Broadway director and drama critic Harold Clurman and I were walking along one of Manhattan's lovelier streets one balmy spring day. A truck barreled by. He winced, and we had to stop conversing. As we progressed, a jackhammer suddenly opened up at a street repair site. He winced again, much disturbed by these noises, but he failed to make any comment.
"Doesn't this noise upset you?" I asked, when I could.
"Upset me! That's putting it mildly. Have you ever tried to direct a show with heavy truck traffic outside the rehearsal hall, not to mention the sirens and the horns? Can you imagine putting together a play review while living next to a construction site? But what can I do? Noise is one of the curses of civilization."
"Would you believe trucks could be designed with less noise, and that jackhammers are made with built-in mufflers?"
"Stop pulling my leg," he replied. "You've been reading too much science fiction."
Noisy equipment is a sign of imperfect design. At the same time that man's inventiveness has enabled him to produce the noisiest epoch in his history, it has also given him tools for eliminating or minimizing all that noise. So refined is the application of these tools, noise control experts have designed a systems approach to any given noise problem. Its formula is "source/transmission-path/receiver."
Translated into action, this formula means that to solve a noise problem the first step is to find a way to eliminate noise at its source. If not, block the sound waves after they are generated. Or, finally, find a way to separate the receiver's ear and the acoustic energy.
The science and technology of noise control are quite sophisticated. Equipment can be designed to meet goals for preventing hearing loss, for permitting speech communication, for minimizing "annoyance." The design engineer has available a limited number of guidelines ranging from voluntary codes adopted by an entire industry to purchase specifications and—rarely—legislation.
Some guidelines set limits for the allowable noise to be emitted by a given machine, others, for the amount of noise allowed at the receiver's ears. Most available guidelines or standards specify what instruments are to be used to measure the noise, and how these measurements are to be taken. The basic measuring instrument is the sound level meter. This device—which comes in all sizes, including portable, hand-held models—makes possible the comparison of various sound intensities ranging from the decibel reading in a bedroom at night to the sound intensity of a passing truck or jet. Sound level meters are in use throughout the world. One manufacturer prints instruction sheets in sixteen languages, including French, Russian, Norwegian, German, Finnish, Japanese, and Chinese.
There are also instruments for analyzing the decibel level of sounds at various frequencies. With these analyzers it is possible to tell how much of the sound energy is in the low pitch, middle pitch, or high pitch range. They can analyze the acoustic energy in hisses, swishes, sizzles, rattles, and buzzes, and can be used for the rating of product noises, the measurement of noises proscribed by local ordinances, and the designing of auditoria.
It should be noted that though they are commonly referred to as noise meters, the sound level meter and the analyzer are, more correctly, means of measuring the two physical qualities of sound—intensity and frequency; they do not measure human response to noise.
For acoustical research and design applications, technology has provided an incredible array of more sophisticated measuring and analyzing instruments based on decibel measurements. There are impact analyzers, to measure impulsive noises too brief to give a reading on the dial of the regular sound level meter. There are oscilloscopes, by means of which sound can be observed and measured visually. There are tape machines that can record noise on location for subsequent analysis in acoustic laboratories. As you listen to your noisy appliances, think of the noise level meter available for factory production lines to assess the noise and vibration output of each item.
The most effective and possibly the least expensive method of noise abatement (as far as society is concerned) is to design machines that generate and radiate little noise. Engineers can be trained to determine if a given noise can be reduced at its source. Machine noise is not "natural." According to one consulting engineer, "Noise is a form of pollution that is not necessarily inherent in the design of larger, more powerful systems and equipment. It is not necessary for design engineers to accept increased noise and vibration as an unavoidable accompaniment to the power, capacity, and efficiency of industrial machinery."
Given the motivation, engineers can select silent operating levels, can specify low tolerances for moving surfaces, can call for the use of bearings with relatively few imperfections. (It is said that bearings of the future will be made in outer space, with the result that each bearing will come out perfect.) Gears can be designed to make less noise, and in some cases they can be fabricated of nylon or other plastic instead of noisier metal.
Engineers can check thin metal housings for vibration, and rotating machinery for imbalance. They can select a type of metal that will not vibrate as readily as another.
With or without guidelines, the motivated engineer can measure the decibels as a diagnostic means of tracing machine parts that are responsible for unwanted or unnecessary sound. For example, in precision design, an air conditioner or lawn mower could be isolated in an anechoic room—a room so quiet that one begins to hear the internal sounds of his own body—in order to determine the total sound power level. The design engineer can isolate tones, obtaining a picture of how the noise energy is distributed, from rumble to high squeal. Moving parts making unwanted sounds can be identified.
One of the most dramatic examples of effective silencing—the car muffler—was developed before there was a science and technology of decibels, even before the invention of the sound level meter. The automobile industry had to develop effective muffling to keep from frightening horses on the roads. Unfortunately, we do not insist that new noise-suppression methods be developed to keep from frightening humans.
Noisy machines can be partially or completely enclosed in barriers. At Baden-Baden a dramatic reduction of noise was achieved by placing what looked like an open-ended privy around a jackhammer and an air compressor. The open end, which faced away from the public, provided light and ventilation, while sound-absorbent lining made life a little more bearable for the operator.
In this country, partial or full enclosures are employed in factories to protect machine operators from noise produced by very noisy automatic screw machines and jolt-squeeze hammers. (Similar enclosures are available for electric typewriters and calculating machines.) Several companies make standardized soundproof panels that are fitted together to shield noise-sensitive areas in factories. This principle could be adapted to enclose suburban electrical generating stations, noisy turbine generators, and other sources of noxious noise.
It is possible to stop the transmission of noise through attached structures by breaking the acoustic path through walls and floors. With vibration isolators, springs, pads, or rubber mounts in use, apartment dwellers need not suffer from noises radiated from elevator or central air conditioning operations. Ford, Bethlehem Steel, and other industrial giants reduce the noise of operating equipment to safe levels by placing shock and vibration mounts under machines.
Window air conditioners operate less noisily if properly mounted: that is, isolated from the window structure. Resilient gasketing keeps the vibrations of rotating parts from being transmitted to the windows, and into the walls, floors, and ceilings. Even the tiny motors used to aerate fish tanks can create quite a hum. If placed on a foam rubber or other resilient base, however, their noise-making is reduced.
Most of the literature of noise control quickly glosses over the most fundamental approach: source substitution. Noisy equipment and processes can be replaced with quieter substitutes. Bolting and welding are quieter ways of putting up a building than riveting. The banging of punch presses can be eliminated by substituting hydraulic pressure to shape the metal.
Electric motors can be substituted for noisy internal combustion engines. Electric vehicles, in use in England, not only make but little noise, they have no exhaust fumes. Small electric lorries, tested at normal town speeds, were much quieter at a distance of 7 meters than diesel and gasoline counterparts:
| diesel | 81 dB(A) |
| petrol | 80 dB(A) |
| electric | 60 dB(A) |
Diesel and gasoline vehicles are even noisier when shifting gears or accelerating.
Another quieter substitute for the internal combustion engine is the gas turbine engine, and turbine-operated buses are undergoing operating tests. Air compressors operated by propane gas engines are strikingly quiet. Fuel cells and steam engines provide quiet power without air pollution.
Other examples of quiet by means of substitution are the use of plastic or paper sacks instead of metal garbage cans, nylon rollers instead of metal. The list of quiet substitutes for dangers and bangers is quite long.