This largely depends on the room itself. For instance, in the bedroom your air purifier should be close to silent for anyone to sleep comfortably. However, in the living room, you can allow for a bit more noise. Considering there are noisy appliances such as the television and simply allowing for conversation should mean a few more decibels. At the lowest, you should be looking at around 20 decibels whereas at its highest around 45 decibels. To put this into perspective, 45 decibels should be the sound level inside a library or for a quiet conversation.