Sound & Hearing

A Conceptual Introduction

R. Duncan Luce

• Publisher: Psychology Press
• Year: 2013
• ISBN:
  9781317759799
• Edition: 1
• Topic: Personal Development
• Subtopic: Writing & Presentation Skills

The major aim of this book is to introduce the ways in which scientists approach and think about a phenomenon -- hearing -- that intersects three quite different disciplines: the physics of sound sources and the propagation of sound through air and other materials, the anatomy and physiology of the transformation of the physical sound into neural activity in the brain, and the psychology of the perception we call hearing. Physics, biology, and psychology each play a role in understanding how and what we hear. The text evolved over the past decade in an attempt to convey something about scientific thinking, as evidenced in the domain of sounds and their perception, to students whose primary focus is not science. It does so using a minimum of mathematics (high school functions such as linear, logarithmic, sine, and power) without compromising scientific integrity. A significant enrichment is the availability of a compact disc (CD) containing over 20 examples of acoustic demonstrations referred to in the book. These demonstrations, which range from echo effects and filtered noise to categorical speech perception and total more than 45 minutes, are invaluable resources for making the text come alive.

PART I

Transmission, Transduction, and Black Boxes

1. SIGNAL TRANSMISSION

For us to know about the world outside ourselves, events at some distance from us must impact one or more of the sense organs underlying the sensations known as sight, sound, taste, smell, touch, pain, and so forth. So, before information reaches the sense organ, something external must happen: A signal must be produced and that signal must be transmitted in some way to the sense organ.

What are the possible modes of transmission? Scientists know of three distinct types of transmission.

(i) A physical object can simply move from the source to the receiver – from the external site to the sense organ.

The crudest example is some form of projectile, such as a stone or a bullet, although neither is often used to transmit information in any usual sense. An informative projectile is a letter, but of course it requires some additional (visual) transmission once it is sufficiently near the intended recipient.

A more subtle example is light transmission. One of the great discoveries of the early 20th century is that light moves about in tiny packets of energy (and mass) called light quanta, which in some respects may be treated simply as projectiles. In particular, this mode of transmission does not depend on there being a physical connection – medium – between the source and the receiver. Light, for example, traverses a vacuum.

(ii) Impulses that are transmitted in a medium.

In this case, the source of a signal and the sense organ receiving it are usually bathed in the same medium,¹ and the signal moves as a pulse through the medium. Perhaps the simplest, most visible example is a surface wave on water that is generated simply by touching the surface of still water, for example, by dropping a pebble into a pond. Note that no material object is moved from the source to the receiver. For example, if a cork is floating in the pond, it will bob up and down as the impulse arising from the stone passes it, but on average it will be no closer to the receiver after the pulse has passed than it was before. Nothing but the ephemeral pulse moves through the medium.

For the purposes of this text, the key example is sound pulses that are transmitted in various materials, including air, water, and solids. Again, this is done without any particle moving from one place to another. Only the sound propagates, which may be thought of as a wave somewhat analogous to a water wave. Note that this method of transmission is not possible in a vacuum, such as outer space.

(iii) Transmission as a field, which acts somewhat like a medium, but is not.

The two most prominent examples of this form of transmission are gravity and electromagnetism. We are all very familiar with both gravitation and magnetic fields (compasses). The exact nature of fields has been the source of some controversy because, from some points of view, they act much like wave transmission, without any medium.² (Until the late 19th century, a medium called ether was postulated for electromagnetic transmission, until experiments established that it would have to have impossible properties.) In addition to gravity and electromagnetism, two other types of fields have been discovered called the strong and weak forces, and one of the concerns of modern physics is how these forces relate. Gravity, despite its being the first known, seems isolated from the other three. Indeed, despite major efforts to detect gravity waves, it remains unclear whether gravity exhibits a wave motion.

We focus solely on pressure impulses and waves in a medium, for it is these changes in pressure that we perceive as sound. Part II is devoted to the physics of generating and transmitting such pressure waves.

2. SIGNAL RECEPTION

What is involved when a signal arrives at a sense organ? At the least, the person must be made aware by the sense organ that the signal has arrived. A basic generalization can be made about all such perception:

Only physical changes are perceived. Anything that is truly constant is never perceived.

Three examples illustrate this fact:

• Most of the time you are unaware that your entire body is under atmospheric pressure (of about 14.7 lbs/in²). But in a rapidly moving elevator or descending airplane the external pressure can change far more rapidly than does your internal pressure, which is somewhat slower to respond, and you become (often acutely) aware of the pressure change.

• You do not notice an image that is held in a fixed location on the retina (the neural receptors at the back) of the eye. This is a difficult experiment to perform because, without any conscious awareness, the eye moves in small jumps – called saccades – several times a second. So some subtle technique and apparatus is needed to take the saccades into account in order to maintain an image at a fixed location on the retina. But when done, the fixed image rapidly disappears. (Is this assertion inconsistent with your subjective impression that you can fixate on something and it does not disappear?)

• You do not notice when there is a high level of static electricity on your body until it is rapidly discharged (preferably not into a computer), at which point you feel an electric shock. This phenomenon is very familiar to those who live in cold, dry climates; it occurs infrequently in southern California.

A corollary of this proposition states that anything perceived as a steady signal must, despite the compelling impression of steadiness, involve some change at the sense organ. Either

• the sense organ must induce continual change, or

• the seeming constancy of the stimulus must be an illusion, and it really is not constant.

As noted earlier, the eye is an example of a sense organ that induces continual change. When you gaze steadily at an unmoving object, the saccades, mentioned earlier, cause the needed change.

As an example of the second, consider a steady sound, such as a pure tone often used to test audio equipment. It sounds completely constant, but that is an illusion. A pure tone actually involves very rapid, very small changes of air pressure at the ear. They are so fast you do not notice them at a conscious level, but without them you would hear nothing.

So, each sense organ must be a device that responds to a certain class of physical changes: the eye to aspects of the light quanta impinging on it, and the ear to the changes in the pressure waves. But if they are just simple detectors of change, one can ask:

In order to understand sound and hearing (and light and seeing) why isn't it sufficient to understand the physics of the sound stimulus?

This is not a silly question. Indeed, until about 100 to 125 years ago, many scientists thought that understanding the physics of sound would be sufficient. Among those who pursued this approach were three of the most famous physicists, Newton, Fourier, and Helmholtz. They first showed how complex light and sound signals can be decomposed into a sum of very simple components: in the case of light, into the spectrum formed in a rainbow. We look into such decomposition of sound waves in Part V. Next, they postulated that if we understand how the human being responds to each of these components, then the total effect of the signal would simply be the sum of the several effects from the components. This is known as the linear systems approach.

Their strategy did not work because the eye and the ear do a far more complex job of processing complex signals than was realized at first. We encounter some of this processing in Part III.

Such complexity is illustrated by the existence of perceptual illusions. These were first recognized in vision, and you are no doubt familiar with some. There are auditory illusions as well, but most of us are less aware of them. The existence of illusions means that the ear and eye do a lot more than passively “transducer” the physical signal, which is discussed in some detail in the next section.

3. TRANSDUCERS

We just spoke of the ear and eye as doing more than “passively transducing” the signal. What does this term transduce mean?

DEFINITION:

A transducer is any device whose (primary) function is to convert energy from one form to another while retaining information about the amount of energy involved.

The term device is intentionally vague, because transducers come in many guises. Some familiar examples listed in Table 1.1 illustrate the breath of the concept.

TABLE 1.1
Types of Transducers

Name of Device	Conversion
MICROPHONE:	sound pressure into electrical current
LOUDSPEAKER:	electrical current into sound pressure
PHOTOCELL:	light into electrical current
LIGHT BULB:	electrical current into light (+ heat)
GENERATOR:	mechanical rotation into electrical current
MOTOR:	electrical current into mechanical rotation
EAR (outer, middle, inner):	sound pressure into neural activity
VOCAL CORDS + MOUTH:	neural activity into sound
RETINA:	light into neural activity
---:	neural activity into light

Note that transducers mostly come in pairs, the one being, in effect, the inverse conversion of the other, except that in each case some of the energy is dissipated in the form of heat loss. Thus, for example, we use a flow of water or steam in a turbine to rotate a generator that converts some of that mechanical (flow) energy into electricity, which is then transmitted to a site, such as one's home, where some of it is used to run motors (in fans, vacuum cleaners, turntables, etc.) that once again provide mechanical power. In both conversions, heat is also generated and lost.

The major exception to such pairing is the last. Mammals simply do not generate light. Of course, some fish and some insects, such as fire flies, do.

4. BLACK AND NOT-SO-BLACK BOXES

4.1 The Black-Box Approach

In describing tra...