Flight Noise Research Articles

A quadrupole model for jet acoustical shielding

A quadrupole model for the prediction of the effects of acoustical shielding by a turbulent jet is developed. This is accomplished by using a reciprocal theorem applied to a known model which employed a monopole source. Results are compared to aircraft flight noise measurement data for shielding jet Mach numbers of from 0.5 to 1.3 with close correlation obtained for frequencies near the noise spectrum peak.

Annoyance and health risk caused by military low-altitude flight noise

Effects of noise of low-flying military jet aircraft were investigated from demoscopic and epidemiological points of view. Areas with different low-altitude flight noise exposure were compared with one another as to subjective annoyance, casual blood pressure and ear symptoms. With the same energy equivalent sound pressure level (Leq), the subjective disturbance caused by military low-altitude flight noise was essentially greater than that due to ordinary flight noise (in the neighbourhood of civil airports). A comparison of several areas revealed that frequencies of ear symptoms (tinnitus lasting more than one hour and permanent hearing threshold shifts of greater than 30 dB) were higher only in areas where maximal flight noise levels considerably exceeded 115 dB (A) accompanied by rapid noise level increases. Blood pressure measurements yielded significantly higher values (group difference 9 mm Hg systolic) in girls living in these highly exposed areas. Acoustic limits are proposed with respect to public health.

Acute circulatory effects of military low-altitude flight noise

Volunteers aged 70 to 89 years living in a senior citizen's home in Haifa were exposed to flight noise via earphones while watching video films. Their blood pressure and heart rates were measured simultaneously. A high-quality recording and reproduction technique was employed. They were exposed to the noise of two to three overflights with Lmax = 99-114 dB(A) and slow sound pressure level increase (aircraft take off) or with Lmax = 95-112 dB(A) and a fast sound pressure level increase (low-altitude flight at high subsonic speed) at intervals of 10 to 15 min. The systolic and diastolic blood pressure was raised at Lmax = 112 dB(A) and high speed level increase at the average of 23 and 13 mmHg, respectively with individual maximal values of about 40 mm Hg (systolic). In order to prevent risks to the subjects' health, the noise exposure was not raised to levels above 112 dB(A) and fast level increase, although Lmax = 125 dB(A) has been measured in 75 m-low-altitude flight areas. The blood pressure response to a repeated single exposure increased in proportion to the preceding noise exposure. At high intensities and fast level increase an up to fourfold reaction intensification was detected in the majority of subjects. This change in reactivity is regarded as the result of sensitization toward the special type of noise and the implications of these observations for the long-term effects of chronic exposure to low-altitude flight noise are considered. On the basis of these results, proposals are made for limiting values for Lmax and for the speed of sound pressure level increase, the implementation of which would lead to a marked reduction in health risks from low-altitude flight noise.

Medically relevant effects of noise from military low-altitude flights—Results of an interdisciplinary pilot study

Laboratory experiments and field measurements were carried out to determine relevant acute and long term effects of noise from low-flying military aircraft. Acute effects on the cardiovascular system were established under simulation conditions. Blood pressure, heart rate, and biochemical stress parameters (cortisol and catecholamines, renin and free fatty acids in blood) were measured as a function of time. Long-term effects of noise from low-flying aircraft were analyzed in 10 to 13-year-old children by comparative studies including a 75 m low-altitude flight area and a neighbouring area with flight altitudes of 150–450 m. Significantly higher blood pressure values were found in girls in the 75 m area. The hearing thresholds were significantly increased in boys and girls in the 75 m area. Altogether, acute effects of noise from low-flying aircraft on cardiovascular functions and hearing ability could be proved, and potential risks of long-term exposure to low-altitude flight noise are discussed.

Measured Far-Field Flight Noise of a Counterrotation Turboprop at Cruise Conditions

Modern high speed propeller (advanced turboprop) aircraft are expected to operate on 50 to 60 percent less fuel than the 1980 vintage turbofan fleet while at the same time matching the flight speed and performance of those aircraft. Counterrotation turboprop engines offer additional fuel savings by means of upstream propeller swirl recovery. This paper presents acoustic sideline results for a full-scale counterrotation turboprop engine at cruise conditions. The engine was installed on a Boeing 727 aircraft in place of the right-side turbofan engine. Acoustic data were taken from an instrumented Learjet chase plane. Sideline acoustic results are presented for 0.50 and 0.72 Mach cruise conditions. A scale model of the engine propeller was tested in a wind tunnel at 0.72 Mach cruise conditions. The model data were adjusted to flight acquisition conditions and were in general agreement with the flight results.

Modulation of auditory information processing in tethered flying locusts

1. The activity and the response to auditory stimulation of the auditory nerve, of single receptor fibres and of thoracic auditory interneurons were recorded and analysed in resting and in tethered flying locusts (Locusta migratoria). During the behavior in the auditory system (I) flight induced activity and (II) modulations of the auditory response to sound pulses occurred. It was analysed whether flight-induced activity and the modulations of the auditory response were due to influences affecting the auditory receptors and/or interneurons. 2. Flight evoked tonic and phasic activity in the tympanal nerve and in single auditory receptors. Receptor activity was caused by thoracic movements stimulating the auditory organ and additionally by flight noise (Figs. 1–3). 3. Auditory interneurons also received excitatory input during flight. The activity of the interneurons was either markedly reduced or totally eliminated when the auditory organ was destroyed. Thus flight activity in auditory interneurons was largely or entirely caused by the auditory receptors (Fig. 4). No evidence was found for the flight pattern generator having a discrete effect on auditory interneurons. 4. The summed nerve response to auditory stimuli (10 ms, 70 dB SPL white noise) was reduced to 19% during flight. This corresponded to a decrease in sensitivity of 20 dB. Appropriate reductions of the auditory response also occurred in single receptor fibres (Figs. 1, 3). 5. The response of interneurons to sound pulses was reduced during flight to 26–35% of the response at rest (Figs. 5, 6). If the auditory organ was destroyed and the auditory nerve was stimulated electrically, no reduction of the electrically evoked response occurred (Fig. 7). Thus the auditory response reduction during flight must be due to influences affecting the auditory organ. 6. In one auditory interneuron flight enhanced the response to auditory stimulation and to electrical stimulation of the auditory nerve due to an additional sensory excitation of the neuron (Figs. 5–7).

A comparison between the SR‐6 propeller noise in flight and in a wind tunnel

High‐speed turboprops offer an attractive candidate for future aircraft because of their high propulsive efficiency. However, one of the possible problems associated with these propellers is a high level at cruise that might create a cabin environment problem for future aircraft powered by these propellers. Models of these propellers have been tested for acoustics in the Lewis 8‐by‐6‐ft wind tunnel and on the Dryden JetStar airplane. This paper shows comparisons between the airplane and wind tunnel data for the SR‐6 propeller. The comparison of the maximum blade passing tone variation with helical tip Mach number taken in flight with that taken in the tunnel showed good agreement when corrected to the same test conditions. Directivity comparisons also showed fairly good agreement. These good comparisons indicate that the wind tunnel is a viable location for measuring the blade passage tone of these propellers.

Noise and detectability characteristics of small-scale remotely piloted vehicle propellers

Several small-scale propeller configurations applicable to a conceptual remotely piloted vehicle (RPV) aircraft were designed, fabricated, and tested to determine their performance, acoustic, and detectability characteristics. The tests were conducted in static and simulated forward flight conditions in a wind tunnel. Propellers tested included tractor, pusher, and ducted configurations. The acoustic data obtained were used to determine the slant range and altitude of no detection of each propeller configuration. The acoustic and detectability characteristics of small-scale RPV propellers were found to be significantly different from those of the large-scale propellers. An increase in forward velocity resulted in a significant drop in the SPL's at higher harmonics of blade passage frequency. As expected, tip speed had a very strong effect on noise and detectability in forward flight. It was found that most of the propellers tested would be detected at one of the first few harmonics (mostly first or second) of their blade passage frequency. Of the propellers tested at the design values of thrust and tip speed, three-bladed propellers were generally less detectable than either twoor four-bladed propellers for most of the forward velocities considered. Pusher and ducted propellers were found to be more detectable than their tractor and open counterparts.

A preliminary comparison between the SR‐3 propeller noise in flight and in a wind tunnel

The noise generated by supersonic-tip-speed propellers is addressed. Models of such propellers were tested for acoustics in the Lewis 8-by-6-foot wind tunnel. One of these propeller models, SR-3, was tested in flight on the Jetstar airplane and noise data were obtained. Preliminary comparisons of the maximum blade passing tone variation with helical tip Mach number taken in flight with those taken in the tunnel showed good agreement when corrected to the same test conditions. This indicated that the wind tunnel is a viable location for measuring the noise of these propeller models. Comparisons of the directivities at 0.6 and 0.7 axial Mach number showed reasonable agreement. At 0.75 and 0.8 axial Mach number the tunnel directivity data fell off more towards the front than did the airplane data. A possible explanation for this is boundary layer refraction which could be different in the wind tunnel from that in flight. This may imply that some corrections should be applied to both the airplane and wind tunnel data at the forward angles. At and aft of the peak noise angle the boundary layer refraction does not appear to be significant and no correction appears necessary.

Mean Wake Characteristics of an Aerodynamically Loaded Fan

O NE of the main mechanisms responsible for the generation of turbofan noise in flight is the interaction of the unsteady rotor wake field with the fan stators. Analysis of the strength of this interaction noise source requires a knowledge of the rotor-produced flow disturbances as a function of radius and downstream distance. Thus the validity of noise-related design choices such as rotor-stator spacing and the determination of the position of the blade span most responsible for noise generation depend on the rotor wake flow field characterization/The parameters usually described are the magnitude of the wake defect and its width. The purpose of the present experiment is to determine these parameters along with the harmonic content of the rotor wakes in order to provide wake parameters of interest to acousticians and fan designers. Contents Existing rotor wake data were primarily obtained from lowspeed laboratory fans.1'2 The present study was conducted to measure wake properties under conditions that are more representative of practical turbofan operation. The experiments were performed on a 0.5-m-diameter fan with a 1.2 pressure ratio in the NASA Lewis 9x15 ft anechoic wind tunnel.3 The fan was tested at a tip speed of 197 m/s, equal to 96% of the design speed (8020 rpm.) Rotor wake properties under static (£7=0) and forward velocity conditions (£7=41 m/s) were measured with a stationary cross-film anemometer circumferentially located midway between two stator vanes and oriented to measure the streamwise and upwash components of the velocity downstream of the rotor, as indicated in Fig. 1. The anemometer signals were analyzed in a Fast Fourier analyzer, utilizing an ensemble averaging technique in which the analyzer was triggered by a once-per-revolution signal. Data from radial locations at the midspan and near the fan tip are presented for three downstream locations ranging from 0.54 to 1.77 rotor

Installation effects on propeller noise

The installation effects on propeller noise and propeller wake flow in flight have been examined experimentally by operating a model-scale propeller in the Lockheed anechoic open-jet wind tunnel. In particular, two aspects of propeller operation in a real situation have been quantified. These are: 1) the effects of nonzero angle of attack or propeller inflow angle relative to the flight path, and 2) the propeller inflow distortion due to the upwash generated by the presence of wing and flap behind the propeller. The results show that not only are these installation effects very important, but they are predicted inadequately using existing methods.

Empirical method for the prediction of business executive jet cabin noise levels

As a result of flight noise measurements made at various locations in the cabin of the standard lined/no interior Westwind model 1124 business executive jet, it was possible to develop an empirical method for predicting the overall sound pressure level (OASPL) at any required location in the cabin. The cabin overall sound level in decibels (linear) may be found from nomographs related to aircraft altitude, mach number or velocity. The noise spectrum at any location may be found from a reference spectrum shape corrected for local parameters. The accuracy of the prediction method, verified by additional tests, was found to be ± 1 dB.

Effect of a boundary layer about a nozzle on jet noise in flight

It is demonstrated that the presence of a relatively thick boundary layer about the outside of a nozzle can explain observed differences in jet noise between flight simulation experiments (usually thick boundary layers) and flyover experiments with engines mounted in nacelles (thin boundary layers). To study the boundary layer influence, the uniform velocity profile of a single stream static jet is replaced by a coannular jet in flight. The momentum loss in flight due to the external boundary layer is considered by reducing the velocity of the annular portion of the jet properly. A scaling law that relates flyover to static coannular jet noise (A. Michalke and U. Michel, AIAA Paper 80-1031) and prediction methods for static coannular jet noise as implemented in the NASA Aircraft Noise Prediction Program are then used to study the effect of the external boundary layer on the flyover jet noise directivities. [Research was accomplished while the first author held an NRC Senior Research Associateship.]

Empirical method for prediction of business executive jet cabin noise levels

As a result of flight noise measurements made at various locations in the cabin of the standard lined/no interior Westwind model 1124 business executive jet, it was possible to develop an empirical method for predicting the overall sound pressure level and the 13rd octave band spectrum level at any required location in the cabin. The cabin overall sound level in dB (linear) may be found from nomographs related to aircraft altitude, Mach number, or velocity. The noise spectrum at any location may be found from a reference spectrum shape corrected for local parameters. The paper further presents computer programs for the TI‐58 calculator for the generation of the nomographs. The accuracy of the prediction method when verified with additional test performed was found to be ± 1 dB.

Flight Effects on Noise Generated by a JTSD Engine with Inverted Primary/Fan Flow

A JT8D-17R engine with inverted primary and fan flows was tested under static conditions and in the NASAAmes 40 X 80 ft wind tunnel to determine static and flight noise characteristics. The major purpose of the program was to evaluate flight effects on noise generated by the inverted flow profile of a large-scale engine. This jet noise suppressor concept is of particular interest to advanced supersonic transport engine cycle studies where high jet velocities create a serious noise problem during takeoff operation. The engine with inverted flow was tested with conical, plug, 20-lobe nozzles, and an acoustic shield. Wind-tunnel results show that forward velocity causes a significant reduction in peak PNL suppression, but only modest reductions in peak OASPL and EPNL suppression. The flight EPNL suppression ranged from 4.0 EPNdB for the basic inverter to 7.5 EPNdB for the inverter with 20-lobe nozzle and acoustic shield. When compared with the JT8D engine with internal mixer, the inverted flow configuration provides comparable EPNL suppression under flight conditions.

Prediction of jet noise in flight from static tests

The sound intensity of jet noise from aircraft in flight is derived in a co-ordinate system fixed to the jet engine. For this reason a convected form of the Lighthill equation is solved, with special care taken of jet temperature effects. Under certain assumptions and approximations, the in-flight and static sound intensities are related in a simple manner. Thus the directivity of jet noise in flight can be predicted. The theoretical result is checked with measurements. The agreement is remarkably good.

Flyover noise of a wide body aircraft

Aircraft noise in the vicinity of airports is a well‐recognized problem. Rules and regulations are in effect that set limits on the noise levels of aircraft. Although the airplanes in the commercial fleet meet the appropriate regulations, there is always a constant effort within The Boeing Company to lower the noise levels. This requires a thorough understanding of the source mechanisms and propagation effects. The total noise of an airplane is composed of engine noise and airframe noise. The engine noise itself includes various components such as jet, core, fan, and turbine noise. There has been a lack of good quality flight noise data base, especially for wide body aircraft with high bypass ratio engines. To alleviate this, several test programs have been completed within The Boeing Company. The results of one of these programs, the test of a Boeing 747 airplane fitted with Pratt and Whitney JT9D engines, are presented in this paper. From the noise results, regions of jet noise dominance, fan noise dominance, and core noise dominance are identified. Methodology that will be used for more accurate breakdown of the noise components in regions where they are not dominant is outlined.

The effect of flight on jet noise as observed on the Bertin Aérotrain

The effect of flight on the noise radiated by the jet exhausting from a J85 engine is studied experimentally on the Bertin Aérotrain. Both convergent and convergent-divergent nozzles have been tested. Data presented include subsonic and supersonic jet noise data up to a jet speed of 670 m/s and a flight speed of 82 m/s. The behaviour of mixing and shock associated noise in flight are analyzed. A detailed comparison of convergent and convergent-divergent data is presented. A generalized experimental description of jet noise in flight is given together with a theory-to-experiment comparison and a tentative explanation of the observed discrepancies.

Flight Effects on JTSD Engine Jet Noise Measured in a 40 x 80 Tunnel

A JT8D-17 turbofan engine was tested in the NASA Ames Reserach Center 40 x 80 ft wind tunnel to determine flight effects on jet noise. The engine was configured as a baseline with conical nozzle, a quiet-nacelle 20lobe ejector/suppressor, and an internal mixer with conical nozzle. Tunnel-off and tunnel-on noise tests were conducted over a range of nozzle pressure ratios (1.2 to 2.1), primary jet velocities (275 to 550 m/sec), and tunnel velocities up to 100 m/sec. Aft quadrant noise data were measured by a pair of traversing microphones located on a 3-m sideline relative to the engine centerline. Unique correlations and analysis procedures were developed in order to define far-field flight effects from the relatively near-field noise measurements. Windtunnel-derived flight effects compare favorably with 727/JT8D flight-test data and model data obtained in a smaller wind tunnel for both the baseline and 20-lobe ejector/suppressor configurations. The study shows the importance of testing suppressor concepts for flight noise. It was found that the ejector/suppressor experienced a significant loss of suppression relative to static measurements during flight, whereas the internal mixer indicated a slight gain in suppression. It is concluded that the wind tunnel is a viable method for studying flight effects on engine jet noise.

Effects of External Boundary-Layer Flow on Jet Noise in Flight

The effects on jet flow of the external boundary-layer flow emanating from the trailing edge of an engine cowl in flight has been shown to be the main reason for the disparity between predicted and experimental results obtained from flight measurements. Flight simulation experiments indicate that the external boundary-layer flow tends to shield the jet flow in flight. This in turn modifies the jet noise source in flight and consequently the radiated noise from aircraft in flight. Close to 0/ = 90° and in the forward quadrant, this study indicates that the far-field jet noise and its spectrum scales approximately with the absolute jet velocity instead of the relative velocity as has been assumed in the existing prediction models.