teensy-audio/examples/Analysis/FFT/FFT.ino

#include <Audio.h>
#include <Wire.h>
#include <SPI.h>
#include <SD.h>

const int myInput = AUDIO_INPUT_LINEIN;
//const int myInput = AUDIO_INPUT_MIC;

// Create the Audio components.  These should be created in the
// order data flows, inputs/sources -> processing -> outputs
//
AudioInputI2S       audioInput;         // audio shield: mic or line-in
AudioAnalyzeFFT256  myFFT(20);
AudioOutputI2S      audioOutput;        // audio shield: headphones & line-out

// Create Audio connections between the components
//
AudioConnection c1(audioInput, 0, audioOutput, 0);
AudioConnection c2(audioInput, 0, myFFT, 0);
AudioConnection c3(audioInput, 1, audioOutput, 1);

// Create an object to control the audio shield.
// 
AudioControlSGTL5000 audioShield;


void setup() {
  // Audio connections require memory to work.  For more
  // detailed information, see the MemoryAndCpuUsage example
  AudioMemory(12);

  // Enable the audio shield and set the output volume.
  audioShield.enable();
  audioShield.inputSelect(myInput);
  audioShield.volume(0.6);
}

void loop() {
  if (myFFT.available()) {
    // each time new FFT data is available
    // print it all to the Arduino Serial Monitor
    Serial.print("FFT: ");
    for (int i=0; i<128; i++) {
      Serial.print(myFFT.output[i]);
      Serial.print(",");
    }
    Serial.println();
  }
}