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Aviation History
1959
1959 - 1794.PDF
FLIGHT, 14 August 1959 11 600 Take-off noise: comparative values expressed in nays 4OO 8OO 1,200 1,600 2.OOO 2.4OO 2.SOO 3.2O0 DISTANCE TO AIRCRAFT (ft) Notes: (1) The distance to the aircraft is measured in each case from an observer on the ground directly beneath the flight-path. (2) The noisiness values in noys are directly comparable; thus, at a distance of 1,200 ft. the noisiness of the Boeing 707-120 and the "composite N.Y.I.A." (a mean propeller-driven aircraft at New York International Airport) are respectively 223 and 90, i.e., the Boeing at this distance on full power is two-and-a-half times as noisy as the composite aircraft. (3) References: Reports (October 19S8) by Bolt, Beranek and Newman to the Port of New York Authority; Flight for November 7,1958; British Standard 3045 :1958 NOISE IN THE JET AGE . . . The experiments carried out by more than one team led to the definitions of "loudness" as an observer's auditory impression of the strength of a sound and of the "sone" as the unit of loudness on a scale designed to give scale numbers proportional to loudness. The numerical value of one sone was assigned to the loudness of a sound judged by normal observers to be equal in loudness to a pure tone of frequency 1,000 cycles per second with a sound pressure level, instrument-measured, of 40db above the standard reference pressure of 0.0002 dynes/sq cm. A further dimensionless unit, the phon, was invented to connect the sone to sound pressure-levels. The loudness level, in phons, is related to the degree of loudness by the equation (P-40)/10 5=2 . where S=soncs and P=phons. It will be noted that where P=40, S=l and an increase of 10 phons doubles the lower value of loudness in sones . It should further be noted that, as sone numbers are proportional to loudness, if one sound has a loudness of 64 sones and another of 128 sones, the second sound is twice as loud as the first. Sounds are generally composite (i.e., are made up of various frequencies), and the loudness of sounds at the various frequencies is different for the same sound pressure-leveL For instance, from Fletcher-Munson curves of loudness levels, with the same sound pressure level of lOOdb, the loudness in sones at 100 c/s is of the order of 64 sones whereas at 4,000 c/s it is about 119 sones, approaching double loudness. This is an example of the futility of quoting sound-pressure levels as a means of comparison be- tween, the loudness of sounds. Even though the development of a method of producing sub- jective units of sound loudness was a great advance, and even though it has become possible by measuring sound pressure-levels at the various frequencies which generally constitute an overall sound and from them derive an overall comparable figure of loud- ness, even so there has still been something lacking. "Noise" is the missing factor. We have had "sound" measured as_ a pressure level, and we have had "loudness" of a sound, a subjective human- ear appreciation of comparable sounds at various frequency. But "noise"—unwanted sound, sound undesired by the recipient—has not up until recently been assessed scientifically. It has been left to the Port of New York Authority—deeply concerned, as Flight has mentioned, at the possibility that action would be taken in the courts by local residents to shut down the International Airport because of noise—to institute tests on the new jets, the Boeing 707-120 and the Comet 4, with a view to regulating the conditions under which they could operate. The firm of acoustic consultants who undertook the task of making the tests, Bolt, Beranek and Newman Inc., faced with a position in which the annoyance caused by aircraft was to be the criterion, decided that basic data were required. They made a very full series of observations of people's reactions to the "noisiness" as distinct from the "loudness" of die sounds from an aircraft. The question asked of the observer was not, as previously, to match a sound to an equal in loudness, but to adjust the intensity of the comparison sound until he or she found them to be equally annoying; and "equally annoying" was to be interpreted in this way: that the subject, the observer, would under ordinary condi- tions of daily life find the comparison sound (as adjusted) to be no more and no less annoying (or acceptable) than the presence of the standard noise (as given). The results were startling. Mention has already been made of the fact that the "loudness" of a sound at 4,000 c/s and a sound pressure-level of lOOdb is nearly twice that of a souqd at 100 c/s and the same pressure-level. When the same comparison is made on the basis of annoyance the units of noisiness are 42 for the frequency band, 75-150 c/s at lOOdb sound pressure-level and 135 for the frequency band 2,400-4,800 c/s at the same sound pressure-level. The noisiness of the higher frequency sound is 3.2 times that of the lower frequency sound. Sound pressure levels show themselves to be even more misleading when noise is dis- cussed than when loudness is considered. The unit of noisiness has been termed the "noy" and a numeri- cal value of 1 noy has been assigned to the perceived noise in the octave band from 600-1,200 c/s of random noise at a sound pressure-level of 40db above 0.0002 dynes/sq cm. The perceived noise level (PNdb) is related to the noisiness of a sound by an equation given in the report in the form 1.2-HogioN 0.03 with PNdb=perceived noise level and N=noys. This is an approximation to the form of equation relating phons and sones already mentioned and can more accurately be given as Professor Stevens of the Psycho-Acoustic Laboratory at Har- wood University developed a method of calculating the total loud- ness of a sound from its loudness in each frequency band. This method has been applied by the consultants to the results they obtained from their subjective tests. Given, therefore, the sound pressure-levels of an aircraft, as measured by instrument at any recording station for each band of frequency within ranges spreading from 20-75 to 4,800-10,000 c/s, it is possible to calculate that such an aircraft is at that location making a noise of so many units of noisiness. If the spectrum of sound from a second aircraft at equal distance is also measured and a figure of noisiness derived, the two figures can be directly compared. For instance, Flight in its issue of November 7,195JS, published a summary table of the perceived noise levels of the Boeing 707- 120, Comet 4 and a composite propeller-driven aircraft, all on full power, at various distances from an observer and under equal con- ditions of wind, temperature and pressure. At a distance of 1,000ft the three comparable outdoor figures of perceived noise levels for Boeing, Comet and a composite propeller-driven aircraft are 121, 120 and 107. From the formula (PNdb-4O)ll0 , JV=2 it will be seen that the noisiness figures of the three were 274,256 and 104. In other words, the noisiness of die two jets is about 2£ times that of the composite propeller-driven aircraft. Even at 2,000ft where the more objectionable higher frequencies would be reduced in their potency, the three figures of PNL were 109, 110 and 100, giving noisiness figures of 119,128 and 64. The jets make double the noise at this distance. (See accompanying illus- tration.) The problem is stated. People around London Airport are troubled by noise. How much noise? What does the Minister mean when he says that the Boeing 707 and Comet 4 will operate into London Airport "under conditions which are designed to cause no more noise disturbance than the heavy piston-engined aircraft currently in use, when passing over the main built-up areas in the neighbourhood"? Does he mean "noise" as defined for the Port of New York Authority? At what height are the piston-engined aircraft passing when the jets are, as jhe Minister guarantees, "at a height of not less than 1,000ft over the nearest built-up area along the flight-path" when taking off? How much noise is coming from the Tu-104 and the Caravelle? What steps are being taken to reduce poise? Has a level of noise at the airport perimeter, denned in units of noisiness, been deter- mined for the future, or arc people to suffer from an ever-increas- ing bombardment from noise in the hope that their dulled senses will cause them to submit? If there are to be clear and unambiguous statements on noise and the present difficulty in interpretation be overcome, a unit of noisiness must be quoted in future which satisfies scientific requirements and the people affected. Shall we start to talk about "noys"?
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