Active Noise Reduction in Aircraft Communication Systems

free essayThe high levels of noise being experienced in some civil jet aircraft, military aircraft, and helicopters usually result in a reduced intelligibility of speech. These noise levels may also damage an individual’s hearing capabilities as it may affect the auditory monitoring tasks including detection of audio presented as warnings to the aircrew. The incorporation of hearing protection ear shells inside the flight helmets are some of the most cost effective and practical methods of reducing the levels of noise at the ears of aircrew. However, there is a technological limit regarding the passive attenuation of such ear shell and therefore the use of active systems has been preferred to help in reducing the levels of noise. Consequently, one of the most potent methods that are used to reduce the noise levels in the aircrafts is the Active Noise Reduction (AN Cancelation). Furthermore, the cabin noise in turbo-propeller engine aircraft cans significantly causes discomfort to the passengers and the flight crew. The level of noise in the interior of turbo engine aircraft mainly results from the excitation of aircraft fuselage through the unsteady aerodynamic pressure field. Since the noise in the interior components is mostly dominated by tones, which can be heard at the blade passage frequency, it is wise to reduce the possible fuselage vibrations and interior noise at these frequencies. The Active Noise Cancellation is an effective and efficient means of reducing noise levels in an aircraft. The ANC helps in reducing the constant loud background noise usually experienced in long plane rides.

Aspects to Be Discussed

The following paper provides an exploration of the use of the Active Noise Cancelation technologies to reduce the levels of noise in the aircraft. The paper will present the concept of the Active Noise Cancelation technology, Sources of the Active Noise Cancellation, the applications of the Active Noise Cancellation in aircraft, and the advantages of the Active Noise Cancellation. The final part of this paper is a summary in the form of a conclusion.

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Active Noise Control (ANC)

It is of highly importance to fully understand the concept of ANC since it facilitates understanding of how ANC technology works and functions. Sounds exhibit properties of waves as they travel through the air. The sound waves that have high amplitudes will be loud while the sound waves which have very short amplitudes will be very quiet (Kochan, Kletschkowski, Sachau & Breitbach, 2010). This phenomenon is as shown in figure 1 below.

The frequency or the pith of the sound is determined by the distance between the crest and the valleys of sound waves. High pitched sounds consist of sound waves which are squished together, and such sounds will be highly pitched and shrilled. In the events that sound waves are prolonged, the sound produced becomes very low. This phenomenon is shown in figure 2 below.

The existence of two sound waves makes the waves interfere or overlap with each other. When the sound waves have the same phase difference and the same frequency, it will result in a constructive interference and such a noise will double in amplitude. If the two waves have the same amplitude and frequency but are out of phase, however, the sound waves will neutralize each other and it will result in a destructive interference. These result in the failure of the sound wave to produce noise, thereby making Active Noise Cancellation achieve its objective of neutralizing the unnecessary sounds or noise.

The Active Noise Cancelation framework makes good use of the destructive interference of sound waves to neutralize the unnecessary noise, as well as the phase, amplitude, and frequency of the undesired sound must be measured, therefore, a different sound wave of the same amplitude and frequencies but opposite is established. The second sound and the initial sound neutralize each other and this results in a destructive interference, which leads to the noise reduction. It is, therefore, important to understand the amplitude, frequency, and the phase of the undesired sounds. The Active Noise Cancelation works best by canceling lower frequency sounds which are continuous, hard to control impulses, and higher frequency sounds.

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The Active Noise Cancelation system is based on a feedforward or a feedback control strategy or a combination of both. Feedforward strategy is applied where the source of noise is well known and where the reference signal or synchronization, which is derived from the sources is available. These signals have a priori information concerning the radiated noise. In the case of the scenario where there is no reference signal, feedback techniques are used. Feedforward control system implies that all noise constituents which are linked with the reference signals are considerably tampered with. Currently, both feedforward and feedback are used as control strategies to control vibrations and noise in various appliances including automobiles (Zhu, Li & Pan, 2011).

To attain maximum attenuation, there must be a significant correlation between the noise required and the reference signal. The performance may be degraded if the acoustical delay time is shorter that the Active Noise Cancelation controller process time between the secondary source and the reference sensor. This causality constraint is specifically important in considering the broadband ANC, and only periodic noise may constantly be reduced if the constraint is not fulfilled. Consequently, it is imperative to establish and recognize the connection between the intervallic signals of the frequency.

The use of broadband feedforward ANC allows responding to the reference signals by microphones situated close to the sources of noise while in the narrowband control the microphones are usually replaced by non-acoustic sensors. These non-acoustic sensors may include an inductive, optical, or tachometer. The non-acoustic signals are specifically beneficial over the reference signals because the ANC system can internally generate the reference signals. Since the reference signals are generated in this way, the adaptive control may become extremely selective thereby making it possible to determine the harmonics which will be controlled and which will not be controlled.

Application of the Active Noise Cancelation in Aircraft

There is a global trend which indicates that there is an increase in the air traffic. However, there is a limitation to this growth of the civil air traffic due to the associated emission of noise which affects communities and residents around the airport. Three types of aircraft noise are well known; these are the aerodynamic noise, engine noise, and the noise from the aircraft systems. The aerodynamic noise is produced by propellers, landing gears, helicopter rotors, and the airframe. The noise from the aircraft systems results from the auxiliary power unit and the conditioning systems. The engine noise results from fans and turbines, the flow in the engine duct, and the combustion chamber.

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Applications of the Active Noise Control in Aircraft

In most of the applications of Active Noise Cancellation, the primary noise is usually periodic. In such scenarios, the use of the Active Noise Control system based on the feedforward technique can be used to reduce noise successfully. This particular problem normally occurs in the aircraft where the main noise problem is the propeller induced. The feedforward controller needs a reference signal from the source of noise to make the propeller work, and the noise attenuation achieved usually depends on the correlation between the noise and the reference signal. For an efficient reduction in noise, there must be a significant correlation. In the aircraft applications where there is a single source of noise, the use of a single–reference controller can be effective. If there are several independent sources which contribute to the primary noise, there should be one reference signal from each of the noise sources. This increases the success of noise reduction.

The Active Noise Cancellation (ANC) can also be used to improve speech intelligibility in the aircraft. Speech intelligibility refers to the understandability or clarity of speech. It measures the match between the response from a listener and the intention of a speaker to effectively communicate in the everyday situations. The production of significantly high noise levels in the aircraft can affect the intelligibility of speech within the aircraft cabin. Although the passive noise reduction technologies have gained prominence over the past few decades the use of Active Noise Reduction (ANR) has been found to be specifically better and more effective. The use of the ANR significantly improves speech intelligibility in helicopters and fast jet noise fields. Companies have invested in this new technology to provide digital technologies which can provide comfort to the ears of the cabin crew and the passengers.

Sources of Noise in the Aircraft

During the nineties, there was more interest developed in the use of the turbo-propeller aircraft which carried up to 50 passengers. The new interest was a result of the fact that the turbo propeller engines are more fuel efficient than the jet engine aircraft (Kochan, Kletschkowski, Sachau & Breitbach, 2010). However, one of the disadvantages of the turbo propeller engine aircraft is that there is significantly higher level of cabin noise than in the jet aircraft. The propeller aircraft had noise levels which measured approximately 90 decibels (Snyder, 2012). Reducing this level of noise can significantly increase the level of comfort. Noise in the interior of an aircraft is mostly derived from two main external sources: the aircraft propulsion system and the fuselage boundary layer. The boundary layer noise is mainly produced through the shaking of the fuselage wall by the external turbulence fluctuations. The boundary layer noise usually has high frequency since it has random and broadband characteristics. The boundary layer noise, due to its properties, is difficult to reduce through the use of active methods. For this reason, currently passive insulation methods are mostly applied to attenuate the boundary layer noise.

How It Works

The noise in the propulsion system is composed of propeller and engine noise. However, propeller-generated noise is considered to be the most dominant. The propeller induced vibrations and noise involves several low-frequency tonal components which are related to the Propeller Blade Passage Frequency (BPF) as well as some of its harmonics. The BPF ranges between 70 Hz and 110 Hz and depends on the type of the aircraft (McIntosh, 2001). The periodic pressure fluctuations which act on the fuselage are the main causes of these noise vibrations. The fuselage noise and vibrations are then transmitted through the aircraft structure and the cabin walls, all of which act to radiate noise back into the aircraft cabin. The turbo-propelled aircraft usually has the Tuned Vibration Absorbers (TVAs). The TVAs are passive methods of the propeller-induced noise within the cabin. Although this method is efficient when properly used, there are still some drawbacks. First, it is important to tune each TVA to a specific frequency. The need for a broader absorption bandwidth dictates that the aircraft must be fitted with a low Q-value which results in less absorption. A larger number of the TVAs must be used to compensate for the reduction in the absorption. However, the absorbers are mostly used during cruise speed.

Advantages of using the Active Noise Cancelation (ANC)

The use of the Active Noise Cancelation (ANC) produces several advantages which overcome the disadvantages of the use of tuned dampers. First, the ANC allows for the tracking of propeller rotational speed during the entire flight envelope, and this reduces the number of harmonics and contributes to a weight reduction. Secondly, ANC helps to reduce the cabin noise significantly and this increases passenger comfort (M?ller & M?ser, 2012). When passengers feel comfortable, they are more satisfied and this may give an airline a competitive advantage. Furthermore, the ANC enables the use of piezoelectric components which act as the structural actuators. These structural actuators exhibit high bandwidths which are low in weight (P?mies, Romeu, Genesc? & Arcos, 2014).

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ANR is a technology widely applied in the aircraft systems. It helps to reduce the levels of noise that emanate from different components and structures of the aircraft. The ANR works by using a phase differential radio wave with the same frequency and amplitude as the reference wave which has the undesirable noise. This second wave neutralizes the initial wave thereby leading to a reduction in noise. ANR can be used to improve speech intelligibility in the aircraft. It can also be used to improve noise produced by the aerodynamic components of an aircraft. ANC allows for the tracking of propeller rotational speed during the entire flight envelope, and this reduces the number of harmonics and contributes to weight reduction.

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