Back in the 1960s the Australian engineer Neville Theile and American engineer Richard Small developed a method of modelling loudspeakers. This was groundbreaking work, allowing the bass performance of loudspeakers to be explicitly designed, rather than developed through trial and error. Forty years later, their work remains the basis of loudspeaker design. To use their design techniques a number of characteristics of the woofer to be used must be known. These include such things as the driver's resonant frequency, its compliance, its 'equivalent volume' and so forth. These are collectively known as the Theile and Small parameters and are stated by all reputable driver makers for those drivers they sell (although not necessarily for drivers that are only sold installed in loudspeakers).