refactor: Extract Frequency

This is to separate concerns: converting the positioned note to a frequency,
which could be done in different ways, e.g. equal tempered, pythagorean
(these methods belong to PositionedNote),
versus properties or functionality related to the frequency itself, such as
isHumanAudible or a hypothetical .play() method.

Closes #107

lib/music_notes.dart

@@ -20,6 +20,7 @@ part 'src/music.dart';
 part 'src/music_item.dart';
 part 'src/note/accidental.dart';
 part 'src/note/enharmonic_note.dart';
+part 'src/note/frequency.dart';
 part 'src/note/note.dart';
 part 'src/note/notes.dart';
 part 'src/note/positioned_note.dart';

lib/src/note/frequency.dart

@@ -0,0 +1,23 @@
+part of '../../music_notes.dart';
+
+/// Represents an absolute pitch, a physical frequency.
+class Frequency {
+  Frequency(this.hertz);
+
+  final double hertz;
+
+  /// Whether this [Frequency] is inside the human hearing range.
+  ///
+  /// Example:
+  /// ```dart
+  /// Note.a.inOctave(4).isHumanAudibleAt() == true
+  /// Note.d.inOctave(0).isHumanAudibleAt() == false
+  /// Note.g.inOctave(12).isHumanAudibleAt(442) == false
+  /// ```
+  bool get isHumanAudible {
+    const minFrequency = 20;
+    const maxFrequency = 20000;
+
+    return hertz >= minFrequency && hertz <= maxFrequency;
+  }
+}

lib/src/note/positioned_note.dart

@@ -82,24 +82,9 @@ final class PositionedNote extends Note {
   /// Note.a.inOctave(4).equalTemperamentFrequency(338) == 338
   /// Note.bFlat.inOctave(4).equalTemperamentFrequency(338) == 464.04
   /// ```
-  double equalTemperamentFrequency([double a4Hertzs = 440]) =>
-      a4Hertzs * math.pow(sqrt12_2, Note.a.inOctave(4).difference(this));
-
-  /// Whether this [Note] is inside the human hearing range at [a4Hertzs].
-  ///
-  /// Example:
-  /// ```dart
-  /// Note.a.inOctave(4).isHumanAudibleAt() == true
-  /// Note.d.inOctave(0).isHumanAudibleAt() == false
-  /// Note.g.inOctave(12).isHumanAudibleAt(442) == false
-  /// ```
-  bool isHumanAudibleAt([double a4Hertzs = 440]) {
-    final frequency = equalTemperamentFrequency(a4Hertzs);
-    const minFrequency = 20;
-    const maxFrequency = 20000;
-
-    return frequency >= minFrequency && frequency <= maxFrequency;
-  }
+  Frequency equalTemperamentFrequency([double a4Hertzs = 440]) => Frequency(
+        a4Hertzs * math.pow(sqrt12_2, Note.a.inOctave(4).difference(this)),
+      );
 
   /// Returns the string representation of this [Note] following the
   /// [scientific pitch notation](https://en.wikipedia.org/wiki/Scientific_pitch_notation).

test/src/note/frequency_test.dart

@@ -0,0 +1,14 @@
+import 'package:music_notes/music_notes.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('.isHumanAudible', () {
+    test('should return whether the frequency is audible by humans', () {
+      expect(Frequency(0).isHumanAudible, isFalse);
+      expect(Frequency(100).isHumanAudible, isTrue);
+      expect(Frequency(400).isHumanAudible, isTrue);
+      expect(Frequency(15000).isHumanAudible, isTrue);
+      expect(Frequency(100000).isHumanAudible, isFalse);
+    });
+  });
+}

test/src/note/positioned_note_test.dart

@@ -313,151 +313,141 @@ void main() {
     group('.equalTemperamentFrequency()', () {
       test('should return the hertzs of this PositionedNote from 440 Hz', () {
         expect(
-          Note.c.inOctave(4).equalTemperamentFrequency(),
+          Note.c.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(261.63, 0.01),
         );
         expect(
-          Note.cSharp.inOctave(4).equalTemperamentFrequency(),
+          Note.cSharp.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(277.18, 0.01),
         );
         expect(
-          Note.dFlat.inOctave(4).equalTemperamentFrequency(),
+          Note.dFlat.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(277.18, 0.01),
         );
         expect(
-          Note.d.inOctave(4).equalTemperamentFrequency(),
+          Note.d.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(293.66, 0.01),
         );
         expect(
-          Note.dSharp.inOctave(4).equalTemperamentFrequency(),
+          Note.dSharp.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(311.13, 0.01),
         );
         expect(
-          Note.eFlat.inOctave(4).equalTemperamentFrequency(),
+          Note.eFlat.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(311.13, 0.01),
         );
         expect(
-          Note.e.inOctave(4).equalTemperamentFrequency(),
+          Note.e.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(329.63, 0.01),
         );
         expect(
-          Note.f.inOctave(4).equalTemperamentFrequency(),
+          Note.f.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(349.23, 0.01),
         );
         expect(
-          Note.fSharp.inOctave(4).equalTemperamentFrequency(),
+          Note.fSharp.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(369.99, 0.01),
         );
         expect(
-          Note.gFlat.inOctave(4).equalTemperamentFrequency(),
+          Note.gFlat.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(369.99, 0.01),
         );
         expect(
-          Note.g.inOctave(4).equalTemperamentFrequency(),
+          Note.g.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(392, 0.01),
         );
         expect(
-          Note.gSharp.inOctave(4).equalTemperamentFrequency(),
+          Note.gSharp.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(415.3, 0.01),
         );
         expect(
-          Note.aFlat.inOctave(4).equalTemperamentFrequency(),
+          Note.aFlat.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(415.3, 0.01),
         );
-        expect(Note.a.inOctave(4).equalTemperamentFrequency(), 440);
+        expect(Note.a.inOctave(4).equalTemperamentFrequency().hertz, 440);
         expect(
-          Note.aSharp.inOctave(4).equalTemperamentFrequency(),
+          Note.aSharp.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(466.16, 0.01),
         );
         expect(
-          Note.bFlat.inOctave(4).equalTemperamentFrequency(),
+          Note.bFlat.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(466.16, 0.01),
         );
         expect(
-          Note.b.inOctave(4).equalTemperamentFrequency(),
+          Note.b.inOctave(4).equalTemperamentFrequency().hertz,
           closeTo(493.88, 0.01),
         );
       });
 
       test('should return the hertzs of this PositionedNote from 438 Hz', () {
         expect(
-          Note.c.inOctave(4).equalTemperamentFrequency(438),
+          Note.c.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(260.44, 0.01),
         );
         expect(
-          Note.cSharp.inOctave(4).equalTemperamentFrequency(438),
+          Note.cSharp.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(275.92, 0.01),
         );
         expect(
-          Note.dFlat.inOctave(4).equalTemperamentFrequency(438),
+          Note.dFlat.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(275.92, 0.01),
         );
         expect(
-          Note.d.inOctave(4).equalTemperamentFrequency(438),
+          Note.d.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(292.33, 0.01),
         );
         expect(
-          Note.dSharp.inOctave(4).equalTemperamentFrequency(438),
+          Note.dSharp.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(309.71, 0.01),
         );
         expect(
-          Note.eFlat.inOctave(4).equalTemperamentFrequency(438),
+          Note.eFlat.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(309.71, 0.01),
         );
         expect(
-          Note.e.inOctave(4).equalTemperamentFrequency(438),
+          Note.e.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(328.13, 0.01),
         );
         expect(
-          Note.f.inOctave(4).equalTemperamentFrequency(438),
+          Note.f.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(347.64, 0.01),
         );
         expect(
-          Note.fSharp.inOctave(4).equalTemperamentFrequency(438),
+          Note.fSharp.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(368.31, 0.01),
         );
         expect(
-          Note.gFlat.inOctave(4).equalTemperamentFrequency(438),
+          Note.gFlat.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(368.31, 0.01),
         );
         expect(
-          Note.g.inOctave(4).equalTemperamentFrequency(438),
+          Note.g.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(390.21, 0.01),
         );
         expect(
-          Note.gSharp.inOctave(4).equalTemperamentFrequency(438),
+          Note.gSharp.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(413.42, 0.01),
         );
         expect(
-          Note.aFlat.inOctave(4).equalTemperamentFrequency(438),
+          Note.aFlat.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(413.42, 0.01),
         );
-        expect(Note.a.inOctave(4).equalTemperamentFrequency(438), 438);
+        expect(Note.a.inOctave(4).equalTemperamentFrequency(438).hertz, 438);
         expect(
-          Note.aSharp.inOctave(4).equalTemperamentFrequency(438),
+          Note.aSharp.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(464.04, 0.01),
         );
         expect(
-          Note.bFlat.inOctave(4).equalTemperamentFrequency(438),
+          Note.bFlat.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(464.04, 0.01),
         );
         expect(
-          Note.b.inOctave(4).equalTemperamentFrequency(438),
+          Note.b.inOctave(4).equalTemperamentFrequency(438).hertz,
           closeTo(491.64, 0.01),
         );
       });
     });
 
-    group('.isHumanAudibleAt()', () {
-      test('should return whether this PositionedNote is human-audible', () {
-        expect(Note.c.inOctave(4).isHumanAudibleAt(338), isTrue);
-        expect(Note.a.inOctave(2).isHumanAudibleAt(442), isTrue);
-        expect(Note.d.inOctave(0).isHumanAudibleAt(), isFalse);
-        expect(Note.d.inOctave(11).isHumanAudibleAt(220), isTrue);
-        expect(Note.d.inOctave(12).isHumanAudibleAt(), isFalse);
-      });
-    });
-
     group('.scientificName', () {
       test(
         'should return the scientific pitch notation name for this '