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音楽の中の数学
2020/2/1 日曜数学会
リュウ
@ryu_paccho
弦長と振動数、(自然)倍音と和音
1
1
2
2
2倍音
1オクターブ
上のド
1
3
3
3倍音
1オクターブ
上のソ
1
5
5
5倍音
2オクターブ
上のミ
2
3
4
5
3
2
5
4
(3倍音) (5倍音)
ソ ミ
弦長(比) 1
振動数
(比)
1
倍音(名) 1倍音
音階 ド
基準
2倍
4倍
長三和音
2
音律
ざっくり言うと・・・
「無数に(連続的に)ある音の中から、
ドレミ…の音階に使う音を選ぶ決め方」
「弦の長さを2分の1、3分の2、5分の4に
した音を使おう。」
3
オクターブ(2倍音)の間隔を均等分割した音律
平均律
 合理化の結晶。転調や多様な和音使用に強い。
 その代わり、和音(オクターブ以外)は必ず濁る。
1オクターブ内の音の数(白鍵と黒鍵の数。半音階)=12音
振動数
(比)
1 2 2 2 … 2 2
音階名 ド ド♯ レ レ♯ … シ ド
4
平均律の楽器:ピアノ・ハープの曲線
弦の並び(長さ)の曲線
⇒指数関数の曲線
5
平均律の半音階ハープの作り方(GeoGebra)
6
オクターブ(2倍音)の間隔と
3倍音(ドに対するソ)との響きを重視した音律
ピタゴラス音律
 ピタゴラス(学派)が構築したと言われる。
 基準音(1倍音)から逐次3倍音を作り、オクターブ内の音を追加して
いく(適宜2で割ってにしてオクターブ内に調整する)。
 問題点
 いつまでも2倍音を作れず、オクターブ内に無数の音を作れる。
3
2
2(m、nの自然数解が存在しない)
⇒m=12で近似を取る(これによりオクターブ内の音が12音になる)。
 基本の3和音(ドミソ)はきれいに聞こえない。
(5倍音のミが調和しない)
音階名 ド
振動数
(比)
1
ソ
3
2
レ
9
4
ラ …
27
16
…
ファ ド?
2?
<2に調整 3
2
7
オクターブ(2倍音)の間隔と、3倍音・5倍音
(ドに対するソとミ)との響きを重視した音律
 (5-limitの)純正律
3倍音、5倍音の合成表
 各種和音や音階を表の中で観察できる。
 その他の音律や純正律自身の不都合の分析もできる。
×
1
9
1
3
1 3 9
25
25
18
25
24
25
16
75
64
225
128
5
10
9
5
3
5
4
15
8
45
32
1
16
9
4
3
1
3
2
9
8
1
5
64
45
16
15
8
5
6
5
9
5
1
25
256
225
128
75
32
25
48
25
36
25
×
1
9
1
3
1 3 9
25 ファ♯ ド♯ ソ♯ レ♯ ラ♯
5 レ ラ ミ シ ファ♯
1 シ♭ ファ ド ソ レ
1
5
ソ♭ レ♭ ラ♭ ミ♭ シ♭
1
25
ミ♭♭ シ♭♭ ファ♭ ド♭ ソ♭
※値は1オクターブ内(1<f<2)となるよう2^nで調整済み。
振動数比表示 音階名表示
長三和音長三和音
⇒ 「オイラー音律」とも呼ばれる
⇒「オイラー格子」
8
オイラー格子(振動数比)の空間表現
オイラー純正律の振動数比
⇒「3 ・5 ・2 」で表される。
⇒素数指数表現< >で各音階を表せる。
※ は整数の範囲を動く。
空間にプロット z軸(オクターブ調整の 2 )を無視
y軸(5倍音)を無視
(拡張ピタゴラス音律)
9
オイラー純正律の特徴
素数を有限個に制限した
有理数の部分集合で表現される。
自然数のような和(後者関数、+1)に
よってではなく、
素数の積(比)で形成される。
これらはまだ、オイラー音律の音一つ一つの特徴。
さらに和音や旋律を和や積で表せば、
少しゼータっぽさ(オイラー積?)も見えてくる…?
指数/対数の関係以上に何か対応を作れないか?
10
今後の検討:音楽の数学化
音楽の三要素
和音
<ハーモニー>
旋律
<メロディ>
律動
<リズム>
音の高さの移り変わり
=積(比)による変化
⇒乗法的?
音・波の合成
⇒加法的?
音の有無の時間的変化
⇒音律外の追加パラメータt?
積と和の合成
⇒IUT理論??
和声進行
11
最後に
モノの形を数学するように
オトの繋がりを数学にできないか?
代数幾何、数論幾何、位相幾何・・・
代数音学、数論音学、位相音学・・・?
「音楽の中の数学」から、「数学の中の音楽」へ
12
参考文献
 音律と音階の科学 小方 厚(講談社 2007)
 数字と科学から読む音楽 西原 稔 安生 健(YAMAHA 2020)
 ピアノの音響学 日本音響学会(コロナ社 2014)
 周波数比の素数指数表現に基づく調性理解モデルとその応用可能
性の検討 白松 俊, 大囿 忠親, 新谷 虎松
http://www.osaka-kyoiku.ac.jp/~challeng/SIG-Challenge-
B303/B303-07.pdf
 クラシカル・ギターを止められない
https://classical-guitar.blog.ss-blog.jp/
 ミーントーン大好き!~音程は自分で作ろう~
https://meantone.blog.ss-blog.jp/
 Music Essay - 音楽ひとりごと
http://www.basso-continuo.com/Musik/Dokuhaku-j.htm
 Huygens-Fokker Foundation centre for microtonal music
http://www.huygens-fokker.org/index_en.html
 浜松市楽器博物館(写真)
13

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音楽の中の数学@20200201日曜数学会