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The Vocabulary of Noise

In 1967 the Scali, McCabe, Stoves advertising agency volunteered to prepare public service advertising for Citizens for a Quieter City. President Marvin Stoves, copy chief Ed McCabe, and creative director Sam Scali spent weeks poring over a small library of noise reference material. They ended up confused as to the technical nature of noise, but clear about one point: the noise problem is easier to solve than to understand.

The scientists who work in the field of noise have tended to concentrate on those aspects of noise which are readily demonstrable and measurable. The emphasis is on the physics of sound, the sense of hearing, and the attempt to find formulae for measuring and predicting “annoyance” reactions.

The public–and many of its elected officials and government administrators–is intimidated by an alphabet soup of decibels and noise scales. It is almost as if the noisy machine is protected by a wall of measuring systems and units. Trying to define noise and quantify human response has become a substitute for seeking to achieve a less noise-stressed civilization. Quality is dictated by statistics and formulae, not by intuition and common sense.

Noise is defined by the American National Standards Institute as:

(1) any undesired sound
(2) an erratic, intermittent, or statistically random oscillation

A Federal report on the subject was titled Noise: Sound Without Value. It would be more accurate to define noise as destructive sound.

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.

Table 1

Typical Decibel [dB(A)] Values Encountered in Daily Life and Industry

	dB (A)
Rustling Leaves	20
Room in a Quiet Dwelling at Midnight	32
Soft Whisper at Five Feet	34
Men’s Clothing Dept. of Large Store	53
Window Air Conditioner	55
Conversational Speech	60
Household Dept. of Large Store	62
Busy Restaurant or Canteen	65
Typing Pool (9 typewriters in use)	65
Vacuum Cleaner in Private Residence (at 10 ft.)	69
Ringing Alarm Clock (at 2 ft.)	80
Loudly Reproduced Orchestral Music in Large Room	82
(Beginning of Hearing Damage, if Prolonged)	85
Printing Press Plant (Medium Size Automatic)	86
Heavy City Traffic	92
Heavy Diesel Propelled Vehicle (about 25 ft. away)	92
Air Grinder	95
Cut-off Saw	97
Home Lawn Mower	98
Turbine Condenser	98
150 cubic foot Air Compressor	100
Banging of Steel Plate	104
Air Hammer	107
Jet Airliner (500 Ft. overhead)	115

These values are unlikely to be repeated as shown here and may vary by several decibels in similar situations. Reprinted by permission of Martin Hirschorn and Sound and Vibration (April, 1970)

However, because sound can set bodies vibrating, it can pack an incredible wallop. It can do work, it can produce heat, it can be reflected, bent, and absorbed.

Given the proper combination of acoustic energy and frequency, sound can destroy rock formations, mix paints, crack plaster, break windows, and wash dishes. In one classic experiment, exposure to intense sound lit a pipe and brewed a cup of coffee in seven minutes.

We live in a sea of sound waves, the vibrations of which may be as slow as three per second or as rapid as millions per second. The most familiar response to these vibrations is the sensation of hearing. Though the entire body “senses” the vibrations, the human sense of hearing responds only to the ones that fall within the range between 20 cycles per second to somewhere in the region of 15,000 cycles per second. (Remember, this is different from the decibel scale, which measures sound intensity, not frequency.) Below 20 cps is infrasound, sound of such low vibration it is inaudible though sensed as a vibration. Sound above 15,000 cps, called ultrasound, is inaudible to most people in industrialized countries.

We are able to “hear” because among the human senses is the ability to detect the very small and rapid fluctuations in the pressure of the air called sound waves. The detection apparatus is called the ear. It is this organ that first bears the brunt of acoustic abuse.

The basic hearing mechanism of the ear involves: the outer ear (external ear or pinna plus the external auditory canal), the middle ear, about one-third of an inch long (containing the familiar hammer, anvil, and stirrup bones), and the inner ear, a system of cavities lying within and protected by dense bone and containing the all-important cochlea. This apparatus must transmit to the brain an accurate pattern of all sound vibrations received from the environment. The human outer ear, from pinna to eardrum, is approximately four centimeters long. (The elephant’s ear canal is eight inches long.) The middle ear has been described as so small it can be filled with five or six drops of water. The inner ear is no bigger than the tip of a little finger. All three parts together are approximately one and one-half inches long, an example of natural miniaturization. The outer ear serves as more than the collecting point for the sound waves. The ear canal can amplify the intensity of certain pitches by means of sympathetic resonance. This is one explanation for the amazing sensitivity of the ear to the sound vibrations. It’s almost as if the ear had a hunger for sound, so great is its sensitivity.

Sound vibrations are condensed, and conducted by the ear canal to the eardrum, a membrane about one-forth of an inch in diameter. The resulting pulsations of the eardrum activate the three tiniest bones in the body, the malleus, incus, and stapes (more familiarly, the hammer, anvil, and stirrup). The three bones of the middle ear serve as a bridge between the eardrum and a membrane at the entrance to the inner ear (the oval window). The stapes, last link of the bony bridge, is attached to the oval window and causes it to vibrate. The vibrations of the oval window set up vibrations in the fluid in the two canals of the cochlea which surround the organ of Corti. Thus the three parts of the ear convert the mechanical waves of airborne sound energy into waves in liquid, and finally into electrical impulses.

It is in the cochlea of the inner ear that, via the all-important organ of Corti, conversion from mechanical to electrical energy takes place. Imbedded in the organ of Corti are some twenty to thirty thousand sensory cells, each of which is capped with fine hair, or cilia. Each hair cell of the inner ear responds only to a specific frequency. The cilia sensitive to high frequencies are at the beginning of the snail-like cochlea, and the low-frequency sensors are at the apex, or far end of the spiral.

Each hair is joined together with the others to become the auditory nerve. By some mechanism, not yet fully understood, the wave-like motion of the hair cells sets up a “coded” electrical signal that is transmitted to the auditory center of the brain.

Not all sound enters the body through the outer ear. The inner ear is capable of receiving acoustic energy via bone conduction and tissue conduction. Intense sound waves can penetrate the skull, the torso, and the groin.

To recognize a sound, the ear has to analyze millions of tones, each with a specific pitch and intensity. The American National Standards Institute defines pitch as “that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from low to high. Pitch depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and wave form of the stimulus.” Loudness is defined as “the intensive attribute of an auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud.” Loudness is also influenced by the frequency of a sound and personal judgment of the listener.

Some sounds are pure tones; most are a combination of several tones. Many can best be described as “broad band” sounds; others as “narrow band” sounds. The whine of a jet is a narrow band sound.

The brain attempts to locate the source of a sound. The sightless human is helped in navigating his way by the echoes of sounds received from nearby reflecting surfaces such as walls and building exteriors.

The ear can hear the infinitesimal vibrations caused by a falling leaf and the vibrations caused by the intense sounds of a rocket engine, a range of about 150 decibels. The ear has been likened to an instrument which can measure in yards at one end of its range and yet detect changes of less than one-thousandth of an inch at the other end.

So sensitive is the human organism to sound that even the mere description of an unpleasant sound can evoke a physical response. Scratching a piece of hard chalk or a fingernail on a blackboard can give one goose pimples, and so can the description of such a sound source.

Sound evokes much more than the sensation of hearing. The sound signal is transmitted, via the brain, to almost every nerve center and organ of the body. Therefore, sound influences not only the hearing center of the brain, but the entire physical, physiological, emotional, and psychological makeup of the human being. The received sound wave evokes a combination of responses-auditory, intuitive, emotional, biological, associative. Sound’s impact is a profound one.

The sound wave is both a message and a transmitter of messages. The message itself may be verbal or non-verbal. A verbal sound signal may transmit a single fact: today is Monday. The fact might be laden with significance and thus arouse emotion in you: you have just lost your job. Non-verbal signals, such as music, can also convey emotions. Non-verbal signals may also cause irritating physiological and emotional states. In short, sounds can make you ecstatically happy or terribly depressed.

Sound, per se, is a desirable and essential part of our lives and of our environment. There are many sounds that are pleasant in themselves; they can convey warmth and desired information. Sound permits communication, the exchange of thoughts and feelings between people. It enables us to enjoy the beauty of music, the voices of nature: waterfalls, birds, the wind rustling in trees, the ocean surf. It is a means of giving and receiving life-saving warnings.

When does sound become noise?

There is no simple answer to this question, any more than there is a simple answer to the question of what is an optimum acoustic environment. We can only hope to approximate an answer.

In general, sound is noise when its physical components disturb the relationship between man and his fellow man, and man and his environment. Or, when the acoustic energy causes undue stress and actual physiological damage.

In conventional terms, sound may be classified as noise when it damages the hearing mechanism, causes other bodily effects detrimental to health and safety, disturbs sleep and rest, interferes with conversation or other forms of communication, annoys or irritates.

Some authorities say that the most obvious effect of noise on man is an interference with the ability to communicate. Sound becomes noise when it masks sounds one wants to hear. Nonetheless, this problem is not necessarily obvious to the victim as a noise problem.

“Speak up,” judges often tell lawyers and witnesses. Sometimes this signifies a hearing problem on the judge’s part; sometimes it means that outside noises are penetrating the halls of justice. A member of London’s Scientific Advisory staff told me that one of the older judges came to him for a hearing test, explaining with embarrassment that he was having difficulty hearing his cases. Shortly thereafter another judge came up to the acoustic lab, and then another. Puzzled, the acoustic scientist did a noise level survey of the courtrooms, and discovered that speech was being masked by intrusive street noises.

Ironically, the masking effect of sound starts a spiral of other objectionable sounds. Fire and police departments, for example, justify making sirens louder and louder, to overcome increasingly higher street noises. Continuous masking is experienced in homes because of noisy air conditioners, appliances, and passing traffic, and perhaps this is one reason why neighbors play radios and TV at high volume.

One way of judging what noise does to behavior is by seeing what happens when it is removed. Researchers at the Ford Foundation’s Educational Facilities Laboratories believe that disruptive noise influences both the dignity and the effectiveness of the teaching process. Donald Barr, headmaster of New York’s Dalton School, has observed that 8- and 9-year-olds–noisy and unruly in the typical noise-box classroom-calmed down and concentrated in that school’s first sound-treated classrooms. Noise and dignity seem interlinked.

Noise-induced hearing damage is described in Chapter 3.

Dr. Gunther Lehmann, former director of the Max Planck Institute, in Dortmund, West Germany, has warned that what threatens man is not the likelihood of auditory troubles or loss of hearing due to noise, “but an incessant disturbance, which under certain conditions creates an intolerable strain.”

The effects of noise other than damage to hearing are called extra-auditory effects. Most of the science of acoustics has been applied to measuring only one category of extra-auditory response, described by the catch-all term of annoyance. Annoyance is probably the most widespread and one of the more complex responses to noise.

The acoustician approaches the annoyance reaction by attempting to separate the acoustic or physical factors (pitch, intensity, duration) from the extra-auditory factors. Of the acoustic factors, loudness has been selected as a key consideration. All other things being equal, the louder the sound, the more annoying it is. Loudness distracts. Loud shouting and yelling–from Indian war whoops to the Japanese banzai–have been used in battle to demoralize, confuse, and frighten.

Though loudness is related to the intensity of the sound, the more decibels the louder, other factors can influence loudness. For example, a given sound appears louder at night, if the background level is lower than during the day.

If one has been shielded from loud noise, sudden exposure is uncomfortable, and can prove startling. People who have had ear surgery find everyday noises inordinately discomfiting. Before the Belgian Congo got its independence, one of its government agencies placed an order for silencers for construction machines being shipped from Europe. It had been observed that Congolese workmen would run away from the construction sites because they could not stand the unmuffled noise.

Extremes of pitch are annoying. Illustrations are sirens (high pitch) and fog horns or boat whistles (low pitch). These intense sounds intrude over the background noise.

Annoyance appears to be a matter of degree and circumstance. For example, irregularity, lack of pattern, makes sound ugly and annoying. The unexpected, unpatterned sounds of jackhammers, auto horns, and helicopters cause unpleasant internal responses. Almost paradoxically, regularity, too, can irritate. The constant roar of an air compressor or a loud air conditioner is not soothing.

A sound which repeatedly changes its point of origin is annoying. When a noisy truck passes another vehicle on the highway, the overtaken driver is immersed in and consequently disoriented by the intense sound that seems to be coming from all directions at once.

Sounds can be annoying even if they are not loud. Proponents of center city heliports claim that helicopters are not as loud as jets, therefore not as annoying. But the distinctive drone and chop-chop of a helicopter is irritating. It is also terribly disturbing to have a relatively low-decibel background noise at night punctured by the clicking of heels on the floor overhead, the hammering in water pipes, the neighbor’s TV.

Sound becomes noise for many non-acoustical reasons. Canned music in a restaurant or an elevator is noise to a man who is preoccupied. This is an invasion of privacy. According to the British Wilson Committee* annoyance can be “essentially the resentment we feel at an intrusion into the physical privacy which we have for the moment marked out as our own, or into our thoughts or actions.”

The fight for a quieter world becomes obscured when feelings about a noise are divorced from the noise itself. We are told that how we react to a given noise may be influenced by our attitude towards the noise source, our state of health and well-being, our personalities, education, income, previous exposure, ad nauseum. Does the transportation noise problem disappear if we all learn to love driving and flying, or the industries that make these activities possible? Would I have been less disturbed by the subway project if I appreciated what the TA was doing for progress? Is a 90-decibel jackhammer really less of a noise because it takes place during the day, or because I’ve heard one before? It is relatively simple to measure the physical quality of the noise signal, its decibel level, frequency distribution, duration, number of occurrences per unit time, etc. It is virtually impossible to measure the significant human response to noise. Schemes for predicting complaints and evaluating annoyance responses are crude guidelines, their effectiveness questioned even by noise specialists.

A number of schemes have been developed to predict the response to a new noise in a given environment, or the response to a noise from a given source. According to acoustician Lew Goodfriend, “All currently used noise rating techniques are based not on what the optimum environment for man should be, but on what the public as a group will put up with.”

One of these noise rating tools is the Composite Noise Rating, or CNR. It supposedly predicts whether a given noise will lead to no response, or provoke a rising degree of protest, culminating in vigorous legal action. “The objective,” states Goodfriend, “is not to produce an enjoyable or even a suitable environment, It is merely to prevent complaints.”

Another example of a noise rating system based on what people will tolerate is the family of “Noise Criteria” curves. Ratings obtained by means of these curves are aimed at “determining the maximum level of noise at which office personnel feel they can accomplish their duties without loss of efficiency.” For secretarial areas (typing) and accounting areas (business machines), the recommended rating is NC-50 to NC-55. The environment for communication at this level is described as “unsatisfactory for conferences of more than two or three people; use of telephone slightly difficult; normal voice one to two feet; raised voice three to six feet.”

Goodfriend is evolving a scheme using computers to obtain a statistical picture of the noise within a community. “From this information it should be possible by mathematical analysis to describe the optimum environment.” Unfortunately for human beings, they do not lend themselves to formulae and equations.

We can measure sound; we can only guesstimate noise. But we know how to reduce noise and noise exposure (Chapter 8). Why not do so? Each day we avoid design for quiet we pay an unreasonable price for progress.

* Noise–Final Report. Committee On The Problem Of Noise, Her Majesty’s Stationery Office, London, 1964.