Humans crave simplicity. We crave complexity too, but under very different circumstances. The beauty of simplicity is that it’s easy to understand, and understanding things easily is important to our survival. Simplicity is especially useful in language, because tight definitions allow us to convey information in fewer words. I once had a friend ask me what the “shape that was like a square but two sides were longer and the other two were shorter” was; he meant, of course, a rectangle. Reducing complex ideas to a single term helps us better understand each other; not all concepts can be so easily reduced, though.
Consonance and dissonance are not things that can be easily described with one definition; their meanings are relative to each other, and relative to culture, upbringing, sense of aesthetics and more. A discussion of consonance and dissonance must also be a discussion of complexity. Instead of offering you an easy answer for what they mean, we’re going to analyze them through a variety of different lenses, to see if we can create a deeper understanding of the terms.
The first lens we’ll employ is purely mathematical. Two notes might be said to be consonant if the ratio between them is simple; our desire for simplicity, back at it again. A perfect fifth is highly consonant; it has a ratio of 3:2, meaning that the upper note makes three vibrations in the same amount of time it takes for the lower note to make two. The major seventh, conversely, has a ratio of 15:8; quite an odd ratio, and quite dissonant to the ears. Without knowing these ratios, you could probably still tell which interval was consonant, and which was dissonant. Things get really strange with the perfect fourth, though. It has a ratio of 4:3, which is quite simple, and to listen to it without context, it sounds quite consonant. In practice, however, the perfect fourth is often considered dissonant. Why?
Put simply, it’s because more often than not, a perfect fourth feels like it needs resolution. It feels inchoate and unstable; we expect it to resolve to a more stable note. You can try this yourself; play an A, then a D, and you’ll almost certainly feel the desire to resolve the two to a third note, something like a C. This shows that simple mathematical ratios aren’t sufficient to explain consonance and dissonance; what else, then, explains it?
The answer to this isn’t simple, though we might very much like for it to be. We can look at cultural understanding to get some idea. When most of the music you’ve listened to for most of your life has treated certain intervals as “tension” and other intervals as “resolution”, you’re likely to interpret perfect fourths or other dissonant intervals as tense, even if you didn’t even understand the concept before reading this. Things that we might find “dissonant” can be seen as desirable in other cultures. For example, if you play two notes whose wavelengths are ever so slightly different, the ear can’t readily distinguish between the notes, but the respective waveforms will construct and destroy each other when overlapped, creating a “beating” or “tremolo” effect. Western musicians strive to eliminate beating, while in Indonesian gamelan music, the beat effect is sought for its added texture.
Here’s where things get really complex: what is music? When we make music, what are we trying to do? There’s no easy answer to this question, but what we are doing, in most cases, is trying to use sounds to invoke an emotional response. We can analyze consonance and dissonance from a variety of different angles, but in the end, it all comes down to how you feel. You can feel dissonance because you want resolution; you can feel consonance because it feels like home, like you’ve arrived. Without dissonance, the music might not be very good, because there’s no real journey to embark on; you never leave home. Without consonance, the music feels listless, unpredictable, not fully formed, because you have no sense of where you’re trying to get to. This means that consonance and dissonance will change constantly over time, as we gain new conceptions of what home is, and what the journey to home feels like. As you learn more about music, through piano lessons, music theory classes, even just listening to new songs, you’ll begin to appreciate how deep the concepts of consonance and dissonance truly are.