# 7 Etudes, Op. 2: Musical Math

Example 7-1. Converting metric proportion into time in seconds.

```(define (bpm->seconds bpm)
(/ 60.0 bpm))

(define (rhythm->seconds rhy tempo)
(* rhy 4.0 (bpm->seconds tempo)))
```

Interaction 7-1. Using the rhythm->seconds function.

```cm> (rhythm->seconds 1/4 60)
1.0
cm> (rhythm->seconds 1/2 120)
1.0
cm> (rhythm->seconds 1/8 90)
0.3333333333333333
cm>```

Interaction 7-2. Calculating octaves as powers of 2.

```cm> (expt 2 -2)
.25
cm> (expt 2 -1)
.5
cm> (expt 2 0)
1
cm> (expt 2 1)
2
cm> (expt 2 2)
4
cm>```

Interaction 7-3. Exponentiation.

```cm> (expt 2 0)
1
cm> (expt 2 1)
2
cm> (expt 2 1/2)
1.4142135623730951
cm> (expt 2 1/12)
1.0594630943592953
cm> (expt 2 -3/24)
0.9170040432046712
cm>```

Example 7-2. Scaling Hertz frequency values by equal tempered amounts.

```(define (scale-hz hz mul)
(* hz (expt 2 mul)))
```

Interaction 7-4. Calculating Hertz values with scale-hz.

```cm> (scale-hz 440 1)
880
cm> (scale-hz 440 -2)
110
cm> (scale-hz 440 1/24)
452.8929841231365
cm>```

Example 7-3. Converting MIDI key numbers to Hertz frequency.

```(define lowest-freq 8.175)

(define (keynum->hertz knum)
(* lowest-freq (expt 2 (/ knum 12))))
```

Interaction 7-5. Converting MIDI key numbers to Hertz equivalents.

```cm> (keynum->hertz 60)
261.6255653005986
cm> (keynum->hertz 69)
439.99999999999994
cm>```

Example 7-4. A version of keynum->hertz based on A.

```(define lowest-freq 6.875)

(define (keynum->hertz knum)
(* lowest-freq (expt 2 (/ (+ knum 3) 12))))
```

Interaction 7-6. Converting MIDI key numbers to Hertz equivalents.

```cm> (keynum->hertz 60)
261.6255653005986
cm> (keynum->hertz 69)
440.0
cm>```

Example 7-5. Random selection between a low and high bounds.

```(define (pick-range low high)
(let ((rng (- high low)))
(if (> rng 0)
(+ low (random rng))
0)))
```

Interaction 7-7. Examples of pick-range.

```cm> (pick-range 60 100)
96
cm> (pick-range -1.0 1.0)
-0.697347603174574
cm> (pick-range 440 880)
789
cm>```

Example 7-6. Random selection from a list of values.

```(define (pick-list lst)
(let* ((len (length lst))
(pos (random len)))
(list-ref lst pos)))
```

Interaction 7-8. Examples of pick-list.

```cm> (define aeolian-mode
'(57 59 60 62 64 65 67
69 71 72 74 3 77 79 81))

cm> (pick-list aeolian-mode)
62
cm> (pick-list aeolian-mode)
79
cm> (pick-list aeolian-mode)
71
cm> (pick-list '(1/16 1/8 1/8 1/4))
1/4
cm>```

Example 7-7. Random selection from a list of values.

```(define (chance? prob)
(< (random 1.0) prob))
```

Interaction 7-9. Examples of chance?

```cm> (chance? 0)
#f
cm> (chance? 1)
#t
cm> (chance? .75)
#t
cm> (if (chance? .8) 'retrograde 'prime)
retrograde
cm> (if (chance? 0) 'retrograde 'prime)
prime
cm>```

## Chapter Source Code

The source code to all of the examples and interactions in this chapter can be found in the file math.cm located in the same directory as the HTML file for this chapter. The source file can be edited in a text editor or evaluated inside the Common Music application.