Mathematically, a decibel is the unit of an arbitrary scale logarithmically proportionate to a scale of physical intensity units of sound. A physical intensity unit of sound is the amount of energy transmitted in unit time through a unit area in a plane normal to the direction of the sound waves. Those interested in such things say that this intensity in microwatts per square centimeter is equal to 2,380 times the square of the root mean square pressure in dynes per square centimeter; and a dyne is the force which, acting on a gram for a second, imparts to it a velocity of one centimeter per second.
To the layman the decibel is usually characterized as a unit of loudness, an identification sufficiently clear for our purpose but not in exact accord with scientific accuracy, as loudness depends in some measure upon the frequency of the sound wave. Since extremely low and high frequencies are altogether inaudible, we can theoretically have extremely loud noises (as measured in decibels) without even hearing them, a condition which does not agree with our usual conception of the term loud. A decibel is supposed to be the smallest increment of sound which the average human ear can detect. A whisper is measured by about 25 decibels, ordinary conversation produces about 60, and loud shouting may generate as many as 90. The decibel level in a railway car is about 65, in a subway about 80, and in the pilot’s seat of a cabin airplane about 100. At about 120 decibels sound becomes actually painful and at about 140 serious physical injury is probable. Since the decibel scale is logarithmic, its values cannot be added algebraically. Thus, shouting in an airplane cabin probably does not increase the noise level by more than 1 or 2 decibels, and the stopping of one of the two engines, which together are producing 100 decibels, reduces the noise level to about 97.
The aerial decibel affects two general classes of people: (1) the occupants of planes and (2) everybody else within earshot. Let us first consider the victims of class one. Mr. Charles Cox, of the New York State Health Department, says
The shock of noise and the strain the body suffers in adapting itself to it results in actual physical injury to the body that seems similar to the bodily changes resulting from old age. City people think they become accustomed to noise but they do so only at the expense of nervous energy and efficiency. Sleep is less restful when there are noises near by. Thinking uses up more energy in noisy places and depletes the nervous vitality needed for other activities. Prolonged, intense noise damages the inner ear and there are at least twenty-five noisy trades in which deafness is common.
Although exhaustive study has been given to the subject, many properties of the ear and their relation to the rest of the human body are not yet clearly understood. It is generally admitted that intense noise is destructive to efficient mental activity, and it is well known that compressional waves of high intensity at supersonic frequencies produce harmful physiological effects. Eminent physicians agree that prolonged exposure to intense noise is one probable cause of shell shock, but they are not in accord as to the degree that noise alone is responsible. Perhaps a mild form of shell shock produced by the aerial decibel is responsible for the alleged mental instability of the average naval aviator. According to Schopenhauer “noise is a true murderer of thought,” and Herbert Spencer wrote that “you may gauge a man’s intellectual capacity by the degree of his intolerance of unnecessary noise.”
The noise level in a naval combat airplane to the knowledge of the writer has never been measured, but it is probably from about 100 to 115 decibels, depending on the type. Consider the pilot and observer of a scouting plane in their struggle with the aerial decibel. Sitting two feet apart they must use telephones, hand signals, or written notes to communicate with each other! The observer is forced to tune his radio signals from the ship to extremely high intensities so that he can hear them clearly. The chances for misunderstood radio signals are certainly enhanced by the noise. A weak signal may easily be altogether lost. It requires no great stretch of imagination to imagine how much more efficiently the mission of the plane might be accomplished if the occupants were inclosed in a cabin whose noise level permitted conversation in ordinary tones. A constantly recurring criticism of our fleet problems and exercises is that they are misleading because they are too short to indicate adequately the prolonged strain on men and machinery which will be inevitably experienced in war operations. And yet even in our short fleet problems, the endurance of the crews of the scouting planes is strained to extents which already make this a serious problem. Visualize the fleet at sea in time of war with the scouting operations forced to limits not reached in peace, which continue day after day and week after week. Consider the crews of the patrol planes working ten and twelve hours a day for months without end under the strain of war and at noise levels allegedly physically and psychologically detrimental to human health. It seems probable that their military efficiency would be greatly increased, with consequent military gain to the fleet, if the aerial decibel could be curbed to some extent and not allowed to operate totally unrestrained as it does in our present types of naval aircraft. German naval authorities are reported to have stated that the Diesel engine installations of the newer “pocket battleships” will be greatly modified in order to reduce the dangerously high noise level encountered in the engine-room of the Deutschland. If increased military efficiency as a whole resulted, might it not be worth while to reduce the noise level of our naval planes, even at the expense of a few pounds of pay load?
Turning now to the second theater of operations of the aerial decibel, let us examine some of the military effects of its misconduct. We especially desire an enemy not to hear a naval airplane under certain circumstances. Everyone knows that aircraft are usually heard before they are seen, and that very frequently they cannot be picked up visually even though they are heard distinctly. It would certainly be a decided advantage to delay as long as possible the knowledge of an impending bombing attack, or to obtain a contact report from an unseen aerial scout. When atmospheric conditions are favorable, an aircraft can often see a battleship without being observed at all, but very few cases are on record where an airplane heard but couldn’t see a surface vessel. A pilot 2,000 feet over a firing ship can distinctly “feel” the shock of each salvo but cannot hear the guns.
Despite many difficulties, sound-locator apparatus is being steadily developed to a high degree of efficiency in shore installations, and has been used actually to direct by mechanical means the train and elevation of searchlights and anti-aircraft guns. Almost all modern schemes embody the extensive use of sound locators for the protection from aircraft bombardment of important and logical shore targets. These locators form the first outpost of the antiaircraft defense and may be so disposed that they can give ample warning to the most sluggish of gun crews and enable retaliatory pursuit planes to take the air. The ships in the distant screen of the cruising fleet of the future may be equipped with such locators. Nor are we in the realms of fancy when we state that sound locators may ultimately be capable of such accurate gun control as to maintain an effective fire against an aircraft squadron proceeding to its objective over a thick cloud layer which completely obscures it to view from the ground. The best apparatus so far developed is reported to be unable to pick up an airplane whose propeller is turning at idling speed. It seems probable that in time of war the heavy bombardment planes at least will be forced to control the nefarious decibel if they are to avoid ceding marked advantages to the anti-aircraft defense. The surprise element has long been one of the principal reasons for the efficacy of aircraft attacks, and that efficacy bids fair to be considerably curtailed by the general use of the sound locator. Plane development in peace time cannot deliberately ignore the grave military consequences of failing to appreciate and keep pace with the peace-time development of anti-aircraft guns, searchlights, and sound- locator equipment.
The noise level on a carrier deck with all engines warming up is certainly very high. So great is the din if a pilot clears his spark plugs by briefly opening the throttle, that it is doubtful if he could hear the fire siren. Speed and precision without unnecessary risk are conducive to efficient carrier operations, but these are difficult of attainment with vision largely handicapped by plane structures and if auditory communication of any sort is entirely denied to all hands.
The aerial decibel may even have a very small part in the constant struggle between the demands of the taxpayer on the one hand and the need for adequate naval appropriations on the other. The average military plane is more noisy than the average civilian plane because of larger horsepower, more strenuous operations, and less attention to noise control. How many are the letters of complaint in the vicinity of military air bases alleging that such and such a plane came so close to such and such a farm as to turn all the cows’ milk sour and scare the hens out of three days’ lays of eggs? Does a letter of regret and apology pointing out that the plane actually was 2 miles distant and 2,000 feet above serve to allay the irritation? Reduction of the noise of military aircraft might go far to reduce the number of irritated taxpayers and cows and hens. The aerial decibel may have only trifling effects on the average citizen, but the good will of the holder of the purse strings is sufficiently important to make every trifle count.
Adequate noise control may involve increased fire hazard and will certainly involve increased weight. Increased weight means decreases in bomb loads, top speed (or increased landing speed), maneuverability, range, and rate of climb.
For every line of development there must be a beginning and military noise control has never really been begun. However, investigations have shown that a simple noise-eliminating device of light weight will probably not be developed, and it is not contended that a comparatively noiseless airplane can be produced without considerable sacrifice in pay load. Airplane noise is attributable to two principal sources, the propeller and the engine exhaust, and to three minor sources, the noise of the engine, and of the wind stream around the various parts of the plane, and the vibration of panels remote in the structure from the cause of the vibration. It is believed that the logical means of attacking these sources of noise is not inconsonant with the lines of aeronautical engineering development along which we expect to find increased performance and military efficiency regardless of noise control.
The actual cause of propeller noise is not clearly understood. A steel propeller has been whirled in a sound-proofed chamber to such speeds that the tip speed was approximately 1,100 feet per second; and the noise was in excess of 100 decibels and extremely painful to the human ear. The tip speed of the propeller bears a definite ratio to noise generation, and it has been found that the noise can be reduced as many as 30 decibels by reducing the tip speed from 1,000 to about 650 feet per second. Thus, large propellers at low speeds produce less noise than small propellers at high speeds, and it is the large low-speed propeller that is most desirable from a purely aerodynamic point of view. The efficiency curves of propellers fall off very rapidly at a tip speed of about 1,000 feet per second, and it is at that speed that the decibel output approaches the painful point. Experiments made in England indicate that a diminution in the section of the propeller blade would give a diminution of noise of the same order, and a similar difference would be obtained if four blades were used instead of two. In this country experiments were made with the view to decreasing engine back pressure by throwing the exhaust gases centrifugally from slots in the propeller blades. This diligent search for increased engine power produced an experimental propeller of the same type. Search for a more silent propeller may produce unlooked for developments in increased propeller efficiency. Aerodynamic efficiency and noise control are not necessarily poles apart.
At cruising speeds the noise level produced by the exhaust gases and the propeller are about the same and approximate 100 decibels. The Bureau of Standards has made several interesting experiments with aircraft mufflers in which noise diminutions as high as 16 decibels were obtained without unduly heavy installations. And 16 decibels is a very considerable amount in the upper ranges of the logarithmic decibel scale; in reducing 100 deibels to 84, 9,640,000,000 sound intensity units are eliminated. The wide variations in the results obtained with several different types of mufflers indicate that herein may lie a rich field for experimentation. So efficiently cooled was one type that immediately after the load had been removed from the engine, the muffler was not uncomfortable to the touch. This would not appear to be a great fire hazard in an allmetal plane. In the night bombers of some continental powers mufflers and collector rings discharge above the top wing, conducing (1) noise reduction by muffling and the interposition of the wing mass, and (2) concealment of the blue exhaust flame from both the enemy and the pilot, impeding the former in spotting the plane and aiding the latter in seeing the ground. In 1931 the N.A.C.A. Laboratories made several experiments in the cowling of aircooled engines. Of some ten types of low drag cowlings tested, the one which gave the second highest speed increase to the plane had an exhaust collector ring in the trailing edge of the cowling. These experiments were made on a standard navy fighting plane whose top speed was increased from 145.2 m.p.h. in the uncowled condition to 161.6 m.p.h. using the collector-ring cowling. Although one lighter and simpler type of cowling gave a greater speed increase, the experiments serve to show that the collector ring is not entirely incompatible with increased performance.
The muffler and collector ring have been much condemned because of the increase in back pressure with consequent decreases in horsepower and speed. In many naval aeronautical activities high speed is not of transcendental importance. Except as one means of protection, for instance, speed is of no great moment to the battleship spotter. In many patrol operations there is reason to believe that speed is a distinct handicap and that such operations can be more efficiently performed over long periods of time by lighter-than- air craft simply because of their slower speed. So rapidly does the patrol plane pass over a fixed line which it desires to maintain under constant observation that inordinate amounts of gasoline are consumed in frequent flights and the logistic supply of fuel, planes, and men develops into a grave problem.
It is apparent that noise abatement sufficient to affect materially military efficiency of the crew can never be secured by the control of engine and propeller noises alone. Insulation and absorption methods must be resorted to before obtaining a working atmosphere in which the noise level is at all reasonable. This means the adoption of the closed cabin as standard military practice. Cabin planes have long been resisted by aeronautical personnel as being destructive to true military efficiency. It has been contended that the fighting plane must have an open cockpit in order that the pilot may sense the “feel of the plane” and thus obtain better dog fighting performance; that the scouting plane must not be handicapped by the visibility restrictions imposed by cabin structures; that inclosed cockpits militate against good performance by imposing increased weight; and that cabins render impracticable the employment of the free machine gun.
There are many arguments other than that of noise for the military cabin plane, and a marked tendency toward inclosed cockpits (uninfluenced, however, by noise considerations) is even now in evidence in service types. The cabin fighter can easily be heated (in connection with an engine muffler, incidentally), thus eliminating the added weight and extreme inconvenience of heavy flying clothing. The inclosed scout offers such greatly improved facilities for navigation and communication as to more than compensate for any visibility losses. It has been conclusively demonstrated that inclosed cabins may be so designed as to reduce considerably the drag now caused by open cockpits with windshields. High speeds have already dictated the use of extremely large windshields in order that the free gunner may be sufficiently protected from the wind blast to train and elevate his guns. The parasitic drag of such windshields is probably as great and possibly greater than that of completely inclosed free gun turrets developed on some foreign planes. In any even cabins may be so arranged as to open up for free gunnery. The average plane even in time of war will fly a good many hours without having occasion to use its guns.
It seems probable that the aerial decibel will continue its destructive and unfettered career just as long as service opinion makes no demand for its confinement. Heretofore the service has apparently accepted without serious question the dictum that noise control virtually cannot be mentioned in the same breath with true military efficiency, and as a result the U.S. naval plane of today gives precisely the same amount of attention to sound elimination as did the famous plane that flew at Kitty Hawk, North Carolina, in 1903. The decibel output of the 1933 model, however, undoubtedly surpasses many times over the puny efforts of the Wright Brothers’ 12-horsepower, 35-m.p.h. machine. The few researches that have been made indicate that noise control in aircraft must receive the consideration and attention of an experienced acoustical engineer from the moment that the first design projects are laid out.
It is submitted (1) that true military aeronautic efficiency is not solely a function of high speed and high pay load; (2) that the lines along which noise diminution may be developed are not necessarily diametrically opposite to those in which to find improved aerodynamic performance; and (3) that the detriment to military efficiency brought about by the aerial decibel justifies a concerted service demand to severely restrict those activities.