In lesson two, I'm going to be introducing you to the concept of Sound Signals and Sound Stimuli. There is a distinction between these two terms, and I want to make that clear. And let's begin by talking about Sound Signals. Sound Signals are the mechanical disturbances that are out there in the world in the atmosphere. And the usual way of talking about these, is in terms of the sine waves generated by a tuning fork. So, a tuning fork is what you see here. And when I whack it on the [NOISE] table [NOISE] and hold it down, you can probably hear the sound wave that's coming from that. This tuning fork is at 440 hertz. Hertz being the term that we're going to be hearing a lot about. That is the measure of sound signal frequency and the tuning fork, as the tines vibrate, setup a disturbance locally in the atmosphere, of course they don't go very far, you wouldn't hear the tuning fork in the next room. And these disturbances are rare factions and condensations that emanate from the source of a sound signal in a concentrically uniform way, out from the source, in a series of waves that are the atmosphere being pressed together as the pressure increases and then being released as the tine of the fork moves in the opposite direction, that's the rare factions. So, as diagrammed here these are actually mechanical disturbances in the pressure of the atmosphere. And the measurement of sound signals is in terms of sound pressure level, which is the same metric that we use in barometric pressure that we think of in terms of weather and weather forecasting. So there are three different parameters of sound signals. I've already mentioned frequency measured in hertz, which is how many times per second the sound wave fluctuates up and down, that's the frequency. The amplitude of the wave, how big, or not so big, the disturbance is in terms of normal atmospheric pressure, baseline atmospheric pressure, which is the black line in this diagram. And then the phase, where we are along the trajectory of these waves, and that's important if you think of two different waves that are happening at the same time that can interfere with one another, and they can amplify each other if they are in phase, in synergy, with each other or cancel each other if their out of phase. Phase is measured in terms of angles, the full phase being 360 degrees from peak to peak, over trough to trough, as we go along here. So, that's sound signals. Again, keep in mind that these signals are out there whether we're there to listen to them or not. They are not stimuli. Stimuli only happen when we introduce, bring into the picture the apparatus that we talked about in lesson one. Now, I'm not gonna go over this again, but remember that the sound signal reaching the ear is amplified, filtered, transduced in the inner ear by the basilar membrane, whereby the sound signal in the atmosphere is transduced into a sound stimulus. That's a biological phenomenon that initiates the signals that travel centrally that eventually allows us to perceive sound. I included this little sentence here, which is one you may have heard before, that based on the differences I've just been describing, you ought to be able to answer easily. The question is, when a tree falls in the forest and there's no one there to hear it, is there a sound? Well, if you keep in mind the difference between a sound signal and a sound stimulus, the answer to this philosophical puzzle, that I think people [LAUGH] have sometimes take seriously is obvious. Of course, when there is a sound signal, but no one there to hear it, no machinery of the human or other auditory system to transduce that signal into a stimulus. There is no sound, which is the term we apply to the perception that we actually hear.
