four-stringed electric bass is E1, two octaves below middle C (approximately 41 Hz), and on a five-string it is B0 (approximately 31 Hz). The requirement to reproduce low frequencies at high sound pressure levels means that most loudspeakers used for bass guitar amplification are designed around large diameter, heavy-duty drivers, with 10", 12" and 15" being most common. Less commonly, larger speakers (e.g., 18") or smaller speakers (e.g., the 8x8" cabinet, which contains eight 8" speakers) may be used. As a general rule, when smaller speakers are used, two or more of them are installed in a cabinet (e.g., 2x10", 4x10" and 8x8"). For 12" speakers, combo amps and cabinets are available with 1x12" and 2x12"; less commonly, 4x12" cabinets are seen. For 15" speakers, combo amps and cabinets usually have 1x15", although 2x15" and even 4x15" cabinets exist. A small number of 1x18" bass cabinets are sold (e.g., Trace Elliot). For 10" speakers, the most common combo amp and speaker cabinet configurations are 2x10" and 4x10". For speaker cabinets, 2x10" and 4x10" are the most widely used, although 8x10" cabinets are used in stadium concerts, especially in louder rock genres. Other configurations with 10" speakers do exist, but they are less common. For example, there are a small number of 1x10" and 3x10" combo amps and speaker cabinets, and a small number of 6x10" cabinets. Bass speakers are usually made with stiff paper cones. Hartke combo amps and speaker cabinets are unique in that the cone is made from paper, except for the middle, which is made of aluminium. Gallien-Krueger's MB210-II combo amp uses ceramic speakers. On the smaller end of the speaker spectrum, some small practice combo amps have 1x3", 2x5", 1x6.5", and 1x8" speakers. Many manufacturers abbreviate the number and size of speakers in the name of their equipment. A cabinet with two 10" speakers may be called a "210". Another abbreviation that is used is to add the wattage to the name, so a 500-watt Yamaha combo amp with two 12" speakers may be called the "Yamaha 212-500". Bassists who want a more powerful low end may use a subwoofer cabinet. Subwoofers are specialized for very-low-frequency reproduction, with typical maximum useful high frequencies of about 150 or 200 Hz, so a subwoofer cabinet must be paired with a full range speaker cabinet to obtain the full tonal range of an electric bass or upright bass. Bass