Now in lesson 4, let's turn to the Perception of Frequency which of course we call pitch in Empirical terms, can the discrepancy between the physics of the sound signal in terms of its frequency and the pitch that we hear be explained in empirical terms the same way that we saw to explain the difference between intensity and decibels and loudness. So here I'm going to use the phenomenon that we talked about before of the missing fundamental to provide some evidence of phenomena, it's long been a puzzle, can also be explained in terms of the empirical accumulation of information about what we expect to hear when we hear any harmonic series. So, in this diagram, and it is just a diagram, there are three harmonics of harmonic series that has a fundamental at 150 hertz. But that fundamental is missing so is the harmonic, rather, at 300 hertz. So the harmonics at 450, 600 and 750 are upper harmonics of a harmonic series that's expected to be beginning at 150 hertz. And here is the time signal of those three harmonics. Remember, this is a diagrammatic spectrum. This is the time signal that those three harmonics would generate. And the question that's being asked is, if you use the TIMIT database again the database that we talked about in the previous lesson to ask does the accumulated experience with the frequencies that present in the routine experience we have with natural sounds. Can you explain the phenomenally of the missing fundamental on the basis of that accumulated experience? And the answer is that you can, and what this represents is that each one of these little blue dots, and there are thousands of them here is the strongest periodicity in hertz when you cross correlate the time signal with a whole bunch of possible time signals that you experience in normal speech. So each blue dot is a correlation with this particular time signal that's being generated by this particular missing fundamental experience. When you cross correlate that, is the experience we have generated by natural sounds, speech sounds indicating what you expect to hear and therefore what you do hear based on that experience. So each one of these blue dots is a hundred millisecond snippet in which the frequencies in this time bearing symbol has been cross correlated. With all the possible frequencies, of signals that you hear in the timid database, in the normal experience that you have with speech. This is just a single line drawn over the variation here to make it a little bit clearer, but the frequency that is occurring, most often in this experience is indeed, in this accumulated experience, is indeed at 150 Hertz. So the argument would be that the reason you hear the missing fundamental when there's no energy at 150 Hertz and you're just exposed to these upper harmonics, but yet are hearing the fundamental. That's what we talked about before that the reason for that is that in your normal experience, that's the common occurrence, the most frequent occurrence of your collected experience over evolutionary and individual time. And you therefore hear the, you sort out the conflation of frequencies that we talked about in the earlier lesson here. You sort that out by using your experience and that accumulated experience to tell you what it is you should hear when that signal is prevalent. That's going to be the signal that's usefully, behaviorally to you, not the signal that there's physically.
