Thiele and Small Parameters (also referred to as T/S parameters) are used to define technical electromechanical parameters for a loudspeaker.  These specifications are used primarily by advanced consumers that are looking for low frequency performance of a subwoofer.  A manufacturer runs a series of tests to determine the parameters of the loudspeaker and will typically print this information in the product manual of the subwoofer.  This information is used by intermediate to advanced car audio box builders to make a subwoofer enclosure designed to the exact specifications of the subwoofer. The ratings are named after A. N. Thiele and Richard H. Small, the two men responsible for these detailed parameters.

Here is an example of the T/S parameter from an RE Audio SEX12D4

  • Qes Electrical Q Value: 0.49
  • Qms Mechanical Q Value: 4.5
  • Qts Total Speaker Q Value: 0.44
  • Fs Free Air Resonance: 25.1 Hz
  • Vas Equivalent Compliance: 72.9 liters
  • Xmax One-Way, Linear Excursion: 18 mm
  • Efficiency (SPL 1W/1m): 86.1 dB SPL
  • Sd Effective Piston Area: 480 cm2
  • Re DC Resistance: 2.8 ohm
  • Znom Nominal Impedance: Dual 4 ohm
  • Pe Thermal Power Handling: 600 W
  • BI Force Factor: 17.7

What exactly do these abbreviations and numbers mean?  Below is a list of the different types of parameters that manufacturers can use to define their subwoofers.

Fs: The free-air resonance frequency of the subwoofer, measured in Hz.  At this frequency the weight of the moving parts is perfectly balanced with the subwoofers suspension system.
Q: This refers to the relative damping of a loudspeaker.  This is the “Q” that is in the other Q parameters. The damping is the effect that reduces the amplitude of oscillations within the subwoofer
Qms: This is the subwoofers relative damping (Q) at the resonance frequency  (Fs), including mechanical losses.  Also called the mechanical damping of the subwoofer.  This unitless measurement usually varies from 0.5 – 10.
Qes: The subwoofers relative damping (Q) at the resonance frequency (Fs), including electrical losses.  Also called the mechanical damping of the subwoofer.  The unitless measurement usually varies from 0.5 – 10.
Qts: This is the subwoofers relative damping (Q) at the resonance frequency  (Fs), including all dimensionless losses.
Mms: The total mass of the subwoofers cone, coil, and other moving parts.  It is measured in grams.
Rms: The mechanical resistance of the subwoofer, taking into account the driver’s suspension losses (damping).  It is measured in kg/s
Vas: This is the amount of air that has the same stiffness as the subwoofers suspension. The greater the rating, the lower the stiffness and larger recommended enclosure.  It is measured in Liters.
Xmax: The maximum amount of linear excursion a subwoofer is capable of.  It is typically measured in millimeters. The excursion is the amount of linear movement that the voice coil can travel.  Also referred to as maximum linear peak or peak-to-peak.
D: This is the effective diameter of the subwoofer.  It is usually measured in metric units, however some manufacturers will list it with different units.
Sd: The surface area of the cone measured in square centimeters.
Vd: This is the maximum amount of linear volume displacement the subwoofer is able to produce.  It is the total air volume the subwoofer’s cone will move, measured in cubic meters. The formula Xmax multiplied by the surface area (Sd) is used.
Re: Amount of DC resistance of the subwoofer when measured with an ohm meter.  Typically, this will read less than the impedance rating of the subwoofer.
BL: Total magnetic strength of the motor structure measured in Tesla meters.
n0: The reference efficiency of the system.  It is often more accurate than the manufacturers sensitivity rating and is expressed as a percentage.
SPLo: Represents the subwoofers Sound Pressure Level (SPL).  The rating is measured at 1 watt, from 1 meter in front of the subwoofer.
Pe: This is the thermal capacity of the subwoofer. Using the driver at higher ratings can cause damage and failure to occur. It is measured in watts.
Pt: This is the thermal power handling of the subwoofer and is typically the recommended RMS rating of the subwoofer measured in watts.

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