So the last lesson in this first module concerns determining the sources of sound stimuli and the huge and fundamental problem that lurks in what would otherwise seem to be a pretty straightforward issue. This diagram is one that you may wanna come back to and spend a little time and thinking about. And again it uses a plug string, as an example of the kinds of sounds that we're gonna be concerned with, not just in thinking about the sounds that we evolve to hear coming from human and other animal vocalizations but from the sounds that are generated by musical strings or columns of air that we're gonna be discussing quite a lot. The problem is how does the listener indicated here and the sound signal that's reaching the listener's ear. So this is a diagram of the time signal that's reaching the listener's ear. How does the listener know what that sound signal is in terms of its physical components which are what we need to know to behave usefully in the world in relation to a sound signal that's translated into a sound stimulus? So, consider the vibrating string. So, what I described to you before, the harmonic series that comes from a vibrating string. You think, well okay, that reaches the ear. That's all that we need to be concerned with, but it's not all that we need to be concerned with. Here is the stimulus. Our transduction of the sound signal at the listener's ear into a stimulus that's going to be useful to us in our behavior. That signal comprises the mechanical force that's acting on the string, the force with which we pluck the string with our pick or whatever it might be or bow the string. The resident properties of the string. What's it made out of? Is it an E string, a C string on the guitar for example? Is it properly tuned? We need to know those things to act appropriately. And then we have the effects of the environment. And all of these things that we need to understand to behave appropriately in the world. The mechanical force that's acting on the string, the resonant properties of the string, the effect of the environment on the string. All of those comprise a stimulus when it's transduced but they're all conflated in the stimulus. All of these things are tied together in the stimulus and we need to know them. We need to separate them to know what's going on. I should mention that the effect of the environment, by the way, is highly significant in this. So here's the sound waves right at the locus of a plucked string. But those are being reflected off whatever objects are in the environment. Walls for example, reflecting surfaces, and they’re being diminished and altered by the distance of the listener from the sound of the source. So again all of these factors, mechanical force, resonant properties, effect of the environment, the reflective surfaces that may be altering the sound signal that reaches the ear. They're all conflated. They're all mixed together and we somehow have to disambiguate that conflation. We have to pull that apart in order to behave usefully. And a huge problem not just in thinking about audition, but in thinking about other sensory this is a very big and obvious problem in vision, where we have to get back to a world with conflated information at the level of the retina. How do sensory systems do this? This is a huge problem. Conceptually, it's a huge problem in terms of biology. How does a listener evolve that and disambiguate all of these factors that are conflated at the listener's ear in these sound pressure disturbances that are illustrated here in this complex wave. So to sum that up, the question is, how does the auditory system provide us the information we need to be have correctly given this conflation? And the answer is that we really don't know. And I'll just give you a preview of the argument that's going to be thematic as we go along in the course. So this series of points is kind of a guide to the kinds of things that we're going to be considering about, emphasizing and coming back to as we try to puzzle out how is it that our auditory systems allow us to use information, mechanical information and disturbances in the environment, mechanical energy that we ultimately hear as sounds. How is it that we are able to use those when this fundamental problem exists? So, here's the kind of guideposts that we're going to be coming back to as we go through the course. First of all, we evolved hearing to take advantage of information in sound energy. It's obvious. All of the sensory systems that we have are taking advantage of some form of energy. Light energy, invision, chemical energy, in taste and olfaction. In the case of audition, it's the mechanical energy in the environment that's out there and the disturbances that we've been talking about. And the reason for evolving this sense of hearing, among other things, but a very important aspect of that, is social communication. You could argue that human beings have the culture that we have and wouldn't have it if we didn't have a sense of hearing, the ability to communicate with each other in vocalizations, in a pre-lingual context. Ultimately as speech and language today. And this is the kind of thing that many, many animals have and have evolved hearing to allow them to communicate socially. Obviously not in the sophisticated way that we do, but this is a fundamental biological need that many animals have gone to some difficulty to evolve an apparatus similar to ours to hear this. For humans this involves identifying and attending to our vocalizations. As I said, arguably this is absolutely fundamental to the evolution of human culture. And in this process, tonality, for reasons that we're gonna be talking about in some detail, the ability to hear, identify tones is absolutely critical in achieving this goal. And, as I said, systematic signal repetition is the definition of tones. And as we'll see, tones are not just a critical part of social communication, but they're a critical part, of course, of music. The way we use experience with tonality to get around signal sound sources, that as I said, are conflated at the ear and ambiguous is a huge problem in thinking about the evolution of hearing and what it accomplishes. And, a final point as a guide post in where we're going from here is the idea that we like music because it presents tonal information in a very essential way and that introduces the whole concept of aesthetics, why we're particularly attracted and addicted to music. Let me end this first module just by reviewing the main points that we've covered in this last 40 or 50 minutes. First, the auditory system generates what we hear for reasons of biological advantage. It's not there for no purpose. It's there for biological purposes that have allowed us to survive and flourish as a species. The second point is it's important to distinguish sound signals from sound stimuli. Again, signals being what's out there. Independent of listeners stimuli being what happens when our auditorium machine transduces those sound signals. Third point is that total signals are relatively rare in nature but they are especially important in social communication and they're of course going to be important in thinking about how social communications related to music. Most natural sources of sound signals are non tonal and noisy. Again, noise being non systematically repeating sound signals at a variety of frequencies. The last point that I made in some detail is that sound signals are conflated at the ear. It's difficult to understand, therefore how does that what we hear, which is subjective, is related to sound signals in the world, which are objective. So that relationship between subjective and objective Is critical. Just to introduce you into what's coming up next, we'll be talking about auditory perceptions, something that we really haven't focused on in this introductory module.
