Though there are many air purifiers that claim to be noiseless, they fail to redeem themselves anyhow. The noise that is often heard, is produced due to the speed of the fan. You can control the speed of the fan, thus reducing the noise of the air purifier. Nowadays, many air purifiers come with built-in sensors. Furthermore, these sensors help in adjusting the fan speed according to the level of impurity in the air. These sensors provide a great degree of comfort as they won't bother you to operate manually.