How?

dsp.mm

// MULTIPLE SIGNAL GENERATOR!
__block float *phases = nil;
[audioManager setOutputBlock:^(float *data, UInt32 numFrames, UInt32 numChannels)
{
   float samplingRate = audioManager.samplingRate;
   NSUInteger activeSignalCount = [tones count];
   
   // Initialize phases
   if (phases == nil) {
       phases = new float[10];
       for(int z = 0; z <= 10; z++) {
           phases[z] = 0.0;
       }
   }
   
   // Multiple signals
   NSEnumerator * enumerator = [tones objectEnumerator];
   id frequency;
   UInt32 c = 0;
   while(frequency = [enumerator nextObject])
   {
       for (int i=0; i < numFrames; ++i)
       {
           for (int iChannel = 0; iChannel < numChannels; ++iChannel) 
           {
               float theta = phases[c] * M_PI * 2;
               if (c == 0) {
                   data[i*numChannels + iChannel] = sin(theta);
               } else {
                   data[i*numChannels + iChannel] = data[i*numChannels + iChannel] + sin(theta);
               }
           }
           phases[c] += 1.0 / (samplingRate / [frequency floatValue]);
           if (phases[c] > 1.0) phases[c] = -1;
       }
       c++;
   }
   
   // Normalize data with active signal count
   float signalMulti = 1.0 / (float(activeSignalCount) * (sqrt(2.0)));
   vDSP_vsmul(data, 1, &signalMulti, data, 1, numFrames*numChannels);
   
   
   // Apply master volume
   float volume = masterVolumeSlider.value;
   vDSP_vsmul(data, 1, &volume, data, 1, numFrames*numChannels);
   
   
   if (fxSwitch.isOn) {
       // FILTER
       // 10 bands EQ
       // vDSP_deq22(<#float *__vDSP_A#>, <#vDSP_Stride __vDSP_I#>, <#float *__vDSP_B#>, <#float *__vDSP_C#>, <#vDSP_Stride __vDSP_K#>, <#vDSP_Length __vDSP_N#>)
   }
   
   // Measure dB
   [self measureDB:data:numFrames:numChannels];
}];