For those who like the idea of adjusting the overall firmness of their mattress, Sleep Number now offers the m7 memory foam series with 3 inches of their gel-infused memory foam over their dual air chamber system. Following this is the m9 with 5 inches of memory foam.
The x12 is Sleep Number‘s highest level mattress that uses 5 inches of total foam, with 3 inches of a layered memory foam that is softer and springier than that in the M7. They do not list the density of the memory foam or the origin, which means that it is not possible determine the overall quality of the foam.
With the m7, m9 and x12, the air replaces the foam support system found in traditional memory foam mattresses. According to Dr. Swartzburg, the mattress does feel like a very firm memory foam mattress when it is on the firmest setting, and when the air level is reduced it does achieve the feeling of a softer mattress. However, reducing the air pressure also allows the air to be displaced by the heavier mid-section of the body and transferred to the lighter legs and shoulder areas, thereby creating a hammock effect.
The solution for those who want an adjustable memory foam mattress, but want to avoid the loss of low back and hip support would be to go with a tri-zone air system where the chambers are partitioned and prevent air from displacing from mid-section to the head and foot sections of the mattress (see the Air-Pedic and Tempur-Choice models above for more info on these types of systems).