30-05-2012, 02:53 PM
The Active Noise Control of a Light Aircraft Cabin Interior; A work in progress.
The Active Noise Control of a Light Aircraft Cabin InteriorA work in progress.pdf (Size: 302.55 KB / Downloads: 124)
ABSTRACT
The effects of aircraft noise, with respect to both passenger comfort and occupational health, have long since
been realised, with many examples of sound control now implemented in commercial aircraft. However, the
single engined four seater aircraft cabin is still an extremely noisy environment, which has apparently been
ignored to date, especially with respect to low frequency sound reduction. Consequently, pilots and passengers
are still exposed to potentially damaging noise levels, with a high dependency on the proper use of personal
hearing protection. With space and weight a premium, the practical application of passive noise control methods
can only provide appreciable benefit at higher frequencies. Feedforward active noise control offers a
lightweight and cost effective method of reducing the low frequency noise, dominated by propeller tones. This
paper, reflecting a work in progress, presents preliminary results and conclusions from the application of an
active noise control system in a four seater Piper Archer.
INTRODUCTION
Single engine light aircraft cabins remain a noisy
occupational environment for the pilots and an unpleasant
prospect for potential passengers. While higher frequency
noise may be addressed by passive means, the longer
wavelength associated with low frequencies means that
effective passive noise control materials would have to be
impracticably large and massive.
The low frequency, enclosed field and periodic nature of
the single engined light aircraft cabin noise, make it an
ideal candidate for adaptive feedforward active noise
control.
For these reasons Southern Aircraft Maintenance have
shown interest in the concept of active noise control and
allowed the University of Adelaide use of their Piper
Archer four seater aircraft for research purposes.
In this paper the aircraft noise levels are initially quantified
to indicate the approximate sound power for the cancelling
(secondary) sources. The aircraft’s noise characteristics,
methods of noise generation, noise transmission paths and
the aircraft physical environment are all addressed as part
of the procedure of identifying an appropriate noise control
strategy. The closing sections draw conclusions from the
work to date and present a brief synopsis of future work.
NOISE LEVELS
The interior cabin noise levels of the Piper Archer aircraft
under operational conditions were recorded for subsequent
playback and control under laboratory conditions. Third
octave noise levels were measured, to estimate the power
requirement for the speaker system (figure 1) and a narrow
band analysis was undertaken to identify specific problem
frequencies (figure 2). Measurements from inside the
aircraft cabin, adjacent to the pilots head at full throttle
conditions, recorded an "A" weighted sound pressure level
of 97dBA and a linearly weighted level of 113dB.
WHY ACTIVE NOISE CONTROL ?
While personal ear protection may be an obvious solution,
the INCE report continues to state that their effectiveness
(personal ear protectors) depends on proper fitting (to avoid
air leaks) and timely use. Other factors that must also be
considered are hygiene and the reduced ability to hear
warning signals. Quantifying their effectiveness is therefore
extremely difficult. Higher frequency noise may practically
be reduced passively.
OBSERVATIONS AND CONCLUSIONS
The work conducted to date is certainly encouraging with respect to the potential of feedforward active noise control
reducing the low frequency tonal noise of a single engine light aircraft. However, the research has also demonstrated that
microphones used as error sensors, produce noticeable noise attenuation only around a very localised region of the sensor.
Energy density shows promise as an alternative for this particular application. The speakers currently being used in the
“brute force” approach would obviously be too heavy leading towards the question of how a practical installation would
compare. Without an obvious dominance of acoustic modes, placing sources at antinodes may offer only small rewards.
With the practical necessity of smaller speakers, zones of silence covering acceptable regions would be more achievable.