The lowest note on the double bass or 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).