Noise cancellation products often claim “18dB reduction” or “reduces up to 90% of background noise” but what does this really mean?
Firstly watch out for the language tricks like “up to”, this necessarily implies there may be much less noise reduction and the stated value is only a possible maximum.
The common practice in the industry is to quote noise reduction level that is the maximum level of reduction at a single frequency point, it does not mean that all sounds are reduced by this amount!
A typical high-end noise reduction headset will offer around 18dB reduction at 200Hz, but as the frequency goes up the reduction level reduces and by the time it reaches, say, 600Hz for an over ear headphone, the noise reduction has reduced to 0dB (similarly as the frequency goes down, below 200Hz, the level of reduction also drops off to 0dB).
Now this bring us to the next type of claim, that being the % reduction. This is really the most deceptive way of specifying amount of noise reduction. This is often simply a mathematical trick of converting the dB number into a % number, there may also be other methods of calculating a % unfortunately no one tells you how they arrive at the figure.
So does an 18dB NC headphone reduce noise by 18dB? No, it reduces SOME noise by 18dB. Does a “reduces up to 90% of background noise” headphone really eliminate 90% of noise? No, it reduces SOME noise frequencies by 90% only. So far there is no international standard for specifying the noise cancellation level of a headphone, the consumer must decide based on reviews, brand reputation or personal experience.