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The intervals between notes are "equal" not in the sense that the difference in Hz between them is the same, but the ratio a between them is the same. Let's say g is one semitone higher than f, then g = a f.

Note Hz Ratio a to previous note, rounded to 3 decimal places
A4 440.00
A#4 466.16 1.059 (466.16 / 440.0 = 1.059, and so on down the column)
B4 493.88 1.059
C5 523.25 1.059
C#5 554.37 1.059
D5 587.33 1.059
D#5 622.25 1.059
E5 659.25 1.059
F5 698.46 1.059
F#5 739.99 1.059
G5 783.99 1.059
G#5 830.61 1.059
A5 880.00 1.059

It might be easier to understand when you think of the frequency of the octaves. The number of Hz between octaves is different (220, 440, 880, 1760, etc.), but the ratio of 2:1 is always the same. The same concept applies to the notes in the scale.

Mathematically, what we are doing is dividing an octave (2:1 ratio) into 12 equal steps (equal in ratio, i.e. a^12=2). Using a scientific calculator, we can solve for a=2^(1/12) = 1.0594630943592952645618252949463, which is the exact ratio between two half steps.

The division of notes has to do with human perception and psychoacoustics. One description of human perception is the Weber-Fechner law, where a human will perceive equal changes in some sensory input, such as sound level or sound pitch, not by absolute level or value difference, but by the ratio of the change. e.g. larger values need a proportionately larger change for the change be perceived (if small) or perceived as about the same, within some reasonable range (e.g. audible, but not causing ear damage, etc.)

Thus, for a semitone (forth, fifth, etc.) interval to sound the same, no matter which base note one starts from, in the equal temperament scale, the notes have to differ not by equal absolute frequency differences (as would be created by equal Hz deltas between notes), but by equal ratio differences (the 12th root of 2, so that twelve equal multiplications will equal one octave).

e.g. the "equality" in equal division has to be in equal in ratio, not additive absolute value.

What happens if you go down by the same steps:

• 440Hz


• 1 step down : 403.33Hz


• 2 steps down : 366.67Hz


• 3 steps down : 330.Hz


• ...


• 11 steps down : 36.67Hz


• 12 steps down : 0Hz


• 13 steps down : -36.67Hz

So, using your "equally divided" logic, we are at zero Hz after 12 steps, and the next step beyond that is minus 37 Hz! What does that even mean? But ok, let's follow your logic a little bit ... what's the frequency exactly in the middle of the octave 440 - 880 Hz, that would be 660 Hz. What's an octave above that? That would be 2 * 660 Hz = 1320 Hz. What would be the steps in that octave - 660 Hz / 12 = 55 Hz? Ok, then let's take one step up from 660 Hz, that's 660 Hz + 55 Hz = 715 Hz. But wait ... the step was supposed to be 37 Hz, not 55 Hz??? Does your step size depend on the start and end points of the octave? Or does it take a sudden jump at 880 Hz - steps below 880 would be 440 / 12, but above 880 they would be 880 / 12? Where does such a divider come from, is it embedded in nature? I thought A = 440 Hz was only an agree-upon convention, not a law of nature.

Where did you get the 880Hz? By multiplying by 2, i.e. one octave higher. I guess the same has to apply for any frequency, not only 440Hz? For example, one octave higher from 880Hz has to be 880Hz * 2? And any other frequency like 1000Hz... one octave above that must be 2000Hz. If the interval of an octave is calculated with multiplication, how could other intervals be calculated with addition?

So, ask yourself: if F1 and F2 are the frequencies of two consecutive semitones, what is the relationship between F1 and F2, if (F1 * 2) and (F2 * 2) has to have the same relationship?

You're looking for a function f(F) such that f
applied 12 times gives 2*F.

 f(f(f(f(f(f(f(f(f(f(f(f(F)))))))))))) = 2 * F

If you step up one semitone from F, you get a frequency f(F). The frequency one octave higher from that is 2 * f(F).

If you first step up an octave, you get F*2. And if you step up one semitone from that, you get f(F*2), which should be the same frequency, so:

 2 * f(F) = f(2 * F)

What might function f be like?

From the subject line "why hertz differences are not the same but element 12th of two?" I assume that you already know that consecutive semitones have a ratio of 2^(1/12).

A simple way is to look at ratios as suggested above. One can divide an interval equally arithmetically such that the length (size, or more technically "measure") of each subinterval is identical. Dividing an interval arithmetically in 12 pieces (I can explain the 12 but it takes more math.) yields, 1=12/12, 13/12, 14/12, 15/12, 16/12, 17/12, 18/12, 19/12, 20/12, 21/12, 22/12, 22/12, 24/12=2. However people's hearing seems (experimentally) to distinguish ratios of frequencies rather than differences as being more identical. For example (Taking A=440cps), the fifth above A is E at 660cps not 19/12*440=696.666....

If we want equal ratios for each half-step, instead of (2-1)/12, we 2^(1/12). The point is that the ratio of G to C is constant for all fifths (A-D, C-F, etc.). Since antiquity, the ratio of a fifth is 3:2 (or 3/2 times the frequency of the lower note.) This comports with dividing a string into intervals and listening to the frequency of the two shorter pieces. (Aside: Vincenzo Galilei suggested using 18/17 as a approximation to the twelfth root of two; it's remarkably good.)

However: for computational work, we can use logarithms; the logarithm of a ratio is the difference in the logarithms of that ratio's constituents. One divides the octave into 1200 cents (the 1200th root of 2) and assigns 100 cents to the equal-tempered semitone. This allows one to easily (at least when using pencil and paper instead of a calculator) compute interval sized for varying tunings.

So, even though our ears hear by ratio (experimentally), we can calculate by ratios or addition. Wiki has a bunch of articles qG (quod Google in analogy with qv) which give a more comprehensive explanation.

Possibly a simple way to look at it is to look at a guitar neck. An octave there is divided into 12 parts - equal as far as each fret is a semitone away from its neighbour. But looking carefully, it's fairly obvious that each fret isn't the same size. In fact, the eleventh fret is very nearly half the size of the first one, from nut to fret 1. Go further, and the 12th (octave) is actually half the size of the first.

Your hypothesis is that they would all be the same size - one twelfth of the half length of the open string? Were that the case, what woud happen at fret 13? And apart, each fret would produce a note that was out of tune. So there needs to be a ratio of each fret against its neighbour, as pointed out in other good answers.

Start by considering the equal division of octaves into one part. That is, think about changing pitch by octaves only.

If we start with A1=55 Hz, we have the following pitches:

 Pitch Frequency
 ----------------
 A1 55 Hz
 A2 110 Hz
 A3 220 Hz
 A4 440 Hz
 A5 880 Hz
 ...

You can see that when you increase the pitch by an equal additive amount, you increase the frequency by an equal multiplicative factor. That is, every time you increase the pitch by one octave, you double the frequency. This means that the relationship between pitch and frequency is logarithmic.

From there, it is fairly easy to reach the conclusion that to divide the octave into some number of equal parts, you need to find the factor that, when multiplied by itself that number times, yields 2. In other words, the frequency factor corresponding to a division of the octave into n parts is the nth root of 2.

Our note system is a logarithmic scale for frequency. A logarithmic scale turns equal fractions into equal distances. You can define equal temperament as a constant step size of 1/12 on the log_2 scale of frequency.

Going back to the linear scale, this means that a semitone translates into a factor of 2^(1/12) (the twelfth root of two).

The reason for this is, that the sound of an interval depends on how the overtone spectra of the two nodes match up.

The octave has the unique feature that all harmonics of the higher note match up with some harmonic of the lower note. Likewise, if you have a perfect fifth (factor 3/2), every second harmonic of the upper note coincides with every third harmonic of the lower note. Similar relations hold for the perfect fourth (factor 4/3), the mayor third (5/4) and the mayor sixth (5/3). And so on, and so forth. The pattern of how the harmonics match up defines the sound of the interval, and the harmonics are defined by factors of frequency.

Thus, only a logarithmic scale can be used to describe intervals well (our note system). And by consequence, equal temperament must be defined on the logarithmic scale.

This is another answer trying to help understanding also the question to people who can't cope with ratios and other abstract terms:

Imagine you have a tone of 12 Hz frequency (a string waving 12 times/second).
How must the 12 half steps between the octava (24 Hz) be tuned, so that the differences between all half steps are equal?

The question implies: If the range between the octava is 12 Hz, why is the difference between the 12 half steps not always just 1 Hz?

root=12Hz

minor second 13Hz

major second 14Hz

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perfect fifth 18Hz

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major seventh: 23Hz

octave: 24

We can see that the difference between the first half 12Hz and 13Hz is just 1/10 of 12Hz (10% of the whole octava), while the additional difference between the octava 24Hz and the precedent half tone (23Hz) would have been almost only a 1/20 (=5%) of the difference between the next upper half tone above the octave will be 2Hz more - because this must be a 1/10 of the next octava of 48Hz, as the difference between of ocatava' (24Hz) and octava'' (48Hz) is 24Hz! (48-24=24) and a half step of 1/12 between octava' and octava'' will be 2?

From this we can derive that the differences between the half steps are not additional of 1/12 but proportional by multiplying each half step with 1/12.

Hope this is not droning and confusing. TLDR?