diff --git a/lib/util/notefreq.go b/lib/util/notefreq.go
new file mode 100644
index 0000000..9f85cd0
--- /dev/null
+++ b/lib/util/notefreq.go
@@ -0,0 +1,8 @@
+package util
+
+import "math"
+
+// Return the frequency of a given MIDI note number.
+func NoteFreq(n int) float64 {
+	return (math.Pow(2, (float64(n)-69)/12)) * 440
+}
diff --git a/lib/util/notefreq_test.go b/lib/util/notefreq_test.go
new file mode 100644
index 0000000..8e3bac6
--- /dev/null
+++ b/lib/util/notefreq_test.go
@@ -0,0 +1,31 @@
+package util
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestNoteFreq(t *testing.T) {
+	type TestCase struct {
+		In  int
+		Out float64
+	}
+
+	testCases := []TestCase{
+		{108, 4186.01},
+		{81, 880},
+		{69, 440},
+		{58, 233.08},
+		{57, 220},
+		{56, 207.65},
+		{55, 196},
+		{36, 65.41},
+	}
+
+	for _, tc := range testCases {
+		out := NoteFreq(tc.In)
+		if fmt.Sprintf("%.2f", out) != fmt.Sprintf("%.2f", tc.Out) {
+			t.Fatalf("%v != %v", out, tc.Out)
+		}
+	}
+}