Tread patterns are the network of grooves that form the outer surface on most tires. These grooves exist to draw water away from the tire's contact with the road and thereby increase handling in wet weather and reduce the risk of aquaplaning.
The design of these grooves is has a large effect on a tire's handling and also the amount of noise it produces. This noise reduction from the addition of grooves is especially pronounced at higher speeds with the opposite extreme of blank slick tires being profoundly louder. Different tread pattern reduce noise in different amounts and tire engineers generally try to make the sound come through as a tolerable white noise. This is generally achieved by a slight degree of randomness in the tread blocks of the tire as this stops the pitch generated by the tire rolling on the road from being one intense pitch .