External noise of a tyre is measured in decibels.
Since many people are not familiar with the values in decibels, noise class is also indicated. It categorizes the tyre to the European limits which will be introduced.

1 black bar: quiet (3dB or more below the future European mandatory limit value
2 black bars: average tyre (between the future limit and 3dB below)
3 black bars: noisier tyre (above the future limit)

A part of the noise from a passing car dues to the tyres. Choosing a tyre with good noise category reduces the impact of the Environment when driving