You'll want one for every person in the house
We have five kids and 4 bedrooms with a Marpac sound machine in each.  These are a must-have for sleep.  They may not be loud enough for those living in an urban context, but we love them in our small town neighborhood.  Our kids sleep longer and we wake up less from outside noises and early rising birds in the morning.  The machine is easy to use and the company is great if you need a repair.
Rating: 5 stars