T/S (THIELE/SMALL) PARAMETERS: terms used to describe the characteristics of a given driver. The most common T/S parameters encountered are:
Fs = resonance frequency of the driver. In free-air, the driver's impedance will peak at this frequency.
Pe = Thermal capacity of the driver, in Watts. If continuously driven above its rated Pe, the driver may prematurely burn out and fail.
Qes = Electrical Q of the driver at Fs. Qes is a measure of the driver's tendency to resonate at Fs, based on its electrical characteristics, e.g. magnet strength, magnetic circuit characteristics, etc.). The driver's overall resonance characteristics are usually dominated by Qes.
Qms = Mechanical Q of the driver at Fs. Qms is a measure of the driver's tendency to resonate at Fs, based on its mechanical characteristics, e.g. surround compliance, the compliance of the spider, weight of the cone, etc.
Qts = Total Q of the driver at Fs. Qts is a measure of the driver's tendency to resonate at Fs, based on its overall characteristics. Qts can be calculated using the equation:
Qts= Qms*Qes/(Qms+Qes))
Re = DC resistance of the driver's voice coil. Re is less than the driver's rated impedance (normally 4 or 8 ohms).
Sd = Effective surface area of the driver. Roughly equal to the area of the cone plus 1/3rd of the surround.
Xmag = [DUMAX] Excursion limit due to the magnetic limitations of the driver's motor. Xmag is defined as the displacement at which the BL product has fallen to 70% of its value at the cone's rest position.
Xmech = Maximum physical excursion capability of the driver. Exceeding Xmech normally results in damage to the driver.
Xsus = [DUMAX] Excursion limit due to the driver's suspension. Xsus is defined as the point at which the compliance of the suspension has decreased to 25% of the value at the cone's rest position.
Xmax = Linear (one-way) travel of the cone. Xmax is used to determine the maximum linear SPL capability of the driver, and can be defined in a number of ways. The DUMAX definition is objectively the best one, and it is defined as the shorter of the Xmag and Xsus values, in each direction of cone travel. This definition is more useful than the older definition of Xmax, which was solely dependent on the length of the voice-coil vs. the length of the gap.
Vas = Equivalent air compliance. The volume of air that has the same compliance ("springiness") as the driver's suspension. Because less air is more "springy" than more air, a large Vas represents a "loose" suspension
Vd = Peak displacement volume. Vd = Sd*Xmax
