Welcome back to this second module. We're going to talk today about the perception of sound stimuli, something that we didn't really cover the last time. And let me begin by making a few general marks. And the first of these is to define perception. It's not a terribly hard concept but you certainly need to be aware of what is meant by using this word. Perception is what we experience subjectively. Remember last time I differentiated between sound signals, sound stimuli, how those signals are transduced into neural signals. And now perception is the end result of what we get when we experience subjectively those stimuli as sounds. So the definition of perception is what we experience subjectively in response to sound signals and sound stimuli. It's important to realize as we go through this that there's always a physical measure that's going to define some parameter, some aspect of the sound source, that's objective. And there's always going to be a corresponding cycle physical parameter that pertains to what we actually hear on the case of audition. So in every sense modality, you can think of vision for example, as being a sense modality in which the things that we are subjectively aware of are lightness, darkness, color, form, motion and so on. Those subjective qualities, those basic qualities define what we hear in audition are loudness, pitch, and timbre. Timbre is spelled kind of a funny way, but it's pronounced as timbre not timber. So let's go in with lesson one and consider the first of these qualities, or this pair of qualities, loudness and intensity. Intensity being the physical parameter that describes the sound signal, sound stimulus, at the basilar membrane that we talked about last time. And, loudness being the perception of the sound signal intensity. So, let me go through a little bit about the genesis of intensity at the periphery. We talked, remember this picture from last time. We talked about the ear, the signal going into the external ear canal, reaching the eardrum, and carrying the mechanical disturbance in the atmosphere into the inner ear. the cochlea which, as you remember, is this structure here, the snail shell that you see. And in this bottom diagram what's happened is that the snail shell has simply been rolled out. It's about 35 millimeters in length. So you can consider this extent on that order. And what you're seeing here is in diagrammatic form of course, the vibration that's transmitted to the basilar membrane as an amplitude. That is here's the resting state of the basilar membrane, this line along here. And the wave is again a series of peaks and troughs that we talked about last time, that have an amplitude. they have a magnitude that can be greater or lesser depending on the intensity of the sound. The intensity of the sound is measured with a sound pressure meter. That's a device, not a complicated device. You can buy one at Radio Shack or similar store for a few dollars, and it measures literally the pressure that a sound single generates. But you need to be aware that the pressure that the sound signal generates has to be equated to the range of human hearing. It's not relevant that there is some pressure, let's say, the atmospheric pressure, that determines the weather. Well, we obviously don't hear those pressure changes. They're measurable, they're measured by barometers, I said last time and terribly important in that context. We don't hear them, why? Because these are slow changes that don't cause any disturbance of the little hair cells on the tips of the neurons that are sitting on the basilar membrane. So it's the amplitude of the vibration of the basilar membrane that is determining the intensity of the signal that's actually coming into the ear. That intensity being measured at the ear by a sound pressure meter of some kind. There are different settings on a sound pressure meter. You might want to measure the safety of the environment in a workplace, for example. In that case you really wouldn't need to worry about the details of adjustment for human hearing, you just need to worry about the intensity over the range of human hearing, and that would be a different setting on your sound pressure meter. But for what we hear, the sound pressure is in decibels which I'm gonna talk about in just a second. Remember as we go forward that the point I'm really going to emphasize today, is that the intensity that exists measured by the sound pressure meter, is not really what we hear. And we'll see the same thing when we talk about pitch and tamber. But that's really a key in the discussion here today. So the decibel is a measurement as you can gather from what I've just said. That's predicated on the sensitivity of human's hearing. So it's not an absolute measurement, it's a measurement that's adjusted to what human being's are capable of responding to, or what the human ear, what the hair cells are capable of responding to. So 0 decibels, incidentally, the decibel is named after Alexander Graham Bell, as you might have guessed. 0 decibels is, by definition, the threshold of human hearing sensitivity. And it goes up from there in a logarithmic fashion because we're capable of hearing an enormous range of sounds from the proverbial dropping of a pin in a quiet environment to as you see on this diagram a jet engine. So this diagram of decibel intensities on the x-axis here and some examples on the y-axis is instructive just to put you in the ball park of what we're talking about. So minimum audible intensity by definition, 0 decibels. A quiet room, 20 decibels. Conversation at a meter's distance from the speaker you're listening to, 60 decibels, and so on up to a jet engine 60 meters away. That's the limit of hearing because that level of sound intensity, and that's why you see the guys at the airport wearing ear protection, at that intensity the hair cells are actually physically damaged. So the range of sensitivity to physical sound intensity is from 0 decibels by definition threshold of human hearing, to the sound intensity that leads to damage to the hair cells. That's incidentally, tremendously important because hair cells don't regenerate. Once they're damaged, and that's why older folks like me are actually wearing hearing aids if you can notice those little things in my ear. Over a lifetime you are exposed to numerous high energy sounds and gradually the wear and tear on your hair cells shows and by the time you're my age you have lost some sensitivity. It's just common place. So let's go back and talk about what I said and what I want to emphasize as critical in these discussions and really thematic in what we're talking about today, which is that what we hear doesn't really correspond to the sound intensity that you measure in the ear. So you would of course expect the sound intensity that you hear to be proportional to the physically measured sound intensity with the pressure meter. Well, roughly speaking, that's true but a diagram like this shows you that mm. There are a bunch of things that affect the loudness of what you hear that make the statement that, oh, this is just a response to sound intensity and it's a proportional relationship, obviously false. So, here is the influence of frequency. We're gonna talk about this in a minute when we talk about pitch frequency you remember is measured in hertz and that's the repetition rate of a sound signal, be it a sign to alter something else this diagram actually is based on the measurement of sign tone frequencies. So with the x-axis frequency's at 20 Hz, 100 Hz, 1,000 Hz up to 20,000 Hz which as we'll talk about later, is the full range of hearing pitch, being able to hear frequency on this axis. On the y-axis is sound pressure level measured in decibels. These are curves at different physical intensities ranging from the red dotted line which is threshold. That's 0 decibels up to 10, 20, 30, 40, 50 decibels, and so on. So this is a test of how sensitive your hearing is to sound level intensity as you go over this range of decibel physical intensities from threshold. Up to 120 again, that's up around the jet engine engine level. And what you see here is that, these curves are telling that low frequencies and a very high frequencies are 20 Hz or 20,000 Hz. You're much less sensitive to the sound than you are in this middle range of hearing, which incidentally is the range of the frequencies that we generate as we speak. And the frequencies that we generate musically, which of course we're going to be talking a lot about. So the point here, is that these U-shaped curves at any level of sound intensity are telling you that what you hear is not just proportional to the intensity of the sound. It varies, and we've evolved to hear sounds with greater sensitivity in the middle range of the sounds that are biologically, ecologically important to us like speech and music. And less sensitive to sounds that are at low frequencies or very high frequencies. That's the first indicator that loudness is not just a translation of the sound pressure at the ear, as you might expect. And the same is true in other sensory modalities, eye work a lot on vision and just remind you that the same phenomenon exists in the sensitivity curve to light. We are not sensitive, we're not very responsive to light, that's at the low end and the high end of a range that we're capable of seeing which is a narrow bit of electromagnetic spectrum as in this case there's a U-shaped curve that falls off. At the extremes and we're most sensitive to light in the range that's ecologically important to us, which of course is the range of sunlight. So, as I told you Ruby Fru, the assistant in the course, is gonna come in with demonstrations from time to time. And here's the first of these, which is a demonstration of how sound intensity varies in music, and the impact that it has. So, what Ruby's going to play here is a familiar tune. It was an 18th century folk tune, before Mozart got a hold of it and turned it into a bunch of variations of a song, a French tune that's called! vous dirai-je, Maman. That has turned into the modern English version in the lullaby Twinkle, Twinkle, Little Star. The same melody but different lyrics. The ABC song that we're all familiar with as kids. So, what Ruby's going to do is play the ninth of Mozart's 12 variations of this didy and show you in this particular rendition how important the intensity of sound is and what an effect it has on the music that you hear and the response that you have to it. >> I'll be playing five of the twelve variations of a piece called Vous dirai-je, Maman, which was written by Mozart in the 1780s. The theme, or melody on which the other variations are based is the Twinkle, Twinkle Little Star melody, which is actually where that melody comes from. The phrases often begin quietly to piano, or soft, dynamic, and end in a louder, or forte, dynamic. There are also interesting differences in articulation, staccato versus legato to listen for here. [MUSIC]
