在iphone中播放与频率和分贝相关的哔声

#import <AudioToolbox/AudioToolbox.h>
#define TONE_SAMPLERATE 44100.

@interface Tone : NSObject {
    AudioQueueRef queue;
    AudioQueueBufferRef buffer;
    BOOL rebuildBuffer;
}
@property (nonatomic, assign) NSUInteger frequency;
@property (nonatomic, assign) CGFloat dB;

- (void)play;
- (void)pause;
@end


@implementation Tone
@synthesize dB=_dB,frequency=_frequency;

void handleBuffer(void *inUserData,
                  AudioQueueRef inAQ,
                  AudioQueueBufferRef inBuffer);

#pragma mark - Initialization and deallocation -

- (id)init
{
    if ((self=[super init])) {

        _dB=0.;
        _frequency=440;
        rebuildBuffer=YES;

        // TO DO: handle AudioQueueXYZ's failures!!

        // create a descriptor containing a LPCM, mono, float format
        AudioStreamBasicDescription desc;

        desc.mSampleRate=TONE_SAMPLERATE;
        desc.mFormatID=kAudioFormatLinearPCM;
        desc.mFormatFlags=kLinearPCMFormatFlagIsFloat;
        desc.mBytesPerPacket=sizeof(float);
        desc.mFramesPerPacket=1;
        desc.mBytesPerFrame=sizeof(float);
        desc.mChannelsPerFrame=1;
        desc.mBitsPerChannel=8*sizeof(float);

        // create a new queue
        AudioQueueNewOutput(&desc,
                            &handleBuffer,
                            self,
                            CFRunLoopGetCurrent(),
                            kCFRunLoopCommonModes,
                            0,
                            &queue);

        // and its buffer, ready to hold 1" of data
        AudioQueueAllocateBuffer(queue,
                                 sizeof(float)*TONE_SAMPLERATE,
                                 &buffer);

        // create the buffer and enqueue it
        handleBuffer(self, queue, buffer);

    }
    return self;
}

- (void)dealloc
{
    AudioQueueStop(queue, YES);
    AudioQueueFreeBuffer(queue, buffer);
    AudioQueueDispose(queue, YES);

    [super dealloc];
}

#pragma mark - Main function -

void handleBuffer(void *inUserData,
                AudioQueueRef inAQ,
                AudioQueueBufferRef inBuffer) {

    // this function takes care of building the buffer and enqueuing it.

    // cast inUserData type to Tone
    Tone *tone=(Tone *)inUserData;

    // check if the buffer must be rebuilt
    if (tone->rebuildBuffer) {

        // precompute some useful qtys
        float *data=inBuffer->mAudioData;
        NSUInteger max=inBuffer->mAudioDataBytesCapacity/sizeof(float);

        // multiplying the argument by 2pi changes the period of the cosine
        //  function to 1s (instead of 2pi). then we must divide by the sample
        //  rate to get TONE_SAMPLERATE samples in one period.
        CGFloat unit=2.*M_PI/TONE_SAMPLERATE;
        // this is the amplitude converted from dB to a linear scale
        CGFloat amplitude=pow(10., tone.dB*.05);

        // loop and simply set data[i] to the value of cos(...)
        for (NSUInteger i=0; i<max; ++i)
            data[i]=(float)(amplitude*cos(unit*(CGFloat)(tone.frequency*i)));

        // inform the queue that we have filled the buffer
        inBuffer->mAudioDataByteSize=sizeof(float)*max;

        // and set flag
        tone->rebuildBuffer=NO;
    }

    // reenqueue the buffer
    AudioQueueEnqueueBuffer(inAQ,
                            inBuffer,
                            0,
                            NULL);

    /* TO DO: the transition between two adjacent buffers (the same one actually)
              generates a "tick", even if the adjacent buffers represent a continuous signal.
              maybe using two buffers instead of one would fix it.
     */
}

#pragma - Properties and methods -

- (void)play
{
    // generate an AudioTimeStamp with "0" simply!
    //  (copied from FillOutAudioTimeStampWithSampleTime)

    AudioTimeStamp time;

    time.mSampleTime=0.;
    time.mRateScalar=0.;
    time.mWordClockTime=0.;
    memset(&time.mSMPTETime, 0, sizeof(SMPTETime));
    time.mFlags = kAudioTimeStampSampleTimeValid;

    // TO DO: maybe it could be useful to check AudioQueueStart's return value
    AudioQueueStart(queue, &time);
}

- (void)pause
{
    // TO DO: maybe it could be useful to check AudioQueuePause's return value
    AudioQueuePause(queue);
}

- (void)setFrequency:(NSUInteger)frequency
{
    if (_frequency!=frequency) {
        _frequency=frequency;

        // we need to update the buffer (as soon as it stops playing)
        rebuildBuffer=YES;
    }
}

- (void)setDB:(CGFloat)dB
{
    if (dB!=_dB) {
        _dB=dB;

        // we need to update the buffer (as soon as it stops playing)
        rebuildBuffer=YES;
    }
}

@end

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该类生成以给定整数频率振荡的 cos 波形（振幅*cos(2pi*频率*t)）；整个工作是通过void handleBuffer(...)使用具有线性 PCM、单声道、float @44.1kHz 格式的 AudioQueue 来完成的。为了改变信号形状，您只需更改该线即可。例如，以下代码将产生方波：
```
float x = fmodf(unit*(CGFloat)(tone.frequency*i), 2 * M_PI);
data[i] = amplitude * (x > M_PI ? -1.0 : 1.0);
```
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对于浮点频率，您应该考虑到一秒钟的音频数据不一定有整数次振荡，因此所表示的信号在两个缓冲区之间的连接处是不连续的，并产生奇怪的“滴答声”。例如，您可以设置较少的样本，以便连接点位于信号周期的末尾。
正如 Paul R 指出的那样，您应该首先校准硬件，以便在实现中设置的值与设备产生的声音之间获得可靠的转换。实际上，此代码中生成的浮点样本范围从 -1 到 1，因此我只是将幅度值转换为 dB ( 20*log_10(amplitude) )。
查看有关实施中其他细节和“已知限制”（所有这些“待办事项”）的评论。Apple 在其参考资料中详细记录了所使用的函数。

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