First we need to understand what frequencies make up the sequence of tones to detect. The spectrogram tool from the previous page is the perfect tool for this analysis.
For example the image on the left is a spectrogram of me whistling up and down slowly. Read the graph starting from the bottom and going up so you see the slow rise and fall in frequencies as I whistle.
If I wanted to detect this sequence I just need to look for a series of strong intensities from the FFT output at the rising and falling frequencies of the whistle.
Taking things a step further, the image on the left represents a spectrogram of 5 notes being played from an instrument. Again, read the graph from the bottom up. Low frequencies are to the left and high frequencies are to the right.
From analyzing the spectrogram I can see the notes I want to detect are highest in intensity between these frequencies:
Here's a video below of the tone sequence detection with the answer. As you hear the tones played, look at the spectrogram to see how each note relates to output on the graph.
Continue on to look at another application of the Fourier transform, attempting to detect cat purrs.