Note frequency calculation in Objective-C

NoteUtility.h

#import <Foundation/Foundation.h>

typedef enum : int {
    C = 0,
    D = 2,
    E = 4,
    F = 5,
    G = 7,
    A = 9,
    B = 11,
} Note;

typedef enum : int {
    None = 0,
    Sharp = 1,
    Flat = -1,
} Modifier;

@interface NoteUtility : NSObject

+ (double)frequencyForNote:(Note)note withModifier:(Modifier)modifier inOctave:(int)octave;

@end

NoteUtility.m

#import "NoteUtility.h"

@implementation NoteUtility

+ (double)frequencyForNote:(Note)note withModifier:(Modifier)modifier inOctave:(int)octave {
    int halfStepsFromA4 = note - A;
    halfStepsFromA4 += 12 * (octave - 4);
    halfStepsFromA4 += modifier;

    double frequencyOfA4 = 440.0;
    double a = 1.059463094359;

    return frequencyOfA4 * pow(a, halfStepsFromA4);
}

@end