refactor: Extract Frequency

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 +1,24 @@
+part of '../../music_notes.dart';
+
+/// Represents an absolute pitch, a physical frequency.
+@immutable
+class Frequency {
+  const Frequency(this.hertz);
+
+  final double hertz;
+
+  /// Whether this [Frequency] is inside the human hearing range.
+  ///
+  /// Example:
+  /// ```dart
+  /// const Frequency(880).isHumanAudible == true
+  /// Note.a.inOctave(4).equalTemperamentFrequency().isHumanAudible == true
+  /// Note.g.inOctave(12).equalTemperamentFrequency(442).isHumanAudible == false
+  /// ```
+  bool get isHumanAudible {
+    const minFrequency = 20;
+    const maxFrequency = 20000;
+
+    return hertz >= minFrequency && hertz <= maxFrequency;
+  }
+}

lib/src/note/positioned_note.dart

@@ -71,35 +71,22 @@ final class PositionedNote extends Note {
     );
   }
 
-  /// Returns the equal temperament frequency in Hertzs of this [PositionedNote]
-  /// from the A4 note reference.
+  /// Returns the equal temperament [Frequency] of this [PositionedNote] from
+  /// the A4 note reference.
   ///
   /// Example:
   /// ```dart
-  /// Note.a.inOctave(4).equalTemperamentFrequency() == 440
-  /// Note.gSharp.inOctave(4).equalTemperamentFrequency() == 415.3
-  /// Note.c.inOctave(4).equalTemperamentFrequency() == 261.63
-  /// Note.a.inOctave(4).equalTemperamentFrequency(338) == 338
-  /// Note.bFlat.inOctave(4).equalTemperamentFrequency(338) == 464.04
+  /// Note.a.inOctave(4).equalTemperamentFrequency() == const Frequency(440)
+  /// Note.gSharp.inOctave(4).equalTemperamentFrequency()
+  ///   == const Frequency(415.3)
+  /// Note.c.inOctave(4).equalTemperamentFrequency() == const Frequency(261.63)
+  /// Note.a.inOctave(4).equalTemperamentFrequency(338) == const Frequency(338)
+  /// Note.bFlat.inOctave(4).equalTemperamentFrequency(338)
+  ///   == const Frequency(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 a4Hertz = 440]) => Frequency(
+        a4Hertz * 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/main.dart

@@ -7,6 +7,7 @@ import 'interval/quality_test.dart' as quality_test;
 import 'music_test.dart' as music_test;
 import 'note/accidental_test.dart' as accidental_test;
 import 'note/enharmonic_note_test.dart' as enharmonic_note_test;
+import 'note/frequency_test.dart' as frequency_test;
 import 'note/note_test.dart' as note_test;
 import 'note/notes_test.dart' as notes_test;
 import 'note/positioned_note_test.dart' as positioned_note_test;
@@ -24,6 +25,7 @@ void main() {
   music_test.main();
   accidental_test.main();
   enharmonic_note_test.main();
+  frequency_test.main();
   note_test.main();
   notes_test.main();
   positioned_note_test.main();

test/src/note/frequency_test.dart

@@ -0,0 +1,16 @@
+import 'package:music_notes/music_notes.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Frequency', () {
+    group('.isHumanAudible', () {
+      test('should return whether the frequency is audible by humans', () {
+        expect(const Frequency(0).isHumanAudible, isFalse);
+        expect(const Frequency(100).isHumanAudible, isTrue);
+        expect(const Frequency(400).isHumanAudible, isTrue);
+        expect(const Frequency(15000).isHumanAudible, isTrue);
+        expect(const Frequency(100000).isHumanAudible, isFalse);
+      });
+    });
+  });
+}