Active Reduction of Aircraft Noise

1
Active Reduction of Aircraft Noise

• by
• Ben Sharp, Micah Downing Chris Hobbs,
• Wyle Laboratories
• Arlington, Virginia USA
• International Military Noise Conference
• Baltimore, MD
• April 25, 2001

2
Characteristics of Low-Frequency Aircraft Noise

• At Low-Frequencies (lt 160Hz) aircraft noise
• Radiates mainly to the rear of the
• aircraft.
• Attenuates slowly with distance.
• Passes easily over barriers
• and berms.
• Transmits well into buildings.
• Causes rattle of windows and
• internal objects.

3
Specific Problem Areas

• Ground Run-ups
• Low-frequency noise from ground run-up operations
  is a major source of community annoyance.
• Maintenance run-ups occur during the nighttime
  hours
• Enclosures, or hush-houses, are expensive and
  often inconvenient to use, and are not suited to
  airports with only localized problems.
• Departing Aircraft
• Low-frequency noise to the rear of departing
  aircraft can be almost continuous during busy
  periods.

4
Passive vs Active Noise Control

• Run-up enclosures and buildings employ passive
  noise reduction technology, such as absorption
  and barriers, which are most effective at medium-
  and high-frequencies.
• Active noise control (ANC) modifies the noise
  radiated by the aircraft, and is most effective
  at low-frequencies.
• ANC does not require expensive fixed run-up
  facilities.
• ANC modifies the exterior noise, thus reducing
  both exterior and interior noise levels.

5
What is Active Noise Control?

• Active noise control is based on the
  interference that occurs when two sound waves are
  combined.
• A secondary noise source is used to generate
  noise in anti-phase (anti-sound?) to that created
  by the unwanted noise, resulting in an overall
  reduction in noise level.
• The reduction occurs over an area where the two
  sound waves are in anti-phase.
• Noise levels may increase outside this limited
  area unless precautions are taken.

6
Concept of Active Noise Control
7
Verification of Model
8
Loudspeaker Source
9
Validation Results
Predicted
Measured
10
Model Prediction for Larger Area
11
Global vs Local Noise Control

• Global
• Global reduction of noise requires placement of
  control source near the source of unwanted noise
  - the closer it is, the more global is the noise
  reduction.
• Global noise reduction requires the control
  source to generate high sound levels.
• Local
• Local area control can be achieved at a distance
  from the aircraft where noise levels are lower
  and can be easily generated by artificial
  sources.
• Active control modifies the sound as it passes
  over the airport boundary - noise reduction
  occurs in the community, not at the source or on
  the airport property.

12
Effects of Control System Geometry
13
Jet Engine Source
14
Aircraft Departure
15
Summary of Potential ANC Applications

• ANC can be a useful tool for the control of low-
  frequency noise from stationary aircraft, and to
  the rear of departing aircraft.
• A computer model has been developed and validated
  to predict noise reduction achievable.
• Noise reductions of 5 to 10dB can be achieved
  over significant areas.
• Noise reductions of gt10dB can be achieved over
  limited areas, such as around a school.
• Current development program in progress to
  optimize the system to achieve at least 10dB
  reduction over a much larger area.