When the embattled farmers stood and fired the shot heard round the world they founded a glorious tradition. This occasion is further memorable because the apocryphal flight of sound waves in this instance must have established an all-time record for long-distance transmission of noise.
A somewhat better authenticated account of long-range transmission occurred in connection with the Krakatoa explosion in 1883. This terrific explosion is placed in nomination for the loudest sound ever heard on earth or ever to be heard until Gabriel sounds his trumpet.
Krakatoa is a small volcanic island near Sumatra. On August 27, 1883, after some preliminary warning, a volcanic explosion took place, the magnitude of which may be adjudged from the fact that the sound was heard approximately 3,000 miles away. Imagine a sound produced in San Francisco being heard in New York. The air waves generated by the explosion spread over the earth and converged in northern South America. All recording barometers in the world were affected by this disturbance. Its passage, as it reverberated back and forth, was noted through four trips from Krakatoa to the antipodes and through three trips returning.
In connection with long-range sound travel it has been noted that the phenomenon of skip distance sometimes observed with radio waves is associated with it. It has been reported that an explosion of a large powder charge may be heard up to an approximate distance of 60 miles, then occurs a zone of silence and the sound is again heard at and beyond 100 miles. The effect is ascribed to meteorological conditions. This peculiarity of sound travel may well explain the often-noted fact that heavy gun firing of the fleet some distance offshore may not be heard on the coast but may be heard some distance inland.
We are, of course, familiar with many aspects of sound. Our usual method of comparing magnitudes of sounds is to say that one is louder or softer than another. In order to express this thought quantitatively, a unit of sound intensity measurement has been developed.
This unit of measurement is called the bel in honor of Alexander Graham Bell, inventor of the telephone. A unit of 0.1 this value, naturally called decibel, is ordinarily used. The decibel is not a physiological unit. Two sounds having intensity levels of 60 and 40 decibels will not, in general, appear to the ear to bear the ratio of 3-2 in loudness. By an understanding of the complex manner in which the ear functions to scale sound in loudness, a decibel measurement combined with a frequency determination may be interpreted in terms of appreciation of the sound by the ear.
The figure shows a scale of decibels applied to certain sounds whose relative sound energies are also expressed.
The power of sound encountered in ordinary measurement is exceedingly minute. The average power of speech, for instance, is 10 microwatts. This value may be increased to 1,000 microwatts for a shout and lowered to 0.001 microwatts for a very low whisper. As an illustration of the extremely broad range of the ear, it may be noted that the average speech might undergo an attenuation of 1010 times before becoming inaudible.
Should the energy developed in sound waves by the regiment of midshipmen in giving a 4-N yell be suitably harnessed, it would provide sufficient power to light an extremely small electric lamp.
Considering the extreme range of intensity within the audible range and the non-linear relationship between loudness and intensity, it would seem an impossible task to devise a sound-measuring apparatus whose scale reading would indicate loudness of sound comparable to the indication of the ear. When we speak of sound we may mean two things either the vibrations themselves or the sensation produced. The measurement of the first lies in the realm of physics while an understanding of the second is in the province of psychophysiology. The problem of physical measurement is relatively simple. The difficulty in the field of noise measurement lies in correlating physical sensation.
The difficulties in developing a sound-measuring apparatus have been quite successfully overcome. The usual scheme is to evaluate sounds in terms of the intensity level of an equally loud 1,000-cycle tone. A typical instrument would consist of a sound pick-up device, an amplifier circuit, some frequency weighting network to approximate the ear's variable response, and some type of indicating meter scaled in necessary to add a frequency analyzing device so that intensities of sounds at various frequencies may be determined separately. These results must be combined mathematically in order to obtain an accurate evaluation of the complex sound. No meter has yet been devised which will accurately give this result directly.
In considering noise from a naval standpoint, two aspects present themselves: the effect of noise on military efficiency, and the effect of noise on personnel efficiency. It may properly be argued that any change in personnel efficiency is directly reflected as a change in military efficiency of the service. This is undoubtedly true, but noise may be considered in itself as having a direct bearing on military efficiency of the fleet.
Recent developments in listening devices may make it possible for a submarine to track and successfully fire torpedoes at a surface ship without exposing her periscope. It then becomes important for a surface ship to limit the noise emitted, thus to make the submarine's task more difficult. It is obvious that a noisy submarine is herself more in danger of detection than a relatively quiet one. A submarine lying on the bottom with her main motors stopped may sometimes be heard a considerable distance if running any of her auxiliary machinery.
Dr. H. C. Hayes, of the Naval Research Laboratory, states that a large part of so-called propeller noise, as heard in underwater listening devices, is engine noise transmitted along the shaft and thrown off the propeller blades. This effect is more noticeable in internal-combustion engine-propelled ships. The actual noise of the propeller itself is said to be minor, but capable of improvement if the noise factor could be considered in propeller design.
It is a common experience during war games or maneuvers, when under way at night without lights, that contacts with destroyers running even at moderate speed are first made by detecting blower noise before the destroyer herself becomes visible. This exposure of the presence of our destroyers during a night engagement might well be of serious consequence.
There are a multitude of sources for internal noise aboard the modern man-of- war. We know too well the annoyance of rivet hammers, chipping hammers, paint scrapers, screaming blowers, grinding gears, noisy ventilating systems, groaning pumps, and some may even include the anachronistic bugle call, rattling mess tables and benches, and the sometimes pestilential wardroom radio.
Steel decks and steel bulkheads offer all too efficient sounding boards for machinery thus supported. A greater proportion of most machinery noise is transmitted through the foundation to the supporting deck or bulkhead which transmits the noise to distant points and sometimes serves to amplify it.
It is an extremely difficult matter to discover the effect of noise on efficiency of personnel. The objective symptoms of noise on the individual are few and difficult of exact measurement. Such measurements as can be made are difficult properly to interpret. Throughout the literature, however, one finds frequent reference to the effect of noise on the health and efficiency of persons subjected to it.
Equipped with a unit and means of measurement, the study of noise in various locations has produced interesting and extremely valuable results. The most outstanding noise survey was made by the Noise Abatement Commission of New York City. This commission, composed of prominent men in the scientific, medical, and industrial fields, made a thorough study of amount and effect of noise in New York City. Many improvements in city noise eliminations have resulted from their work. The report of this commission, “City Noise,” created world-wide interest.
Under the Noise Abatement Commission, the committee on the Effect of Noise on Human Beings reported some extremely valuable discoveries.* Their report is believed of enough interest to warrant quoting in part.
(*Personnel of committee on the Effect of Noise on Human Beings: Dr. Bernard Sachs, chairman; Dr. Foster Kennedy; Dr. Arthur B. Duel; Dr. Samuel J. Kopetzky; Dr. Frederick Tilney; Dr. Alexis Carrel.)
The necessity of putting forth extra effort to overcome noise creates a continual strain on the nervous system which must ultimately show itself in reduced capacity for sustained work, clear thinking, and energetic action. ...
The effects of noise have been described ... as “emotional.” Long before the emotions are actively disturbed, however, there are undoubtedly disturbances expressed in heightened pulse rates, increased blood pressure, irregularities in heart rhythm, and most important of all, in the increase of pressure on the brain itself. . . . Emotion is only the end product of the process; the undoubted effect of constant noise is disturbance of the blood-vessel apparatus and the increase in the degenerative processes of the heart and arteries.
Hearing is apt to be impaired in those exposed to constant loud noises.
Noise interferes seriously with the efficiency of the worker. It lessens attention and makes concentration on any task difficult.
In the attempt to overcome the effect of noise, great strain is put upon the nervous system, leading to neurasthenic and psychasthenic states....
Noise interferes seriously with sleep even though in some cases it appears that the system is able to adjust itself so that wakefulness does not result.
Reporting along similar lines, the Noise Commission of London finds in part, “that neurosis may be attributed to noise just as in the case of shell shock.”
Many attempts accurately to measure the direct effect of a noisy environment on the efficiency of workers have yielded unreliable results because of improper control of the many variables involved in the problem. Many general statements as to the deleterious effects of noise appear in reports of investigations. A great mass of data, however, indicate that, although there is in many cases an exaggerated importance attached to the effects of noise, there is no doubt that it has an adverse effect on personnel efficiency and individual health.
Some interesting results have recently been obtained by Dr. Donald A. Laird investigating results of noise on workers in a "factory" established in a laboratory.* In his factory he employed several young men in excellent health to work daily in accomplishing certain tasks. He states the results of his investigation as follows:
Measurements of the effect of accurately controlled noise upon production in work duplicating the neuromuscular activities involved in a wide array of industrial and office tasks indicates the following:
- A varying complex noise has the most serious effect upon production.
- A complex but steady noise is worse on production than a relatively pure tone.
- A pitch above 512 cycles cuts into production more than a low pitch of the same loudness; above 512 cycles each increase in frequency is for by a loss in production.
- A reduction in a complex noise regardless of the original loudness is accompanied by increased output.
(*"The influence of Noise on Production and Fatigue, as Related to Pitch, Sensation Level, and Steadiness of the Noise," by Dr. Donald A. Laird, The Journal of Applied Psychology, Vol. XVII, No. 3, p. 320.)
He also notes an abnormal fatigue tendency exists when the noise level in his factory is above 40-50 decibels.
An extremely interesting report on noise studies has recently been published in England.* The report embodied the results of two investigations of the effects of noise on individuals. The first study was made in a laboratory and the second in a factory. As a result of the laboratory work, the investigators found that, although the effects of noise on individuals psychologically and physiologically are likely to be greatly exaggerated, noise is prejudicial to efficiency and the harmfulness is roughly proportional to the loudness. The second study made in a factory yielded some pertinent results. The factory workers, in this case weavers, were equipped with ear protectors which reduced the noise level from 96 to 87 decibels. The study extended for many months and results indicated that the personal efficiency of the workers was increased 12 per cent. It was also discovered that even after years of working in the same noisy environment the worker did not become acclimatized to the noise but underwent adaption daily. These results indicate that measurable increase in efficiency might result from a reduction in noise level of the worker's environment in various naval industrial activities such as navy yards.
(*Report No. 65 of the Industrial Health Research, Board—Medical Research Council. (1) "Psychological Experiments on the Effects of Noise," K. G. Pollock and F. C. Bartlett. (2) "The Effects of Noise on the Performance of Weavers," H. C. Weston and S. Adams.)
We have unfortunately taken for granted that considerable noise is a necessary concomitant of a highly mechanized ship. Little or no thought has been utilized in design or installation of machinery to limit or nullify noise.
It is commonplace today for an automobile to possess a power plant insulated from the frame by rubber slabs and bushings. Shackle bolts are insulated and various parts of the body are insulated from each other. Quietness and lack of vibration have become highly important selling points. This noiseless construction is being extended to railway and to subway cars.
The elimination of extraneous noise has been a vital problem in the commercial aviation field. Public demand is forcing the development of less noisy planes.
The abnormal fatigue tendency noted in noisy environments has a very direct bearing on naval efficiency. No one will deny that the fatigue of the flying personnel involved is a serious consideration in extended flight operations such as patrolling. By making our airplanes less noisy we could undoubtedly keep the air longer. Such vital matters as this cannot be neglected in seeking for increased efficiency of our flying forces.
City governments are adopting and enforcing ordinances limiting city noise from street cars, subways, trucks, automobile horns, street radios, and the like. Industry and local governments have found that noise does not pay. The lesson may profitably be carried to sea.
“Men fight, not ships,” hence it is vital to protect the health of our personnel. It is equally obvious we should take all reasonable precautions to make our ships safe from detection by sound source.
The transmission of a great deal of machinery noise may be materially reduced by suitable sound-proof mounting. The development of sound insulating material for foundations of engines, pumps, gears, blowers, and the like, has passed the experimental stage and its use is now well established.
An insulating clutch could well be placed in the line shafting between engine and propeller of such internal-combustion engine-driven types as submarines, to lower the amount of propeller noise.
Emphasis on noise limitation in forced draft blower design would soon result in greatly lessened noise from that source. Gears may be properly designed for noiseless running. If ships’ ventilating systems were designed with properly low outlet, air velocities and quiet fans and dampers, much annoyance would cease.
In view of recent successful developments for eliminating or reducing machinery noise and based on physiological reasons as well as on possible protection from detection in war time, it seems clear that a high level of noise aboard ship will not long be tolerated.