Note frequency calculation in Swift

NoteFrequency.swift

enum Note: Int {
    case C = 0
    case D = 2
    case E = 4
    case F = 5
    case G = 7
    case A = 9
    case B = 11
}

enum Modifier: Int {
    case Sharp = 1
    case Flat = -1
}

func frequencyForNote(note: Note, modifier: Modifier?, octave: Int) -> Double {
    var halfStepsFromA4: Int = note.rawValue - Note.A.rawValue
    halfStepsFromA4 += 12 * (octave - 4)

    if let modifier = modifier {
        halfStepsFromA4 += modifier.rawValue
    }

    let frequencyOfA4 = 440.0
    let a = 1.059463094359

    return frequencyOfA4 * pow(a, Double(halfStepsFromA4))
}