a critical band, and tones need to be farther apart to avoid dissonance. At the bottom end of a piano keyboard, tones as much as a fifth apart (seven half-steps) are dissonant. Harmonious music can hardly be written in this frequency range, and composers normally keep bass tones well separated.

It's good to finally have some of my "unconscious" questions about the piano answered. What I mean by unconscious is this: I've always accepted that when you play in the lower register of the piano it begins to sound "muddy". It was always clear that it was because the frequencies of the notes became increasingly lower but I never knew how the ear actually handled these tones.




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