iOSで特定の周波数の音を鳴らす方法

指定した周波数の音を鳴らすためのコードです。
ご参考までに。
AudioToolBox.frameworkを追加する必要があります。

```objc:ViewController.h
#import <UIKit/UIKit.h>
#import <AudioToolbox/AudioToolbox.h>

@interface ViewController : UIViewController{
    AudioUnit aU;
    UInt32 bitRate;
}

@property (nonatomic) double phase;
@property (nonatomic) Float64 sampleRate;
@property (nonatomic) Float64 frequency;

static OSStatus renderer(void *inRef,
                         AudioUnitRenderActionFlags *ioActionFlags,
                         const AudioTimeStamp* inTimeStamp,
                         UInt32 inBusNumber,
                         UInt32 inNumberFrames,
                         AudioBufferList *ioData);

@end

```

```objc:ViewController.m
-(void)playSound{
    
    //Sampling rate
    _sampleRate = 44100.0f;
    
    //Bit rate
    bitRate = 8;  // 8bit
    
    //周波数（音程）
    _frequency = 261.62;
    
    //AudioComponentDescription
    AudioComponentDescription aCD;
    aCD.componentType = kAudioUnitType_Output;
    aCD.componentSubType = kAudioUnitSubType_RemoteIO;
    aCD.componentManufacturer = kAudioUnitManufacturer_Apple;
    aCD.componentFlags = 0;
    aCD.componentFlagsMask = 0;
    
    //AudioComponent
    AudioComponent aC = AudioComponentFindNext(NULL, &aCD);
    AudioComponentInstanceNew(aC, &aU);
    AudioUnitInitialize(aU);
    
    //コールバック
    AURenderCallbackStruct callbackStruct;
    callbackStruct.inputProc = renderer;
    callbackStruct.inputProcRefCon = (__bridge void*)self;
    AudioUnitSetProperty(aU,
                         kAudioUnitProperty_SetRenderCallback,
                         kAudioUnitScope_Input,
                         0,
                         &callbackStruct,
                         sizeof(AURenderCallbackStruct));
    
    //AudioStreamBasicDescription
    AudioStreamBasicDescription aSBD;
    aSBD.mSampleRate = _sampleRate;
    aSBD.mFormatID = kAudioFormatLinearPCM;
    aSBD.mFormatFlags = kAudioFormatFlagsAudioUnitCanonical;
    aSBD.mChannelsPerFrame = 2;
    aSBD.mBytesPerPacket = sizeof(AudioUnitSampleType);
    aSBD.mBytesPerFrame = sizeof(AudioUnitSampleType);
    aSBD.mFramesPerPacket = 1;
    aSBD.mBitsPerChannel = bitRate * sizeof(AudioUnitSampleType);
    aSBD.mReserved = 0;
    
    //AudioUnit
    AudioUnitSetProperty(aU,
                         kAudioUnitProperty_StreamFormat,
                         kAudioUnitScope_Input,
                         0,
                         &aSBD,
                         sizeof(aSBD));
    
    //再生
    AudioOutputUnitStart(aU);
    
}

static OSStatus renderer(void *inRef,
                         AudioUnitRenderActionFlags *ioActionFlags,
                         const AudioTimeStamp* inTimeStamp,
                         UInt32 inBusNumber,
                         UInt32 inNumberFrames,
                         AudioBufferList *ioData) {
    
    //キャスト
    ViewController* def = (__bridge ViewController*)inRef;
    
    //サイン波
    float freq = def.frequency*2.0*M_PI/def.sampleRate;
    
    //値を書き込むポインタ
    AudioUnitSampleType *outL = ioData->mBuffers[0].mData;
    AudioUnitSampleType *outR = ioData->mBuffers[1].mData;
    
    for (int i = 0; i < inNumberFrames; i++) {
        // 周波数を計算
        float wave = sin(def.phase);
        AudioUnitSampleType sample = wave * (1 << kAudioUnitSampleFractionBits);
        *outL++ = sample;
        *outR++ = sample;
        def.phase += freq;
    }
    
    return noErr;
    
};
```