There are several types of construction methods used in the making of double basses.
Fully carved means that there is no laminated wood in the bass. The top is carved, the ribs are solid wood and the back is either carved or is of solid wood flatback construction. Woods typically used are some sort of spruce variety for the top, and maple for the ribs, back and neck. Willow and Poplar are becoming more popular lately for use in the ribs and back. When a top or back plate is carved, there is not only a tremendous amount of labor involved; there is also a large amount waste. The big block of wood that the top or back is carved from ends up mostly as a big pile of wood shavings on the floor! This is in part why carved basses are so much more expensive than laminate.
Laminated basses have laminate tops, ribs and backs although sometimes the ribs are solid wood because plywood can be difficult to bend. The top and back plates are shaped in forms and can therefore be cranked out in large quantities. Most laminated tops and backs have a spruce or maple surface veneer respectively, as well as some other kind of fill veneers such as poplar or birch. Also, some solid wood basses utilize one thin laminated wood face to hide the cosmetic flaws of an otherwise usable top. Laminate construction is much less wasteful of wood. The thin layers of laminate are either sliced off the rotating log much like the way an apple is peeled, or single passes are made to produce wood with quarter sawn figure.
Hybrid basses usually have a carved top, laminate back and either laminate or solid ribs. This method of manufacturing saves a lot of money in materials and labor but takes advantage of a carved top, arguable the most acoustically critical part of the instrument. Hybrid basses will generally sound better for bowing than laminates.
Flatback vs. roundback
For fully carved basses there are two methods of back construction: flatback and roundback. Roundback construction is just like the top: it is carved with an arched shape out of solid wood. Flatbacks are made from thin sheets of solid wood edge glued and reinforced on the inside with 3-6 braces which usually run perpendicularly to the grain of the back. The braces keep the large flat surface of the flatback from warping and buckling and give the back more strength. Roundbacks with their arched shape and harder wood construction almost never crack, flatbacks almost always do! Traditional bracing designs on the flatback don’t allow for the wood to expand and contract and eventually the back breaks free from the braces or cracks or both. That’s not to say that flatback basses are inferior; indeed some of the finest old and new instruments are of flatback construction but they are less stable than roundbacks. New flatbacks are especially prone to problems as the wood settles and adjusts.