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  Enabling Technologies - Noise Technologies

Noise Abatement Technologies

News stories related to environmental noise problems are all around us. Some are dramatic, some are not, but a huge effort and great sums of money are often invested in conflicts involving environmental noise.

Sound is defined as any pressure variation that the human ear can detect.  Just like dominoes, a wave motion is set off when an element sets the nearest particles of air into motion.  This motion propagates to the next particle, then the next particle, and so on.  This propagation is about 340 meters/second.  As the sound gets farther and farther way from the element that set the particles in motion, the sound level will reduce due to dispersion.

Noise can be described as a sound that does not carry useful information, or it can be a sound that the listener does not want to hear, such as the sound of aircraft flying through the air.  This aircraft noise is created by the aircraft itself: engine noise, propeller noise, drag, and vibrations of various parts of the aircraft.

Passengers flying in an aircraft will hear the noise generated by the aircraft they are flying in.  It is called cabin noise.  Since the passengers are very close to the noise source, it can be very loud. 

People on the ground will hear the aircraft passing through their area.  This is called community noise.  Community noise can vary in intensity a great deal, depending on many factors discussed below.

Here are examples of aircraft noise as compared to other noises you are used to hearing:

Noise is measured with a sound level meter.  Noise is typically measured with an “A” weighting and is measured in dB(A).  Since aircraft community noise is typically transient in nature, this noise is measured over a specific time period with an “A” weighting, and the equivalent noise level is calculated for this time period (T).  This is shown as LAeq(T).

Community Noise

Community noise is what causes most of the complaints against airports and airplanes. This noise must travel from the noise source, the airplane, to the listener. Many people have become sensitive to community noise and are particularly sensitive to this type of noise.  The most important factors affecting how the listener will hear the noise are as follows:

  • Listener’s sensitivity to various types of noise
  • Repetitive nature of noise, such as: impulse, intermittent, continuous, tones, and high and low frequency
  • Distance from the source
  • Atmospheric absorption
  • Wind
  • Temperature and temperature gradient
  • Obstacles such as buildings and barriers
  • Reflections
  • Precipitation

Community noise is generated by these items:

  • Piston engine noise
  • Propeller noise
  • Jet engine noise
  • Aerodynamic noise

Although community noise does not cause physical damage to the listener on the ground, this noise can cause poor relationships with the local airport neighbors.  It can also upset people and animals in wilderness areas that are distant from an airport.  Both the FAA and international standard setting bodies have set up regulations for community noise.  The FAA FAR Part 36 sets the standards for the USA.

Cabin Noise

Cabin noise affects the comfort of the passeners in the cabin and should be less than 85 dB(A).  Cabin noise can cause physical discomfort and/or tiredness to the passengers.  It can also cause long term and permanent hearing loss.  For this reason it is very important that the noise levels are significantly reduced inside aircrafts.

Cabin noise is generated by these factors:

  • Piston engine noise
  • Propeller noise
  • Jet engine noise
  • Aerodynamic noise
  • Vibrations in the airframe and/or other hardware
  • Aircraft systems, such as fans, air conditioning, and radios

These are the most important factors affecting how the listener will hear the cabin noise:

  • Source and intensity of the noise
  • Contributions of the airframe
  • Sound absorption or reduction technique used in the aircraft

The cabin noise level may be affected by various regulations and different governmental organizations both nationally and internationally, such as the FAA and OSHA.

Noise Reduction

There are many different things techniques available for reducing the noise generated by the aircraft.  It is of primary importance to reduce the creation of this noise.  This can be done with any of these techniques such as:

  • Engine mufflers
  • Changing the propeller
  • Balancing the propeller
  • Reducing turbulence
  • Reducing vibrating surfaces

There are also various techniques available to modify or reduce sound levels:

Summary

It is very important to reduce the noise levels generated by an aircraft for both the community surrounding the aircraft, as well as the passengers riding in the aircraft.  World wide government agencies have setup regulations to encourage this noise reduction.

Noise Reduction Technologies

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# Title Author Year Description Size
1 A Theoretical and Experimental Investigation of Propeller Performance Methodologies K. D. Korkan et al 1980 From AIAA/SAE/ASME. 22 pages on NASA's mathematical prediction of propeller performance compared to actual measured performance, examining the most prevalent blade sections. 2.7MB
2 An Acoustic Sensitivity Study of General Aviation Propellers K.D. Korkan et al 1980 A 32 page noise study that examines several methods of reducing prop noise on Cessna aircraft. Good empirical results using metal propellers. Shows the effectiveness of reducing RPM and increasing number of blades. 2MB
3 Noise Control Predictions for High-speed, Propeller-Driven Aircraft J. F. Wilby et al 1980 Finds that a double wall cabin structure can effectively reduce noise, with some weight penalty. 1.3MB
4 Noise from a Vibrating Propeller H. L. Runyan 1980 An important paper exams the noise contribution of 'tuning fork' action of propeller blades. Engine smoothness is a key factor in propeller noise. 748KB
5 Effects of Acoustic Treatment on the Interior Noise Levels of a Twin-Engine Propeller Aircraft--Experimental Flight Results and Theoretical Predictions T. B. Beyer 1984 Looks at passive noise reducing methods for the cabin of a turbo-prop twin. Many methods are explored. 828KB
6 Noise Control Characteristics of Synchrophasing - An Experimental Investigation J. D. Jones 1984 Synchrophasing the blade positions of propellers on twin engine aircraft was shown to substantially reduce cabin noise. 532KB
7 Noise Control Characteristics of Synchrophasing - An Analytical Investigation C. R. Fuller 1984 High-speed turbo-prop aircraft need synchrophasing to quiet the cabin. This study examines the effects obtained. 476KB
8 Effects of Acoustic Treatment on the Interior Noise of a Twin-Engine Propeller Airplane T.B. Beyer 1984 Exams the noise reduction due to placing interior trim panels in a turboprop twin. 684KB
9 Noise Control Characteristics of Synchrophasing, Part 1: Analytical Investigation C.R. Fuller 1986 Synchrophasing on a high-speed turbo-prop. Up to 8 dB of quieting. 764KB
10 Noise Control Characteristics of Synchrophasing--Part 2: Experimental Investigation James D. Jones et al 1986 Showed 10-34 dB of quieting, depending on location in cabin and frequency band. 1.4MB
11 A Numerical Method for the Design and Analysis of Counter-Rotating Propellers S. C. Playle et al 1986 Counter-rotating propellers are numerically analyzed regarding their flow fields and thrust. 2.3MB
12 Validation of an Interior Noise Prediction Model for a Composite Cylinder T.B. Beyer 1987 A composite cylinder is used to measure fuselage noise patterns from several sources. 484KB
13 Noise Reduction of a Composite Cylinder Subjected to Random Acoustic Excitation F. Grosveld et al 1989 A composite cylinder's interior noise is measured after adding double wall sound-proofing. 1.3MB
14 An Acoustic Experimental and Theoretical Investigation of Single Disc Propellers E. A. Bumann et al 1989 2, 3 and 4 blade props studied by a theoretical formula to predict noise. (Succi's) 738KB
15 Determination of Near and Far Field Acoustics for Advanced Propeller Configurations K.D. Korkan et al 1989 More theory on propfan noise, with discussion of blade flex as an impediment to predictions. Looks at both near and far measurement. 1.1MB
16 Noise Transmission of a Large Scale Composite Fuselage Model T. Beyer et al 1990 Examines the noise absorption/transmission properties of a mock composite fuselage. 1.3MB
17 High-Speed Propeller Noise Prediction—A Multidisciplinary Approach Mark H. Dunn and Feri Farassat 1992 Noise prediction theory for high speed propellers by the great Feri Farrassat at NASA. Includes discussion of blade deformation as a noise source. 1.6MB
18 Small Aircraft Propeller Noise with Ducted Propeller R. Duane Oleson et al 1998 A ducted propeller noise study indicates that inflow disturbance adds significantly to noise. Paves the way to noise study of ducted propellers. 1.5MB
19 Advanced Turboprop Noise Prediction Based on Recent Theoretical Results F. Farassat et al 1987 Computer mathematical analysis of advanced high-speed turbo-propeller noise and performance. 1.3MB
20 The Problem of Noise Reduction with Reference to Light Airplanes Theodore Theodorsen 1946 An early paper by the great Theodorsen examines general aviation prop noise and specifically urges use of multiple blades to quiet them. Applies the Gutin Formula, an early predictor of prop noise. 1.0MB
21 Comparison of Sound Emission from Two-blade, Four-blade, and Seven-blade Propellers Chester W. Hicks et al 1947 An early look at the noise reducing effect of lowered propeller tip speeds and use of multiple blades. Note: blades used had too much tip chord by today's math. 1.6MB
22 Propeller Loudness Charts for Light Airplanes Harvey H. Hubbard et al 1947 A nice collection of charts plotting noise v. RPM and other parameters of GA props. 2.1MB
23 Sound-level Measurements of a Light Airplane Modified to Reduce Noise Reaching the Ground A.. W. Vogeley 1948 Noise tests of 150-200 BHP engines and props including a large diameter, slow-turning 5 blade indicated that takeoff thrust suffers as blade number goes up unless a geared engine is used to allow slow-turning props of larger diameter. 616KB
24 Experiments in External Noise Reduction of Light Airplanes Leo L. Beranek et al 1950 122 pages on the several ways to reduce noise from a single engine piston aircraft, including multi-blade propellers, RPM reducers and engine mufflers. 4.5MB
25 Experiments to Determine Neighborhood Reactions to Light Airplanes With and Without External Noise Reduction Fred S. Elwell 1952 An elaborate survey study showing that reducing aircraft noise substantially improves neighborhood acceptance of GA. 5MB
26 A Theoretical Study of the Effect of Forward Speed on the Free -space Sound-pressure Field Around Propellers I. E. Garrick et al 1953 The sound pressure fields calculated around a prop and their sources. Discusses flow separation on the blade as a noise source and uses extensive mathematical analysis. 2.4MB
27 A Method for Calculation of Free-space Sound Pressures Near a Propeller in Flight Including Considerations of the Chord-wise Blade Loading Charles E. Watkins et al 1956 An elaborate study of the mathematical model of prop noise. 2.7MB
28 Measurements of Free-space Oscillating Pressures Near Propellers at Flight Mach Numbers to 0.72 Max C. Kurbjun et al Noise measurements of a P51 prop show that tapered tips were quieter and that noise rose rapidly as the blade tip speed exceeded 0.5 Mach. 808KB
29 A Review of Aerodynamic Noise From Propellers, Rotors, and Lift Fans Jack E. Marte et al 1970 An extensive review of noise calculation methods for props, fans and heli-rotors, includes a huge bibliography on V/STOL. 58 pages in all 3.9MB
30 Advanced General Aviation Propeller Study Rose Worobel et al 1971 204 pages of extensive material on GA prop design, including the full computer program code for designing the prop. 14.9MB
31 Computer Program User’s Manual For Advanced General Aviation Propeller Study Rose Worobel 1972 74 pages of User Manual for the Fortran program on propeller design for GA, whose code listing is included here. Developed by Hamilton Standard. 2.4MB
32 Aeroacoustic Wind-Tunnel Tests of a Light Twin-Boom General-Aviation Airplane With Free or Shrouded-Pusher Propellers H. Clyde McLemore et al 1980 87 pages on the full scale wind tunnel testing of a Cessna 337 for prop noise using 2, 3 and 5 blade metal props, with duct shrouds. The non-ducted propeller was quieter than the shrouded one. 4MB
33 On the Design and Test of a Low Noise Propeller George P. Succi 1981 The M.I.T. low noise propeller design is studied and compared to conventional props. An early attempt at proper span-wise blade lift distribution, but without the full flow field slow-down math of Jack Norris. 1.1MB
34 Experimental Study of the Effects of Installation on Single- and Counter-Rotation Propeller Noise P. J. W. Block 1986 A beautiful color 33 page study of the flow fields and noise patterns of counter-rotating propellers. 18MB
35 Prediction of Helicopter Rotor Discrete Frequency Noise Kenneth S. Brentner 1986 A computer program named WOPWOP by the great Feri Farassat for predicting heli-rotor blade noise. 1MB
36 An Experimental Investigation of the Interior Noise Control Effects of Propeller Synchrophasing J.D. Jones 1986 Virginia Tech examines fuselage quieting by prop synchrophasing. 2.5MB
37 Propfan Test Assessment Propfan Propulsion System Static Test Report 1987 A 237 page report on large propfan noise measurement. 7.6MB
38 Aircraft Interior Noise Reduction by Alternate Resonance Tuning Progress Report for the period ending June, 1990 Dr. Donald B. Bliss et al 1990 A 41 page study of Alternate Resonance Tuning, alteration of fuselage structure to alter its resonance with sound sources and quiet noise. 1.1MB
39 The Radiation of Sound from a Propeller at Angle of Attack Ramani Mani 1990 A study of the radically altered sound propagation from a propeller operating in a non-axis-symmetric flow, as in a positive angle of attack. A valuable 56 page glimpse of the real-world flow streams in which props operate. 1.4MB
40 Computational Methods in the Prediction of Advanced Subsonic and Supersonic Propeller Induced Noise--ASSPIN Users' Manual M.H. Dunn et al 1992 The Farassat ASSPIN program for predicting high-speed propeller noise. A pivotal 48 page paper in noise theory. 2.3MB
41 An Assessment of Propeller Aircraft Noise Reduction Technology F. Bruce Metzger 1995 128 pages of comprehensive review of the many ways of quieting propeller noise. This is a key paper. Many of the promising methods of noise reduction described herein have not yet been converged into one super quiet prop. 5.8MB
42 General Aviation Interior Noise: Part III Noise Control Measure Evaluation James F. Unruh 2002 60 pages of excellent material on the many contributors to cabin noise and how to quiet them. 4.7MB
43 The Noise of a Forward Swept Fan James H. Dittmar et al 2003 A multi-blade turbine engine forward swept fan blade shows promise as being quieter. 8.5MB
44 Derivation of Formulations 1 and 1A of Farassat F. Farassat 2007 Dr. Farassat's definitive prop noise formulae, derived. Elegant current math in 25 pages. 616KB
45 Advanced Turboprop Noise Prediction Based on Recent Theoretical Results F. Farassat et al 1987 Advanced turbo-prop noise prediction. 1.3MB
46 Noise Reduction System for General Aviation Aircraft, Phase II Sangvavann Heng et al 2005 A key current paper on GA muffler design, with the promise of a new foam material for silencing higher frequencies. The Ultramet muffler shows great promise for GA piston aircraft. This is the most substantial paper on silencing GA engine noise: 238 pages. 22.4MB
47 Active Attenuation of Propeller Blade Passage Noise J. M. Zalas et al 1984 ANR in a fuselage of a turbo-prop. 2.5MB
48 Environmental Noise Measurement Brüel & Kjær A primer on environmental noise. 628KB
49 Basic Concept of Sound Brüel & Kjær 1998 Basic concepts of sound and its measurement in 32 pages. 740KB
50 The Connection Between Acoustics and Unsteady Aerodynamics Feri Farassat, NASA Langley
2008 The Connection Between Acoustics and Unsteady Aerodynamics 101MB

 

 

 

 

 
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