Acoustics made easy-phase inverted anti noise
Have you heard that there are aircrafts that have microphones installed in the cabin recording the cabin noise and speakers playing this noise back in the cabin and that all this is done to reduce the noise in the cabin.
This technique is also used in more trivial things like earphones. I know that Sennheiser’s got a nice pair using this technique for instance.
You add noise to actually reduce noise – how is this possible?
Sound are often illustrated as a wave, we are talking about sound waves.
These waves have their tops and bottoms.In reality sound as well as noise for that matter, are not waves lurking around in the air surrounding us. Instead sound/noise are variations in the air pressure where a top of a sound wave actually is an increase of air pressure and a bottom of a sound wave is a decrease of air pressure.
This is then picked up my a membrane in the human which is moving back when it is hit by the higher air pressure and forth when it is exposed to the lower air pressure. The human brain then interpreters this as sound or noise.
For the brain to understand complete silence it must not get any signals at all from the membrane in the ear. This in turn means that the membrane must be completely still. No movements back and forth at all.
In order to get the membrane in the ear to stop moving there must be no variations at all in the air pressure. Zero variations in air pressure is actually equal to complete silence.
What we need to do then is to fill up the gaps, the bottom of the waves so they will have the same pressure as the tops.
This is what the microphones and speakers in the cabin are doing. They record the pattern of tops and bottoms of the cabin noise. Then they transmit an exact copy of this but they turn it upside down. Every time the cabin noise is at a top, the upside down copy is at a bottom and opposite.
This is not so far from what is going on, at a micro-level in sound absorbing materials such as Fellert. Of course there are no microphones and speakers installed inside the Fellert systems, but if you would look at the surface through a microscope you would see that there would be millions of pores where the differences in sound pressure can be leveled out.
More on how that actually works later.