You can also read a more polished version in the article:
Edward Dunne and Mark McConnell, Pianos
and Continued Fractions,
Mathematics Magazine, vol. 72, no. 2 (1999), 104-115.
There are two pieces of acoustics that matter now:
By inverting the rule that says that the note one octave than another must have double the frequency, we can fill-in the perfect fifth in the first octave. It should have half the frequency of the G in the second octave.
Following Pythagoras, we can now attempt to use these two rules to construct `all the notes', i.e., a complete chromatic scale.
The perfect fifth in the key of G is D. Thus we have, by tripling then halving, then halving again:
Repeat again: the perfect fifth in the key of D is A:
We can shorten this by looking at the Table of Fifths, also known as the Circle of Fifths:
| Tonic | Fifth | Tonic | |||||
|---|---|---|---|---|---|---|---|
| C | D | E | F | G | A | B | C |
| G | A | B | C | D | E | F# | G |
| D | E | F# | G | A | B | C# | D |
| A | B | C# | D | E | F# | G# | A |
| E | F# | G# | A | B | C# | D# | E |
| B | C# | D# | E | F# | G# | A# = Bb | B |
| F# | G# | A# = Bb | B | C# | D# | E# = F | F# |
| C# | D# | E# = F | F# | G# | A# = Bb | B# = C | C# |
| G# | A# = Bb | B# = C | C# | D# | E# = F | G | G# |
| D# | E# = F | G | G# | A# = Bb | B# = C | D | D# |
| A# = Bb | B# = C | D | D# | E# = F | G | S | A# |
| E# = F | G | A | A# | B# = C | D | E | E# = F |
| C | D | E | F | G | A | B | C |
If we use the rule of doubling/halving for octaves, we arrive at the following frequencies for the twelve notes in our basic octave:
| Frequency | Tonic | Fifth | Tonic | |||||
|---|---|---|---|---|---|---|---|---|
| 1 | C | D | E | F | G | A | B | C |
| 3 / 2 | G | A | B | C | D | E | F# | G |
| 9 / 8 | D | E | F# | G | A | B | C# | D |
| 27 / 16 | A | B | C# | D | E | F# | G# | A |
| 81 / 64 | E | F# | G# | A | B | C# | D# | E |
| 243 / 128 | B | C# | D# | E | F# | G# | A# | B |
| 729 / 512 | F# | G# | A# | B | C# | D# | E# = F | F# |
| 2187 / 1024 | C# | D# | E# = F | F# | G# | A# | B# = C | C# |
| 6561 / 4096 | G# | A# | B# = C | C# | D# | E# = F | G | G# |
| 19683 / 8192 | D# | E# = F | G | G# | A# | B# = C | D | D# |
| 59049 / 32768 | A# | B# = C | D | D# | E# = F | G | S | A# |
| 177147 / 131042 | E# = F | G | A | A# | B# = C | D | E | E# = F |
| 531441 / 262144 | C | D | E | F | G | A | B | C |
However, there are some rules of acoustics that might also be used:
Using these rules and combining them as efficiently as possible, one arrives at the following list of frequencies for the notes in the C major scale. (That is, I'm leaving out five of the notes from the chromatic scale.)
| Note | Acoustics | Up by fifths, down by octaves |
|---|---|---|
| C | 1 | 1 |
| D | 9/8 | 9/8 |
| E | 5/4 | 81 / 64 |
| F | 4/3 | 177147 / 131042 |
| G | 3/2 | 3/2 |
| A | 5/3 | 27 / 16 |
| B | 15/8 | 243 / 128 |
| C | 2 | 531441 / 262144 |
Notice that some of these fractions are not equal! In particular, the final C in the scale ought to have frequency twice the basic C. Instead, if we go waaaay up by fifths, then back down again by octaves, we have this strange fraction, whose decimal approximation is: 2.027286530. If we took half of this to return to our starting point, we'd have:
So what is the problem? To answer this, it is time to consider some mathematics.
.
This should produce another copy of the note C, but
seven octaves up. Thus, we should compare this frequency with
.
The problem is that we are mixing a function based on tripling (for the fifths) with a function based on doubling (for octaves). More abstractly, we are trying to solve an equation of the type: 2^x = 3^y, where x and y are rational numbers. (With minor finagling, we could restrict to just integers.)
Notice that for different notes in our chromatic scale, we will be using different (and inequivalent) values of x and y. The first issue to contend with regarding the difficulty of notes not agreeing with themselves (that is to say, enharmonics that have different frequencies) is to make a choice of where to concentrate the errors. There are ways of tuning an instrument so that some keys have only slight problems, while other keys have rather bad discrepancies. (See `the wolf' below.)
Even temperament spreads the error around in two ways.
It is natural to use a logarithmic scale for measuring intervals in our musical/acoustical setting. The basic unit in our diatonic scale, the semitone, in equal tempering is equal to 100 cents. Thus, one semitone equals 100 cents and an octave equals 1200 cents.
We can measure the Pythagorean comma in terms of cents. The discrepancy was:
| (3/2)^12 | ~ | 129.746... |
| 2^7 | = | 128 |
Recall that cents are measured in a logarithmic scale with base the twelfth root of two. Therefore, our discrepancy, in hundreds of cents is:
After a little algebra, we see that this is equal to
This difference is so small that most people cannot hear it. (There are stories of violinists being particularly sensitive to such differences, however.)
Following up on the algebra of the preceding problem, we see that an interval corresponding to the ratio I equals 1200 log[2](I) cents. This will simplify the formulas given below for the other commas
There are other commas: The syntonic (or Didymic) comma is the difference between four perfect fifths and two octaves plus a major third. The syntonic comma occurs more easily than the Pythagorean comma or the schisma, since one doesn't need to go through particularly many chord progressions to move through four perfect fifths. The schisma is the difference between eight perfect fifths plus one major third and five octaves. The diaschisma is the difference between four perfect fifths plus two major thirds and three octaves . The computations are given below.
It should be clear at this point that most (indeed almost all) of the acoustic intervals will be imperfect in an equally tempered scale.
Let me come back to some of the details of even temperament after addressing another issue. Namely, why should we have twelve half steps in an octave anyway?
If I take logarithms base 2 of both sides of the troublesome equation, I am left with the equation
Of course, since log[2](2) = 1, the equation reduces to:
We then try to solve this for integer or rational values of x and y. Unfortunately, log[2](3) is not a rational number. The best we can do is to try to approximate it by a rational number. A decimal approximation is: 1.584962500721156181.
A good (and well-known) way to approximate an irrational number by a rational number is by continued fractions.
A continued fraction is an expression of the form:
where
are integers.
Using this form
(with only 1s in the numerators) means we will only be
considering simple continued fractions.
For notational convenience, write [a_0, a_1, a_2, ...] for the infinite continued fraction above. Of course, it is also possible to consider finite continued fractions. It is an exercise to see that any rational number can be expressed as a finite continued fraction. I refer you to Hardy and Wright's book for a discussion of the uniqueness of such an expression. If we cut off an infinite continued fraction after N terms, we have the Nth convergent. For the infinite continued fraction given above, this is
which is denoted 
.
This is obviously a
rational number, which we write (in reduced form) as
There is a convenient algorithm for computing the continued fraction expansion of a given number x, called the continued fraction algorithm. For any positive number A, let [A] denote the integer part of A. To compute a continued fraction expansion for x, take a_0 = [x]. So
and 0 <= x_0 < 1.
Now write
and so on.
, we write
.
=[1,2,2,2,...]
= 
with convergents: 
=[1,1,1,1,1,...]
= 
with convergents: 
.
That is to say, this particular continued fraction does indeed
converge to the irrational number it is supposed to represent.
.
=[3,7,15,1,292,1,1,1,2,...]
with convergents: 
.
is unknown.
There are two pertinent theorems (see Hardy and Wright):
Theorem 1 If x is an irrational number and n >=1, then
Since the denominator of the (n+1)st convergent is strictly larger than the denominator of the nth convergent (and they are all integers), we see that the continued fraction expansion does indeed converge to the irrational number it is meant to be approximating.
Theorem 2 If x is an irrational number, n>=1,
0 < q <= q_n, and 
with p, q integers, then
That is to say, the nth convergent is the fraction among all fractions whose denominator is no greater than q_n which provides the best approximation to x. It is common to use the size of the denominator as a measure of the `complexity' of the rational number. Thus, we have that the nth convergent is optimal for a given complexity.
What does this say for our musical problem?
Recall that the troublesome equation 2^x = 3^y is equivalent to the equation 2^x = 3, provided we use rationals, and not just integers. The obvious solution is x=log[2](3). We want to approximate this by a rational number.
The continued fraction expansion for log[2](3) is [1,1,1,2,2,3,1,5,2,23,2,2,1,...] (see sequence A028507 of the On-Line Encyclopedia of Integer Sequences for more terms). The first few convergents are:
| 1, | 2, | 3/2, | 8/5, | 19/12, | 65/41, | 84/53, | 485/306 |
Thus, taking the fourth approximation (start counting at zero):
That is to say, we obtain the perfect fifth, one octave up, by nineteen semitones. Moreover, the denominator, being twelve, forces us to have twelve semitones per octave. Thus, western music has adopted, quite by accident I assume, the fourth best approximation to a Pythagorean scale using equal temperament.
Obviously it is possible to have scales that come from dividing an octave into other than twelve pieces. For instance, the typical Chinese scale has five `notes' to the octave. Remarkably, this corresponds to the third convergent of the continued fraction expansion.
Going in the other direction, we could use the next more accurate continued fraction approximation of log[2](3), which would lead to an octave consisting of forty-one pieces. Below is a comparison of what happens to some standard intervals in these three systems. The fundamental interval for our standard twelve-tone chromatic scale is the semi-tone. There is no name for the basic intervals of our other chromatic scales. So we will refer to their basic intervals merely as `basic intervals'.
If we compute exactly in a twelve-tone scale, we find:
basic intervals (semitones).
basic intervals (semitones).
basic intervals (semitones).
If we used a five-tone scale, we would have:
basic intervals.
basic intervals.
basic intervals.
basic intervals.
If we used forty-one semitones per octave, we would have:
basic intervals.
basic intervals.
basic intervals.
basic intervals.
Interestingly, around 40 B.C., King Fang, in China, discovered the sixth best approximation given above. It is unlikely, of course, that he actually used continued fractions to do this, which makes it all the more remarkable. In particular, Fang noticed that fifty-three perfect fifths are very nearly equal to thirty-one octaves. This leads to what is sometimes called the Cycle of 53. It can be represented by a spiral of fifths, replacing the more usual circle of fifths.
The Pythagorean Hammers and Acoustics
Western music has adopted certain intervals as basic to
acoustics. It is not always the case that there the choices made are
more natural than their alternatives. The legend about the source of
some of these intervals involves Pythagoras. The story has him
listening to the sound of the hammers of four smiths, which he found
to be quite pleasant. Upon investigation, the hammers weighed 12, 9,
8, and 6 pounds. From these weights, Pythagoras derived the
intervals:
I don't know. Maybe. It's hard to say what really happened twenty-six centuries ago. But this certainly seems lucky. Maybe he was sitting in the same bath tub that Archimedes was sitting in four hundred years later.
In the present, we can look to see what might be natural intervals to construct. Firstly, the octave is quite natural, as a doubling of frequency. As usual, we will also take its inverse, halving of frequency, as equally natural. The next integral multiplication of frequency is tripling, which leads to the perfect fifth when combined with halving. Multiplying the frequency by four is just going up two octaves, so we already have that in our system.
The next natural operation is, then, to multiply the frequency by five. To remain in the original octave, we need to combine this with two halvings, leading to the interval of the major third.
Now it is not simply a preference for integers that leads to these
intervals. There is also the phenomenon of overtones. A
vibrating string has a fundamental tone, whose frequency f can be
calculated from its length L, mass m and tension
T according to a basic formula of acoustics:
But the
string also vibrates in other modes with less intensity. These other
modes are vibrations at integer multiples of the fundamental
frequency. The increasing sequence of such frequencies is called the
harmonic series based on the given fundamental. The
fundamental is called the first harmonic. The frequency of the
octave (twice that of the fundamental) is called the second
harmonic. The third harmonic is the perfect fifth one
octave up from the fundamental. And so it goes.
Thus, the argument for preferring
intervals based on doubling, tripling and multiplying by five is
actually based on acoustics, not just a fondness for the numbers 2, 3
and 5.
The phenomenon of overtones is an important factor in the quality of the sound of any particular instrument. Now, in theory, it may appear that the harmonic series for a particular fundamental frequency continues through all the integers. However, this would surely produce unbearable dissonance. What actually happens is that the intensity of the higher harmonics decreases quite rapidly. Indeed, on some instruments it is difficult to discern beyond the third harmonic. (My guitar, for instance.) Violins and oboes have strong higher harmonics, leading to a `bright' tone. Flutes and recorders have weak higher harmonics. Apparently the clarinet has strong odd-numbered harmonics, which is why has a `hollow' tone. Before valves were added to brass instruments, it was only notes corresponding to harmonics that could be played on these instruments.
After the intervals based on multiplying by two, three and five, our choices become more arbitrary.
For the sake of curiosity, we could investigate what we obtain using the major third as the basis for our computations. The acoustic major third is 5/4. Thus, the critical quantity is log[2](5/4) = log[2](5) - log[2](4).
Since log[2](4) is an integer, the crux of the approximation is that of log[2](5). The continued fraction expansion is [2, 3, 9, 2, 2, 4, 6, 2, 1, 1, 3, 1, 18] The convergents are: 7/3, 65/28, 137/59, 339/146, 1493/643, ...
Since three is certainly too few for an octave, we would have been stuck with octaves of twenty-eight notes!
Four perfect fifths correspond to
.
In the key of C, this is
.
Two octaves plus a major third correspond to
.
Using the logarithmic scale,
Eight perfect fifths plus one major third correspond to
.
Five octaves correspond to 
.
Using the logarithmic scale:
Mean-tone system
One alternative to equal temperament is
the mean-tone system, which seems to have begun around 1500. In
mean temperament, the fifth is 697 cents, as opposed to 700 cents in
equal temperament or 701.955 cents for the acoustically correct
interval. The mean-tone system for tuning a piano is satisfactory in
keys that have only one or two sharps or flats. But there are problems.
For instance, G#=772 cents and Ab=814. They ought to be the same! This
discrepancy is called the wolf. While the
Pythagorean comma, at 23.5 cents, is not discernible by most listeners,
the wolf, at 52 cents is quite noticeable.
Before equal temperament was widely accepted, keyboards had to accommodate these problems. One solution was only to play simple pieces in the keys your instrument could handle. A second solution, which was certainly necessary for large and important organs, was to have divided keyboards. Thus, the single key normally used today for G# and Ab would be split into two keys. Often, the back of one key would be slightly raised to improve the organist's ability to play by touch. The most extraordinary keyboard I was able to find a reference to was Bosanquet's `Generalized Keyboard Harmonium' built in 1876, which had 53 keys per octave!
Edward Dunne (egd@ams.org)