Editing Thiele/Small parameters (section)

==Lifetime changes in driver behavior==
The mechanical components in typical speaker drivers may change over time. Paper, a popular material in cone fabrication, absorbs moisture easily and unless treated may lose some structural rigidity over time. This may be reduced by coating with water-impregnable material such as various plastic resins. Cracks compromise structural rigidity and if large enough are generally non-repairable.  Temperature has a strong, generally reversible effect; typical suspension materials become stiffer at lower temperatures. The suspension experiences [[Fatigue (material)|fatigue]], and also undergoes changes from chemical and environmental effects associated with [[aging]] such as exposure to ultraviolet light, and [[oxidation]] which affect foam and natural rubber components badly, though butyl, nitrile, SBR rubber, and rubber-plastic alloys (such as [[Santoprene]]) are more stable. The polyester type of [[polyurethane foam]] is highly prone to disintegration after 10 to 15 years. The changes in behavior from aging may often be positive, though since the environment that they are used in is a major factor the effects are not easily predicted. Gilbert Briggs, founder of Wharfedale Loudspeakers in the UK, undertook several studies of aging effects in speaker drivers in the 1950s and 1960s, publishing some of the data in his books, notably ''Loudspeakers: The Why and How of Good Reproduction''.<ref name="Briggs_2018"/>

There are also mechanical changes which occur in the moving components during use.<ref name="Klippel_Aging_Presentation"/><ref name="Klippel_Aging_Paper"/> In this case, however, most of the changes seem to occur early in the life of the driver, and are almost certainly due to relaxation in flexing mechanical parts of the driver (e.g., surround, spider, etc.). Several studies have been published documenting substantial changes in the T/S parameters over the first few hours of use, some parameters changing by as much as 15% or more over these initial periods. The proprietor of the firm GR Research has publicly reported several such investigations of several manufacturers' drivers. Other studies suggest little change, or reversible changes after only the first few minutes. This variability is largely related to the particular characteristics of specific materials, and reputable manufacturers attempt to take them into account. While there are a great many anecdotal reports of the audible effects of such changes in published speaker reviews, the relationship of such early changes to subjective sound quality reports is not completely clear. Some changes early in driver life are complementary (such as a reduction in <math>f_{\rm s}</math> accompanied by a rise in <math>V_{\rm as}</math>) and result in minimal net changes (small fractions of a dB) in frequency response. If the performance of speaker system is critical, as with high order (complex) or heavily equalized systems, it is sensible to measure T/S parameters after a period of run-in (some hours, typically, using program material), and to model the effects of normal parameter changes on driver performance.