8.1 Harmonics and just intervals
8.1.1 Human voice and importance of harmonics and just intervals
Human voice is created by the periodic vibration of
vocal cords. From
Fourier analysis, there are harmonics in every periodic signal like the human voice, a sound of a string instrument, a flute or a trumpet, etc.
Harmonics are therefore present in the human voice and other musical sounds.
Just intervals are the intervals between different harmonics. Our brain is well trained to analyze and recognize human voice via identification of these just intervals, so we can understand that just intervals play an important role in music. This is what we will be concerned with this program.
Auditive illusion:
This
video (read the given description) illustrates how the brain creates virtual fundamentals from just intervals, as it would like to give the information to consciousness that a human voice is present. See
wikipedia about Missing fundamental.
8.1.2 What are the harmonics
The harmonics of the fundamental note
for example (subscript
here means octave
), is the following series of notes (and correction in half-tone)
where the index is the harmonic number
and the subscript is the correction with respect to the equal temperament. For example, harmonic 3 is
with a correction
of halftone. Harmonic
is
with a correction
of halftone, that is very perceptible. What is important to observe and enough to remember for after is:
- Harmonic
is called the fundamental.
- Harmonic
is an octave from the fundamental.
- Harmonic
is a fifth (plus an octave) from the fundamental with
of halftone (not perceptible).
- Harmonic
is a major third (plus two octaves) from the fundamental with
of halftone.
- Harmonic
is a minor seventh (plus two octaves) from the fundamental with
of halftone.
- Harmonic
is a augmented fourth (plus three octaves) from the fundamental with
of halftone.
- Harmonic
is a major sixth (plus three octaves) from the fundamental with
of halftone.
Numbers
are
prime numbers and are enough to construct every other numbers, e.g. harmonic
is one octave above harmonic
, hence
octaves and a fifth above the fundamental. Harmonic
is a fifth and an octave above harmonic
, hence two octaves and two fifth above the fundamental, etc.
Frequency of the harmonics:
if
is the frequency of the fundamental, then the frequency of harmonic
is
, and frequency of harmonic
is
, etc. These frequencies are slightly different from the frequency obtained in equal temperament (more on this below). More informations about harmonics are in the appendix
sec:Harmonics.
8.1.3 What are just intervals
Between the first harmonics, there are the following intervals:
- a fourth between harmonics
,
- a minor third (
) between harmonics
,
- another different minor third (
) between harmonics
,
- a tone (
) between harmonics
,
- another different tone (
) between harmonics
,
- a triton (
) between harmonics
,
- another different triton (
) between harmonics
, etc
These intervals between the harmonics of a given fundamental are called just intervals and will have important roles after.
Ratio of frequencies of just intervals:
In the example above, if
is the frequency of the fundamental, then the frequency of harmonic
is
, and frequency of harmonic
is
. Consequently we get that the ratio of frequencies of the major third just interval
is
Similarly the ratio of frequencies of the fifth just interval
is
More generally, there is a just interval between two frequencies
if the ratio is
with two small integers
(say
).
Dissonance of a just interval:
We define the
dissonance of the just interval
as
for an
irreducible fraction
, and with “
” being the
natural logarithm function. (There will be a simpler expression of
later).
8.1.4 Just intonation
Just intonation in musics means that the we play music only with just intervals.
We will use two different and complementary representations for the notes constructed with just intervals: the pitch space and the tonnetz space. We will explain after the concept of temperament that explains how we choose 12 (or less) notes per octave.
8.1.5 Equal temperament
In every culture of every time people played with just intonation, i.e. just intervals. The only exception comes from Europe in the mid of XIX-th century, where they introduce the
equal temperament, where the octave is divided in logarithmic scale in 12 equal intervals. The consequence is that in the equal temperament, only the octave remains a just interval. The other intervals are not. But the fifth and fourth are very close to just intervals, so that the difference is unperceptive.
Later, we will not consider the equal temperament (except in the pitch space representation, giving some dashed lines of references).
8.2 Tunplot: the pitch space
The pitch is very well known by musicians, it represents the position of a note on a piano keyboard. Precisely the
pitch of a note of frequency
is defined by
where
Hz is the frequency of the
tuning fork and
is its (MIDI) key number. In other terms
represents the position of a note on a MIDI keyboard, where integer values are exactly on the keys (ex:
is for
) but non integer values of
are also allowed, between the keys.
For example the pitch of
is
and the pitch of
is
, hence
is a pitch at “half-distance” between
and
, sometimes called
quarter-tone.
In the program, the representation of the pitch space is called: “tunplot”.
Exemple 8.1.
On the following example, three notes have been played with the keys
. They are represented by the green disks. Their horizontal position gives their pitch
, and the
dashed lines are is the equal temperament, i.e. for integer value of
. (In black are other accessible notes by the temperament, but not yet played).
In this example, we observe that
is a little lower than the equal temperament line. The number on the left of each line is the octave range number.
8.3 Netplot: the tonnetz space
In German, ton = “note”, netz = “lattice”, so “tonnetz” is the lattice of notes.
Euler in 1750, proposed a
tonnetz to represent just intervals, that is a two-dimensional lattice. He used it to represent just intervals with fractions of the form
with integers
. This is not sufficient for our purpose, we will need to use all the
prime numbers
in our fractions, so we define the tonnetz space as follows: a fraction is written uniquely as
with some integers
. We represent
as a point in a lattice. In practice we will represent the axis of
, the axis of
are written in small fonts. The value of
(the octave) is not represented, instead we represent the octave position of the note on a circle.
In the program, the representation of the tonnetz space is called: “netplot”.
Exemple 8.2.
On the following example (same as
8.1), three notes have been played with the keys
. They are represented by the green domains. (In grey are other accessible notes, but not yet played). The horizontal axis is for
value, the vertical is for
and the octave range value is the position on each circle. From this picture, up to a factor
, we can read that the ratio of frequencies are
and
. We also read that
are in the same octave range
and
is in the octave range
. We can deduce actually that
is a just fifth and
is a just major third. This was not visible from the pitch representation.
Résumé 8.3.
We not need to be a mathematician to read this picture. We only need to observe that the notes
are neighborhoods on the horizontal
axis which is the axis of just fifths and that
are neighborhoods on the vertical
axis which is the axis of just major third.
Exemple 8.4.
Here is an example with the three axis
. This is a chord with
. The note
is along the axis
which is the axis of just minor seventh.
Exemple 8.5.
In this more complicated example, we have played the keys
. On the pitch axis, we have added the fractions, where
has the fraction
, this is the reference note. For example
has the fraction
with respect to
. On the tonnetz it is represented on the axis
, that starts from
on the upper-left. Similarly the axis
is on the upper-right, the axis
is on the right, the axis
is above.
8.4 Formula for pitch values
We have seen of the tunplot figures above that the pitch of notes differs slightly from the equal temperament shown by the vertical dashed lines. Here we explain how to compute the pitch corrections from the position of the note on the tonnetz.
Here is a table and we explain after how to use it with an example.
Axis |
interval |
Number of half-tone |
pitch correction
|
|
|
in equal temperament |
with respect to equal temperament |
|
fifth |
|
|
|
major third |
|
|
|
minor seventh |
|
|
|
augmented fourth |
|
|
|
minor sixth |
|
|
|
half-tone |
|
|
|
minor third |
|
|
Consider the following example, called harmonic temperament later:
In this example the reference note is
and his pitch is given by the equal temperament, i.e.
. We see on netplot that
is a major third from
, so
as can been observed on tunplot. Similarly we deduce the following table of pitchs for this “Harmonics scale”
Note |
pitch (i.e. half-tones) w.r.t.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
What is important to remember is that:
- Intervals along the
axis, i.e. fifths, do not differ from the equal temperament intervals by a perceptible quantity. And similarly for
axis.
- Intervals along the
axis of major third get a strong pitch correction
(of half-tone) for each positive step.
- Intervals along the
axis of minor seventh get a strong pitch correction
(of half-tone) for each positive step.
8.5 Temperaments
There are an infinite number of rational numbers, so in practice if we use an instrument with 12 keys per octaves, we have to choose which frequencies to play. Such a choice of frequency for each key is called a temperament.
8.5.1 Fixed temperament
If this choice is fixed, we say that we play with a
fixed temperament. The program will allow to play with different fixed temperament, like the famous “
Pythagorean temperament”, the “
Zarlino temperament”, the “Harmonic temperament” etc.. but also on some fixed temperament build by the user. The advantage of fixed temperament is that some fixed “musical mode color” can be explored. The dis-advantage of fixed temperament is that not all the just intervals are available, and some intervals sometimes sound bad.
Zarlino temperament in C:
Here is the picture for available notes
We observe on the tonnetz space that it is made with just fifths and just major thirds. On the pitch space, we observe that the temperament is the same on each octave range. One advantage of the Zarlino temperament is that we have just major triads chords like
,
. But beware that some other triads are not the usual just like:
or
.
Harmonics temperament in C:
Here is the picture for available notes
We observe on the tonnetz space and pitch space that it is made from the first few harmonics of
(up to harmonics 25).
8.5.2 Adaptative temperament
Adaptative temperament has been defined and proposed in the paper [
1].
Instead of using a fixed temperament, it is also possible to adapt the frequency of some key to the present situation, depending on the notes that are played currently (or recently) and depending on some (deterministic) rules. We say that we play with adaptative temperament. The advantage of adaptative temperament is that all the just intervals may be available. The disadvantage is that the frequency associated to a key may change, and this can be a difficulty to the player. To overcome this difficulty this is very useful for the player to look at the available notes on the tonnetz space and on the pitch space.
Definition of the adaptative temperament:
Suppose that there is already a set of notes, i.e. a chord
that sounds. The adaptative temperament depends on this chord
. We have to choose how many keys per octave we want to be able to use. This is
. Then the octave range is divided in
equal intervals and in each of these intervals, we choose the note that is the most consonant with the notes already present in the chord
(among the infinity of rational numbers in this interval).
More precisely we use the dissonance function
of a note
with respect to the chord
made of
notes.
where
is the dissonance of the just interval
made by the pair of notes
. (On the tonnetz
is just the
distance). The resonance function is just the opposite
.
Exemple 8.6.
In this example, the chord is made of the only note
. We choose
notes per octave. For this we are allowed to use only the keys
in each octave. On the first pitch space we represent a vertical bar at (almost) every rational number, i.e. possible new note
, and the hight of the bar is the resonance
. Each octave is decomposed in
equal intervals and the black lines is the selection in each interval that gives the highest bar, i.e. the most resonance note, with respect to
. This is the adaptative temperament with
notes per octave, for the chord
. The other pictures are as above.
Observe that the temperament is not the same at each octave. We observe interestingly that the scale in octave
is the “blues scale”.
Exemple 8.7.
In this example, the chord is made of four notes
. We choose
notes per octave.
We observe that the adaptative temperament depends indeed on the chord
.