Consonance and Dissonance
Two notes close together do not just sound wrong. They sound rough, and you can hear exactly where that roughness comes from. Move one of them apart by hand and the slow throb speeds up, turns into a buzz, and then lets go into two clean tones.
- roughness is physical
- beats you can count
- taste enters later
read next
Two notes close together do not sound wrong. They sound rough.
Why do two close notes scrape?
You put the second tone one hertz above the first and heard one throb a second. At three hertz apart there were three. At sixteen you could not count them any more, and the sound had turned into a buzz.
Two tones beat as many times per second as they differ in hertz, which is why that counting worked. Somewhere around twenty per second your ear gives up counting and files the whole thing as roughness. Move them far enough apart and they land in separate places in your inner ear, and the roughness lets go.
Drag the slider to 233 and then to 330 once more. Nothing changed along the way except the distance, and that is the entire subject of this page.
What you just heard
- Close together, the two waves take turns
They reinforce each other, then cancel, then reinforce again. That cycle is what you hear as a throb, and it repeats as many times per second as the two frequencies differ.
- Past about twenty per second you stop counting
The throbs blur into one sound, and your ear files it as roughness rather than as a rhythm.
- Far enough apart, the roughness lets go
The two tones land in separate places in your inner ear and stop interfering. That is the moment a pair starts sounding like one thing.
- Consonance is mostly the absence of that roughness
Which is why it is not purely a matter of taste, and also why it is not the whole story: what a culture does with the rough ones is taste.
Smooth to rough, same bottom note
The lesson used two bare tones, because that is where the beating is clearest. These are real notes, which is the form you actually meet it in.
Where you run into this
- Two singers slightly out of tune, wobbling against each other
- A chorus effect, which is deliberate mild beating
- A cluster of adjacent notes on a piano, which is roughness on purpose
- Any chord that wants to move somewhere, which is roughness doing its job
Keep going
Everything in sound and hearing
- Pitch Perception
- Psychoacoustics
- Consonance and Dissonance
- Music and Emotion
- Why Songs Get Stuck in Your Head