I did a TON of research for the quietest, yet powerful, air purifier out there. The Alen BreatheSmart75i is exceptional! 
We have a RabbitAir, which is incredibly quiet, and easy to operate, but wanted something more powerful. We purchased an Oransi EJ120 which is supposed to be the quietest - "the Tesla of air purifiers". We sent the Oransi back because it was too loud. 
The BreatheSmart 75i is the easiest, best looking, quietest, and most powerful machine out there. Changing the filters is a snap. The value per dollar is outstanding.
I never, ever write reviews, but I just had to chime in on this one. Well done, Alen!!