used with acoustic instruments with built-in transducer pickups or microphones. The amplifiers often come with a simple mixer, so that the signals from a pickup and condenser microphone can be blended. Since the early 2000s, it has become increasingly common for acoustic amplifiers to provide a range of digital effects, such as reverb and compression. As well, these amplifiers often contain feedback-suppressing devices, such as notch filters or parametric equalizers. Stacks An amplifier stack consists of an amplifier head atop a speaker cabinet—a head on top of one cabinet is commonly called a half stack, a head atop two cabinets a full-stack. The cabinet that the head sits on often has an angled top in front, while the lower cabinet of a full stack has a straight front. The first version of the Marshall stack was an amp head on an 8×12 cabinet, meaning a single speaker cabinet containing eight 12" guitar speakers. After six of these cabinets were made, the cabinet arrangement was changed to an amp head on two 4×12 (four 12" speakers) cabinets to make the cabinets more transportable. Some touring metal and rock bands have used a large array of guitar speaker cabinets for their impressive appearance. Some of these arrangements include only the fronts of speaker cabinets mounted on a large frame. There are many varieties of speaker combinations used in guitar speaker cabinets, including one 12" speaker, one 15" speaker (this is more common for bass amplifiers than for electric guitar cabinets), two 10" speakers, four 10" speakers, four 12" speakers, or eight 10" speakers. Less commonly, guitar cabinets may contain different sizes of speakers in the same cabinet. Cabinets with eight 10" speakers are large and heavy, and they are often equipped with wheels and a "towel bar"-style handle for transport. Some cabinets use mixed speaker types, such as one 15" speaker and two 10" speakers. Cabinet design Combo guitar amplifier cabinets and guitar speaker cabinets use several different designs, including the "open back" cabinet, the closed back cabinet (a sealed box), and, less commonly, bass reflex designs, which use a closed back with a vent or port cut into the cabinet. With guitar amps, most "open back" amp cabinets are not fully open; part of the back is enclosed with panels. Combo guitar amp cabinets and standalone speaker cabinets are often made of plywood. Some are made of MDF or particle board—especially