Sennheiser HD400 manual

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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