A sofa bed or sofa-bed, (in the US often sofabed, hide-a-bed, couch bed or sleeper-sofa), typically is a sofa or couch which has underneath its seating cushions a metal frame and thin mattress that can be unfolded or opened up to make a bed. A futon differs from a sofabed, although sofa beds using futon mattresses are common.
Sofa beds can be found not just in many styles, but many sizes as well. They are categorized accordingly, from Chair Sleepers (cot size bed) to Queen Sleepers. Almost all sofa beds contain the standard bar and canvas mechanism seen for years, however a few new lines can be found with alternative mechanisms.
Sofa beds are usually three-person couches. However sofa beds that are two-person couches are also available. These types of sofa beds are known as loveseat sleepers. They are known as loveseats since the smaller size means one needs to be comfortable with how close they will be to the person next to them.
Sofa bed mattresses
Although most sofa beds utilize a queen short mattress measuring 60"x72", other common sizes include 58"x72", 54"x72", and 52"x72". The average sofa bed mattress thickness is 4.5", thus being half as thick as a good standard mattress. Sofa mattresses can be produced out of many different materials, the majority on the market being comprised of springs or foam.
Spring sofa mattress
Although springs can make a satisfactory sofa bed mattress, the following aspects have to be considered regarding a spring sofa mattress:
- Free Length represents the overall length of a spring when no load is applied.
- Solid Height is the length of a compression, when the spring has been deflected enough to allow each adjacent coil to touch each other.
- Working Length represents the difference between the free length and the solid height.
- Active Coils is the number of coils that are free to deflect under load, i.e. the total number of coils minus the number of closed coils forming the ends is a measure of the spring quality. The more active coils a mattress has, the less working length it has, which means that the springs are the less likely to deteriorate.
- Hydrogen Embrittlement represents the hydrogen absorbed in electroplating of carbon steels causing the spring material to become brittle, leading to cracking and spring failure. Plated springs are a problem, and un-plated springs are more likely to rust. Plastic coating is an alternative, but that’s extremely expensive.
- The best springs are Stress Relieved, which means that the springs were treated at a temperature so as to relieve stresses. Not going through such a procedure might cause mattress failure. Unfortunately, many mattress springs are not stress relieved because it is expensive. 
Foam sofa mattress
Quality foam sofa bed mattresses compress as needed, the foam acting like an infinite number of springs. A high quality foam gives better support, molds to the body and snaps back to shape. Consider the following factors of a high quality sofa bed mattress:
- Density is a measurement of the pounds of polyurethane foam per cubic foot. High density foams can be made very soft, whereas low density foams can be made very firm.
- Indentation Force Deflection (IFD) is the measure of the foam’s support, representing the pounds of force required to indent a foam sample by a specified percentage of its original thickness. Support is the foam’s ability to “push back” against your body weight, and prevent the foam from “bottoming out” on the frame or base.
- Foam Durability, Flex Fatigue and Compression Set are other terms related to foam performance.
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