Clemans A. Powell

Community reactions to helicopter noise: Results from an experimental study

Reactions to low numbers of helicopter noise events (less than 50 per day) have been studied in a community setting utilizing a new type of study design. Community residents were repeatedly interviewed about daily noise annoyance reactions on days when helicopter noise exposures had, without the residents’s knowledge, been controlled for study design purposes. The effects of maximum noise level and number of noise events on helicopter noise annoyance were found to be consistent with the principles contained in Leq‐based noise indices. Although the best estimate of the effect of number of noise events is very nearly the same as that represented by the energy summation principle contained in Leq‐based indices, the possibility that the number of noise events has only a small effect on annoyance cannot be rejected at the conventional p<0.05 level. The effect of the duration of noise events was also found to be consistent with Leq‐based indices. After removing the effect of differences in duration and noise le...

Effects of conversation interference on annoyance due to aircraft noise

The annoyance and interference effects of aircraft flyover noise on face‐to‐face conversation were investigated. Twenty 5‐rain sessions of three flyovers for each session were presented to each of 20 pairs of female subjects in a simulated living room. Flyovers varied in noise level (55–79 dB A‐weighted) and spectrum (low‐or high‐frequency components). Subjects engaged in conversation for 10 sessions and in reverie for the other 10 sessions, and completed subjective ratings following every session. The ratings concerned the annoyance of the noise, the difficulty of conversing in the noise, and acceptability of the noise for conversation. Conversation interference was defined as an increase in vocal effort or cessation of talking during flyovers for each session. Annoyance was affected by noise level, but was not significantly different for the activities—reverie and conversation. A noise level of 77 dB (A‐weighted) was found to be unacceptable for conversation by 50% of the subjects. Furthermore, conversa...

Annoyance caused by propeller airplane flyover noise

Two laboratory experiments were conducted to examine the annoyance response of people to the noise of propeller airplane flyovers and to assess potential method of quantifying that annoyance. In each experiment, 64 subjects judged the annoyance of recordings of propeller and jet airplane operations presented at D‐weighted sound pressure levels of 70, 80, and 90 dB in a testing room which simulates the outdoor acoustic environment. The first experiment examined 11 propeller airplanes with maximum takeoff weights greater than or equal to 5700 kg. The second experiment examined 14 propeller airplanes weighing 5700 kg or less. Five jet airplanes were included in each experiment. Perceived noise level predicted annoyance better than A‐, D‐, or E‐weighted sound pressure level. Corrections for tones greater than or equal to 500 Hz generally improved prediction ability for the heavier propeller airplanes. Tone corrections generally degraded prediction ability for the light propeller airplanes. Duration correction...

A new aircraft interior noise simulator for psychoacoustic testing

NASA is conducting a research program in passenger response to aircraft interior noise to develop tools for use in design decisions for interior noise treatments. A new interior simulator has been built at NASA–Langley to be used in this program. The simulator is a shell fitted with interior trim and seats from 737/727 aircraft. It contains five listening stations, each having a pair of headphones for binaural signal presentation. Binaural recordings made in interiors of a number of aircraft were processed on a workstation into 50 sound stimuli. These were played back over the headphones and analyzed for repeatability within and between headphones. Initial results indicated that ranges averaged within headphones were 0.3 dB (A weighted), 0.3 phons and 0.7 dB (unweighted). When averaged between headphones, ranges were 1.0 dB (A weighted), 0.9 phons and 1.4 dB (unweighted). A first test in the simulator presented stimuli from propeller airplanes digitally modified to reduce the tonal components. Regressions...

Effects of a traffic noise background on judgments of aircraft noise

Subjects were asked to rate their annoyance to recorded aircraft noises when combined with several levels of road traffic noise in two sets of numerical category scaling tests. The aircraft and traffic noise levels were typical of a closed‐window indoor environment. For one set of tests, the mean background noise level was continuous, whereas for the other set of tests the mean background noise level was changed between each aircraft noise. Under the continuous background noise conditions, the rated annoyance was found to decrease consistently with increasing background noise level. For example, when the aircraft and traffic noise levels were of equal Intensity, the reduction in annoyance was the equivalent of a 5‐dB reduction in aircraft noise. For the discontinuous background tests, no consistent change was noted. Regression analyses of the subjective annoyance ratings showed high correlation with the incremental noise pollution level, LNP, developed by D. W. Robinson [NPL Acre Report AC49 (1971)].

Public acceptance of urban rotorcraft operations

Even though tiltrotor operations from city center to city center could greatly shorten travel times over moderate ranges, public opposition to intense urban rotorcraft activity has kept this possibility from being realized. One significant factor in this opposition is rotorcraft noise. Over the last 25 years, NASA has explored the subjective response to rotorcraft noise and developed low noise design concepts and noise abatement flight procedures. While low noise designs can be applied for future rotorcraft, this is not an effective near‐term means of reducing rotorcraft noise, because of the costs associated with replacement of helicopter rotor blades. Recent noise abatement research, which has been focusing on the development of tools and techniques to facilitate the design of quieter flight procedures for existing vehicles, has much more immediate application. While very little subjective response work has occurred recently, prior work at NASA in this area from the 1970s and 1980s is discussed. Lastly,...

Anticipated Effectiveness of Active Noise Control in Propeller Aircraft Interiors as Determined by Sound Quality Tests

Two experiments were conducted, using sound quality engineering practices, to determine the subjective effectiveness of hypothetical active noise control systems in a range of propeller aircraft. The two tests differed by the type of judgments made by the subjects: pair comparisons in the first test and numerical category scaling in the second. Although the results of the two tests were in general agreement that the hypothetical active control measures improved the interior noise environments, the pair comparison method appears to be more sensitive to subtle changes in the characteristics of the sounds which are related to passenger preference.

Aircraft interior noise research at the NASA Langley Research Center

The Structural Acoustics Branch at the NASA Langley Research Center has been a major contributor to the field of aircraft interior noise research for the past 15 years. In addition to contributions from its in‐house program, the branch has supported many contributions from other organizations through an active grant and contract research program. Although the current emphasis of the program is on advanced turboprop airplanes and helicopters, the goal has always been to develop and improve interior noise prediction methodology. This allows for the incorporation of appropriate control measures in new aircraft at the design stage rather than through the use of add‐on acoustical treatments with potentially large weight penalties and reduced effectiveness. The proposed presentation will concentrate on some of the more recent basic research activities concerned with the transmission of noise into aircraft interiors through both airborne and structureborne paths. These include modeling of the structural response...

Multiple‐event airplane noise annoyance

Two experiments were conducted in which subjects in a simulated living room environment judged the annoyance of sessions of airplane noise which contained different noise levels and numbers of flyovers. In the first experiment one, two, or four high noise level flyovers occurred at the beginning, middle, or end of 30‐min test sessions each of which contained a total of eight flyovers. In the second experiment, one, four or 16 flyover noises occurred in 15, 30, or 60‐min test sessions. The time‐of‐occurrence of the high noise level flyovers in the sessions did not significantly affect annoyance, but annoyance increased with the number of such flyovers. Annoyance decreased with test session duration but increased with the total number of flyovers in the test sessions. These results support an average energy model better than total energy, annoyance decay, or peak noise level models.

Effects of impulsiveness on noisiness of helicopter noise

An experiment was conducted to examine the effects of impulsiveness on the noisiness of helicopters. Subject groups located outdoors and inside two houses listened to and judged the noisiness of two helicopters and a propeller airplane, which was used as a reference. The impulsive characteristics of one of the helicopters was controlled by varying the main rotor rotational rate while maintaining a constant airspeed which resulted in other characteristics of the noise being held relatively constant. Other controlled variables included altitude and sideline distance. Preliminary analyses indicate that within each altitude and sideline distance condition, the level of impulsiveness is positively correlated with noisiness. Across helicopter types and flight conditions, however, the addition of an impulsive correction factor did not significantly improve the correlation between the noisiness judgments and noise units such as EPNL which is being considered as an international standard. Comparisons of results be...