The Tyranny of Noise

Robert Alex Baron

Part I — Chapter 2 — The Vocabulary of Noise

• 1
• 2
• 3
• 4
• 5
• 6

Since almost any sound can at some time be a noise, noise is, first and foremost, sound. The study of sound phenomena has been the domain of the physicist, and we owe a debt of gratitude to the physical sciences—and the audiologists—for uncovering the nature and behavior of the sound wave.

Sound is a three-fold phenomenon: the source—a vibrating object or material, something that has been excited; the transmission of the vibration; and the effect—the sensory perception called hearing, plus a complex of physiological and psychological reactions.

One way of understanding how sound is generated is to visualize a vibrating object. When, for example, a tuning fork is struck, it vibrates back and forth. When it moves in one direction, it compresses the air molecules in its path; when it reverses direction the air in its former path becomes less compressed, or rarefied. Each time the vibrating object moves back and forth there is one complete cycle of pressure change, and this pulsating compression/rarefaction is radiated outward from the source. The effect of this oscillation on the medium on which the vibration takes place—in this case air—is called the sound wave.

Acousticians use the term cycles-per-second (cps) to express the frequency of sound waves—and thus their pitch.

The relative intensity of sound is usually measured in decibels. Although decibels are not easy to understand, some facts, and Table 1, will make it possible to feel a little more comfortable with the term.

So sensitive is the human ear that it can hear a wide range of sound pressures, with a spread of many millions of pressure units. This wide range has been compressed into a more workable range of 0 to 140 decibels (somewhat higher if we include such sounds as cannon fire and rocket noise). Unlike inches or cubic centimeters, decibels are on a dimensionless scale of values.

The decibel system has several unique features. Though its basic reference point is 0 decibels, zero is not silence or the absence of sound. Zero decibels represents the threshold of audible sound for a healthy young set of ears.

More important, decibels do not progress arithmetically; decibels are logarithmic. This means that each change of decibel level represents a sizeable change in acoustic energy, enabling the system to cope with the wide range of audible sound. Another rationale for the use of a logarithmic system is that the ear perceives differences in sound intensity logarithmically. A decibel represents the smallest change in sound intensity detectable by the human ear.

To give an idea of decibel progression, a given sound at 10 decibels has ten times the intensity of a sound at 0 decibels; at 20 decibels it has 100 times the intensity at 0 decibels; and at 30 decibels it has 1,000 times the intensity at 0 decibels, etc. We can say then that decibels are small numbers that represent large quantities of sound energy. Decibels are measured with a sound level meter.

Originally the "C" decibel was in common usage, expressed as dB(C). This measured the sound pressure on a flat scale. Then it was discovered that the human ear was not as sensitive to the lower frequencies as to the higher, and so the "A" decibel, dB(A), was born. When a decibel meter is switched to the "A" scale, some 40 decibels at the lower frequencies are filtered out of the measurement. This means that a noise source having much of its acoustic energy in the lower frequencies will have a lower dB(A) reading than dB(C). By coincidence, some of the most disturbing noise sources, such as transportation, have much of their acoustic energy in the lower frequencies.

The decibel levels of some familiar noise sources and environments are shown in Table 1. Tables such as this indicate the relative intensities of various sounds. We should remember that magnitude, or "how many decibels" is only one dimension of noise.

The transmission of sound waves must take place in a medium—gas, liquid, or solid. According to the American National Standards Institute, "The medium in which the source exists is often indicated by an appropriate adjective: e.g., airborne, waterborne, structureborne." Your neighbor's footsteps overhead are creating structureborne sounds. Your neighbor's lawn mower is creating airborne sounds.

Sound is commonly produced by the vibration of a solid: solids striking other solids, solids rubbing against solids (friction). Musical sounds, for example, may be created by the vibration of a plucked string, the friction of bow against strings, or the vibration of a struck surface, as a drumhead. Airborne sound may be caused by turbulence, the agitation produced when a rapidly moving air stream hits still air, as in jet exhaust; or when a rapidly moving air stream hits an obstruction, as when the stream of air from a fan hits a poorly designed fan guard or grille. Other examples of sound created by turbulence in the air are the notes of wind instruments, created by fluctuations in an air column, and the sounds created when rapidly flowing air strikes the open window of a speeding car.

Compared to other forms of power, such as electricity or a combustion engine, the sound wave's power appears minuscule.