Noise Cancellation

Recently I purchased a set of noise cancelling headphones for a twenty-hour plane trip to Europe. Remembering the headache I had on previous occasions from the constant, low-level drone of the engine (no pointy end of the plane for me I’m afraid), I figured that this might be a good investment. I did some quick research on the Internet and found what seemed like a good pair and duly purchased them from the local store that carried them. They actually had them in stock too! A quick test to make sure they worked and it was on the plane the next day. Of course I was eager to give them a try so straight after taking off I put them on and hit the switch.

Wow! Very highly recommended I must say. The noise level dropped significantly but I was still able to hear everything necessary. They work by reversing the phase of the audio signal. A microphone on the unit receives the ambient noise and a circuit reverses it by 180° and feeds it back at a similar volume level into the headphones. So you end up with a positive signal from the real world and an inverted signal from the headphones and they almost cancel each other out. Imagine at the beach a wave coming in and another trough on the way out. When they meet they tend to combine and the result is virtually no wave at all. Of course nothing is perfect, it only operates at low frequencies to about 300Hz and not everything is silenced but the effect is substantial enough to make you think you are travelling in a car such were the reduced noise levels. The only real issue I had was that when it came time to sleep I couldn’t rest my head on the side as the headphones were in the way, so I ended up with a sore neck!

Several companies make these noise-cancelling headphones; the ones I had were from Sennheiser, but Sony and Bose also make them along with some other brands. There are differences between them, the more you pay the better the attenuation and the better the sound quality. Some cover the entire ear hence give protection in the same way as earmuffs do but these are also much bigger and tend to be more expensive. Higher frequencies receive more attenuation with these too. These types also have the electronics and batteries within the headphone section directly so save having to have a separate module to look after, but of course will still have a cord for plugging into the audio source.

Usually they will be supplied with various adapters for the plug, this way you wont have any problems plugging into your own equipment nor aeroplane sockets. Note that they do not have to be plugged into the audio socket to work, you can have them reducing noise but nothing playing if you like. Conversely, most will still work as normal headphones even when the batteries are flat. Battery life in the ones I had was great, I took some spare batteries with me but the initial set lasted there and back and are still going strong.

As to sound quality, these types of headphones can also be used for normal listening, even without the noise-cancelling feature turned on. The sound quality will vary and price is no indication compared to normal headphones as it obviously includes the noise cancelling parts as well. If you have the time ask the stores for a demonstration. That said they could make an excellent spare set of headphones or even your primary set. There is no reason they cannot be used in cars (passengers only!), buses or trains when using portable music players. Actually they can even help protect your hearing since as the ambient noise level is reduced, you do not have to turn the music up so far to be heard clearly. Music fed into the headphones is not affected as no cancellation is carried out on that signal. On the plane I had the volume of the audio system turn to minimum a lot of the time and had no problems hearing the movies.

Noise cancellation is a recent technology that had its roots in the military. Air Force pilots first used this system many years ago and now it has reached a level maturity where the price and compactness makes it suitable for personal and domestic use. New areas are now coming out for its use too. Honda has recently implemented it in some of their cars and this is a very sensible area. All cars have audio systems so it would not be difficult to integrate the two together and make the car cabin a lot quieter. As the systems work only at low frequencies, horns, sirens, rail crossing signals and the like will still be easily heard but road and engine noise is reduced. I can also image uses in pubic areas such as hospitals, shopping centres and the like to reduce noise. With so much industry, transport etc, it may be a cheaper solution that adding a lot of soundproofing materials.

So next time you are going on a long trip, give these a try, you’ll be very surprised at the result!

Written by Leon Gross, originally published in Audio & Video Lifestyle magazine.