How Active Noise-Cancelling Headsets Work

Active, noise-cancelling headsets are favoured by many in aviation for the enhanced reduction of engine noise provided over and above passive headsets, but how do they work?

Sound is the transmission of pressure waves through a medium. In this case, that medium is mostly air. Passive headsets do what they do by providing a barrier through which most of the energy contained within these sound waves is eliminated, but the human auditory system is a very advanced system that can detect a wide range of sound energy levels. This means that even though passive headsets are very good at reducing, or attenuating, the level of energy present, there is still much more that they could do.

Active headsets start by using the principles of passive headsets and build upon them. In addition to providing a barrier to the external unwanted engine noise present, they also monitor that noise and feed a signal through the headphones that is essentially the opposite of the external noise monitored. This signal is added to any sounds already desired to be transmitted, such as radio telephony from air traffic services, or conversation from passengers.

The principle of superposition is one of the properties that applies to sound waves. There exists an excellent Wikipedia article on the subject. Essentially, the principle of superposition can be summarised as follows: If one were to take a wave and add it to itself, the result would be a wave with twice the amplitude of the original. If, however, one instead takes a wave and subtracts it from itself, the result would be nothing.

It is not possible to simply subtract a waveform from itself when you are using a microphone to detect the sound and a loudspeaker to emit new sounds simply because these are located in different places. Each loudspeaker is hence provided with a delayed inverse copy of the waveform. Digital signal processing techniques are usually most effective in terms of providing this delayed inverted signal. The amount of delay required depends on the distance between the microphone and the loudspeaker, and each of the two channels has to be treated separately.

Noise-cancelling headsets are often called active headsets because they require a separate power source. Usually, they use rechargeable battery packs. If the battery pack becomes depleted during a flight, it is usually the case that the noise-cancelling function of the headset will stop working, but the headset itself will still function like a passive headset does.