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Noise generated in aircraft engines is usually suppressed by acoustically treating the engine ducts. The optimization of this treatment requires an understanding of the transmission and attenuation of the acoustic waves. A critical review is presented of the state of the art regarding methods of determining the transmission and attenuation parameters and the effect on these parameters of (1) acoustic properties of liners, (2) the mean velocity, including uniform and shear profiles and nonparallel flow, (3) axial and transverse temperature gradients, (4) slowly and abruptly varying cross sections, and (5) finite-amplitude waves and nonlinear duct liners.

In this study, indoor and outdoor measurements of jet aircraft noise were made at seven schools beneath the eastern approach paths to the Los Angeles International Airport. At five elementary schools, the jet noise in the schoolyards ranged from 96 to 118 dBC and constituted a high risk of hearing damage for the children. The frequency of jet aircraft overflights averages one per 2‐min interval for a total of about 200 per school day (8:30 a.m.–3:30 p.m.). The corresponding jet‐noise levels in the classrooms (80% of which are not air‐conditioned or acoustically treated) ranged from 80 to 96 dBA and caused continual disruption of communication and the learning processes. An investigation of aircraft operation alternatives indicated that the noise could be reduced by 10 dB or more if engine power were properly managed when the aircraft approaches the school areas, or if the aircraft approached at higher altitudes and landed further to the west on the unused 6000‐ft‐long portion of the 12 000‐ft‐long runways.

In aviation we are concerned with vehicles which, for the majority of their journey time, are well separated from the community by a vertical distance of some 5-8 miles. Over most of their flight they are the cleanest and quietest form of transport. The aircraft noise problem therefore is confined to the regions around the terminal area. The object of this paper is to define aircraft noise sources and to discuss means by which the terminal noise of advanced air-transport systems can be limited, in order to minimize the noise nuisance. From the mid-1950’s to the mid-19603 there had been a steady build-up in the noise of commercial aircraft operations due to the introduction of jet transports and the steady increase in both aircraft size and number of movements. In the early part of the last decade public reaction to the noise situation was slow. Jet transports were seen as an advance in technology and many were proud to be associated with their introduction. Although manufacturers were aware of the undesirable noise characteristics of the jet engine, technological advances were aimed more at improving thrust performance and cruise efficiency in order to achieve significant increases in range. The first step towards noise improvement was the introduction of the low by-pass/turbofan engines in the early 1960’s, which although quieter, did not make a significant impact on the noise scene. However, the introduction of the advanced high by-pass ratio engines which power today’s wide-bodied airliners has improved the noise situation dramatically (see Figure 1). Initially the use of high by-pass ratios was probably aimed more at the improvement of performance and economics than at noise reduction. However, as time went by and the environmental lobby became stronger, the emphasis on design for low noise increased. Regulatory bodies like the FAA and ICAO formulated noise requirements and certification rules which reflected public pressures to curtail the aircraft noise.

The Deutsche Forschung sqemenschaft and study of airport noise is reviewed, some further results are included. The study, which considered sociological, psychological and physiological impacts, also describes statistical and analytical methods.

Airports and airways around airports have until now produced more and more noise affecting people living in the vicinity. In order to solve this problem, or at last relieve the disturbances, airports have been located further and further away from city centres, and aircraft movements around airports have been more and more restricted to narrow airways. I think this way is a cul-de-sac. You cannot go on forever producing more and more noise and moving airports further and further away. Neither can you abandon ever-larger built-up areas because the air traffic development makes them unsuitable for living. And certainly you cannot alter the human way of reacting to stress--what remains is to construct aircraft producing less noise. Around each airport there ought to be established a demarcation boundary outside which air traffic will not be allowed to cause more than clearly defined noise values. If these values wre exceeded, traffic has to be cut down or certain types of aircraft have to be prohibited. Airports are likely to have a noticeable effect on the physical structure of a region. Therefore, the location of airports should be determined as a function of the desired regional structure. The following principle for siing an airport can be derived from the experience with Arlanda. If one wants to (a) make the most of the sites with good communication possibilities near an airport; and (b) make full use of the communication system established between the city centre and the airport, the corridor between the two should be developed extensively. But to do so, it is imperative to restrict flight-noise disturbances in this corridor. The best way of achieving this seems to be by placing the runway directions at the airport perpendicular to the direction of the corridor.

Wind-tunnel acoustic investigations were performed to determine the acoustic characteristics and the effect of forward speed on these characteristics of four propulsive lift concepts: the over-the-wing externally-blown jet flap (OTW), the under-the-wing externally-blown jet flap (UTW), the internally-blown jet flap (IBF), and the augmentor wing (AW). The data in this paper represent the basic noise generated by the powered-lift system without acoustic treatment, assuming all other noise sources, such as the turbofan compressor noise, have been suppressed. Under these conditions, when scaled to a 45,500 kg (100,000 Ib) aircraft, the OTW concept exhibited the lowest perceived noise levels because of dominant low-frequency noise and wing shielding of the highfrequency noise. The AW was the loudest configuration because of dominant high-frequency noise created by the high jet velocities and small nozzle dimensions. All four configurations emitted noise 10 to 15 PNdb higher than the noise goal of 95 PNdb at 153 m (500 ft). The AW is the only powered-lift concept which has shown the capability of acoustic suppression to this level. The effect of forward speed did not approach that expected from the relative velocity increments investigated. The dominant low-frequency noise of the OTW and UTW was reduced 2 db by an 80 knot free-stream velocity. The dominant high-frequency noise of the IBF and AW was unaffected by forward speed. Nomenclature OASPL = overall sound pressure level, db PNL = perceived noise level, PNdb SPL = sound pressure level, db referred to 2 x 10'5 N/m2 (0.0002 microbar) Vj = aver age isentropic jet exhaust velocity, m/sec (fps) V^ = freestream velocity, knots (fps) a = angle of attack with respect to wing chord plane, deg df = flap deflection with respect to wing chord plane, deg

NEF contours for aircraft noise have generally been computed assuming three or four well‐defined flight paths from a given runway. In fact, a whole continuum of flight paths normally exists. In this paper, methods are developed to treat this continuum of flight paths. Simplifications are also suggested which provide realistic approximations and which allow preliminary calculation of NEF values without the need for major computer programming. These have the form NEF = NEFC + correction, where NEFC assumes all flights are overhead, and a (negative) correction term is added for the splayout. Results of the conventional discrete path computations are contours shaped like fingers. Using a continuous spread of paths results in a single fanned‐out contour. We feel the latter is more realistic.

Data from controlled flight testing conducted for 11 aircraft noise certification programs under FAA FAR 36 show typical standard deviations for sets of EPNL and PNL values ranging from 0.8 to 1.4 dB. Much greater variability was found in the noise levels measured near the approach path to runway 21R at Detroit Metropolitan Airport during two ten‐day periods of routine operations. EPNL, SEL, and maximum A‐level data, classified in jet aircraft groups by number of engines (regardless of type), showed typical standard deviations ranging from 3.8 to 4.9 dB at the different ground measuring points. Correlation of the Detroit noise data with slant distance gave moderately high correlation coefficients, with 95% prediction intervals about the regression lines varying from ±5.4 to ±9.5 dB. The regression lines show quite good agreement, particularly in slope, with generalized noise level versus distance curves often used for noise exposure calculations. Correlations between different noise measures were high, as...

The response criteria problem in annoyance surveys, particularly with respect to prognosis, is dealt with in this paper. The criteria problem is illustrated with data from self-rating questions in a survey conducted in five areas with different exposures to aircraft noise. The results show that the respondents' response criteria cannot be pressupposed to be independent of the exposure conditions. If one wants to measure mean degree of annoyance evoked by an environmental agent in different areas, calibrated scales must be used. It was shown that the response criteria variation in itself may be used as an indicator of the exposure condition. The simultaneous measurement of response categories and stimulus reactions is recommended in annoyance surveys.

1. Aircraft Noise—Its effect on people OF all the environmental noise sources, aircraft noise has received the most notoriety over the longest period of time. Aircraft noise is generally assumed to be a product of the jet age, but in 1939 in the United Kingdom, the Gorrell Committee on Control of Flying gave noise control the highest priority. However, it was not until post‐World War II that the public became disturbed by the continued high level of military aircraft operations, and this disturbance was aggravated by the rapid introduction of jet‐powered aircraft. Civil aviation developed rapidly and, as airports increased in size, and as aircraft movements doubled every five years, the surrounding communities expanded to the airport boundaries. These conditions of growth provided the ingredients for the aircraft noise problem. In the United Kingdom, all the major airports grew from military installations where little regard had been given to the environmental impact of these sites on the surrounding communities.

Various studies of sound transmission in urban areas that involve multipath propagation or multiple reflections are reviewed. Studies of propagation in a single street or corridors display important descrepancies between theoretical analyses and field experiments. Model studies suggest that scattering may play an important, but as yet unexplained role. Sources above the street show good correspondence between field and laboratory data and theory. Intrusion of aircraft noise into an urban area shows that sound may be slightly amplified above an open‐terrain condition due to channeling effects. The general urban background noise appears to be accounted for by surface transportation sources distributed over the city, aside from a shielding factor of 10–15 dB of as yet undetermined origin.

A telephone survey was conducted to assess the effects of cessation of late-night overflights on communities to the east of Los Angeles International Airport. A total of approximately 1400 interviews were conducted before, immediately after, and a month after the cessation of flights. No appreciable effects were observed in terms of reduction of annoyance, or lessened speech or sleep interference. However large, consistent and statistically significant differences in response patterns as a function of level of noise exposure were observed both before and after cessation of late-night flights. These results can be accounted for either in terms of the relatively small decrease in 24-hour exposure that the elimination of flights between 23.00 and 06.00 hours represented, or in the light of a parallel study which failed to show recovery in the abnormal sleep patterns of long term area residents. The significance of these data for such issues as the proper weighting of nocturnal noise events or the appropriate weighting for numbers of events depends on which of the two alternative interpretations is favored.

The effects of aircraft noise on teaching and classroom activity were studied in a number of schools close to Heathrow Airport, both by direct observation and by a small sample survey of teachers' opinions. The principal changes in observed behaviour result from interference with speech and this finding corresponds with the survey of teachers' opinions. The study was unable to identify any other consistent or systematic changes in class activities directly related to aircraft noise. Teachers speaking to whole classes pause more frequently with increasing peak levels over a wide range of flyover levels. Since they pause during at least one flyover in four of those which peak at or above 70 dB (A) one may assume such flyovers cause appreciable discomfort. When talking to individuals or small groups teaching is less vulnerable to interference and is not seriously affected during flyovers which peak below 75 dB (A). Above this level there is a rapid increase in pausing and in the masking of the teacher's speech when addressing the whole class. The nuisance caused by flyovers peaking below 70 dB (A) depends on the nature of the activities and the level of background noise in the classrooms which was observed to vary between 55 and 70 dB (A).

Interest in helicopter noise has increased dramatically over the past few years. The Greater London Council, in connection with a public inquiry into a proposed small heliport in London, issued noise standards for helicopters which they considered would be acceptable for use at this site. These standards were inferred by many to be the requirements for all helicopters suitable for use in London. This is not the case and, of course, the noise standard for other heliport sites would to some extent be dependent on their size and location. Thus, in the opinion of the author, the GLC standard should not be taken as a typical requirement, since in many instances it would be over demanding. Publication of these noise limits, however, had the impact of focusing attention on helicopter noise. The Federal German Republic has recently introduced a law for controlling aircraft noise around airports. Even though this specifically excludes helicopters, the FGR have indicated that the noise standards will be applied to evaluating all sites including heliports/helipads and helicopter operators, both military and civil, will be expected to comply with the requirements. Westland Helicopters Limited have in the past few years been attempting to standardize measurements/flight test procedures for the evaluation of helicopter noise. Close relationships have been established with the Greater London Council and Aerospatiale in France and these have lead to an increased awareness of the problems involved, particularly in rating the actual noise level. More recently the issue of the ANPRM for noise certification of short-haul aircraft, including helicopters, by the FAA has intensified interest. To date methods of assessing helicopter noise have simply been adopted from traffic noise or aircraft noise standards. There are, however, some fundamental aspects associated with helicopter noise which must be taken into consideration, and in this paper the limitation and implication of using conventional methods are discussed. Emphasis is placed on rating helicopter noise in terms of a “single event” and no attempt is made to correlate the findings on a noise zone or ground contour basis. The relative merits of using digital and analogue analysis techniques are also examined.

An investigation was conducted to determine subjective response to aircraft noise in different road traffic backgrounds. In addition, two laboratory techniques for presenting the aircraft noise with the background noise were evaluated. For one technique, the background noise was continuous over an entire test session; for the other, the background noise level was changed with each aircraft noise during a session. Subjective response to aircraft noise was found to decrease with increasing background noise level, for a range of typical indoor noise levels. Subjective response was found to be highly correlated with the Noise Pollution Level (NPL) measurement scale.

Noise masking of television audio signals by flyovers is the most frequently mentioned problem of airport neighbors. This problem was studied in the laboratory using artificial noise. Three studies varied the intensity, duration, and rate (noises per hour) of the noises. Acceptability was found to approximate a logarithmic function of noise energy for changes in intensity, duration, and rate. Thus, the equal‐energy principle, which has been generally observed in abstract psychometric experiments, was confirmed in this more realistic and representative task. A fourth study showed aircraft‐flyover recordings to be more acceptable than artificial noises even though they had equivalent peak levels and masking durations. This difference can also be explained by the equal‐energy principle, since the flyovers had lower average (or integrated) energy.

In order to examine the effects of aircraft noise on the mentality of growing children, a simple search task and an adding task were applied to 1144 elementary school pupils who live around an airport, and in a quiet area, under the conditions of no stimulus sound, and jet noise stimulus 90 ± 5 dB(A) respectively. The result was that children from relatively noisy living areas tended, when performing tasks, to show occasional short periods in which they produced substantially less than their own average rate of work. A similar difference did not appear when working in noisy rather than quiet conditions, and it was considered to be something chronic about the children themselves. These results were independent of the sex of the subjects and the feelings of the subjects about aircraft noise.

A re-analysis program has been undertaken to determine the relation between annoyancedue to aircraft noise and noise exposure expressed as the dB(A) peak value. Areas reexamined from a Dutch, German and Japanese study were found to be exposed to more than 50 overflights per 24 hours, thus placing them in the high exposure category, as defined by the principles developed in the Scandinavian aircraft noise investigation. When the mean exposure effect in the population was expressed as percent interference of conversation a close similarity was found in the data from different countries and a high correlation was demonstrated betweethe noise exposure and the mean effect in the population. The practical implications of the results are discussed.