How we perceive how high or low a note is (Physics of Sound)

By Tracey 1 min read

How fast a particle moves back and forth -> how high the note sounds

So human ears and other creatures' ears hear frequency as the pitch of a note. Pitch is how high or low a note sounds.

Frequency is how many cycles per second, that a particle moves back and forth, in the case of sound. (Pitch of note is measured in Hertz, Hz which is exactly that, "per second")

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It turns out that "whole number ratios" between frequencies sound really pleasant. I.e 2:1, 3:2, 4:3

If a note is playing at twice the frequency of an original note, we actually perceive it as an octave. Cats can also recognise octaves apparently.

If another note is playing at 3 times the frequency of an original note, the interval between the 2nd and 3rd notes is what we refer to as a fifth, in western music (3:2 ratio)

So

• 2:1 ratio is an octave

• 3:2 ratio is a fifth

• 4:3 ratio is a fourth

• 5:4 ratio is a major third

• 6:5 ratio is a minor third

Picturing sound waves

A while back, we did sound in physics class, and in my own time, I tried to make a model of sound waves, and a project to play with different frequencies

You can play with the model here, to picture frequency (pitch).

https://c.gethopscotch.com/p/12nbko3v8p

There is also amplitude that you can picture in this project, on the iOS app (how far the particle moves back and forth -> how loud a note sounds). However, it seems to be not working in the website version, I will have to take a look.

Hearing frequencies

Here is a project I did, where you can hear multiples of a frequency

Instrument tuning

You can also see this minutephysics video, on why it is impossible to tune a piano to these perfect whole number ratios (it has too many strings, basically)

But you can do it for other instruments e.g. violin (uses 3:2 ratio, or fifths), or guitar (mostly uses 4:3 ratio, or fourths)

As for why whole number ratios sound pleasant,

  • sound waves get superimposed together (added together) when they are played together
  • with whole number ratios, they eventually have periodic lowest common multiples, which is pleasant-sounding
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Tracey