Sennheiser HD400 Instructions For Use Manual page 66

The above diagram shows noise compensation with NoiseGard™: Passive hearing
protectors effectively attenuate noise from the middle and upper frequency range, the
effect decreasing sharply in the lower range. However, active noise compensation with
NoiseGard™ combined with passive hearing protectors results in a reduction of noise of
approx. 25 dB in the 25–500 Hz frequency range. The total attenuation resulting from
active and passive noise compensation is about 30 dB over the entire audio range.
A 10 dB reduction in noise is perceived subjectively as a halving in volume. A further
reduction in noise of 10 dB again results in a decrease in unwanted noise by 50 %.
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