What are T/S parameters?

Do you know all those numbers that are listed beneath the subwoofer and speaker models online or listed in the owner’s manual? Those are the thiele/small (T/S) parameters for that specific driver model and size. They are a set of electromechanical specifications used to display the performance capabilities of the driver. Understanding these parameters is useful in making comparisons between drivers. There are three categories of T/S parameters.  

1. Mechanical – Define the physical properties of the cone and suspension. 

1. Electrical – Define the relationship between the voice coil and magnet. 

1. Electro-Mechanical – Describe the relationship between the mechanical and electrical parameters.  

Mechanical Parameters 

Free Air Resonant Frequency (FS) Parameter  

The frequency that the soft parts (cone, surround, voice coil, and spider) resonate or vibrate the most efficiently at. Largely determined by the weight of the soft parts in the driver.  

General Rule: It becomes increasingly difficult to play frequencies below the driver’s FS. However, with suitable damping it is possible to play well below the driver’s FS.  

Note: Once the driver is installed into an enclosure, the resonant frequency of that setup changes. 

Q (Qms) Parameters 

The quality of the driver’s damping (resistance to or control of motion) capabilities. This is primarily affected by the driver’s suspension. 

Mechanical Damping (Qms) 

Damping is controlled by the driver’s surround and spider. 

General Rule: As the Q factors increase, the mechanical damping decreases.  

Note: Once the driver is installed into an enclosure, the Q parameters of that setup changes. This can be attributed to the air within the enclosure assisting in damping the movement of the driver’s soft parts. 

Volume to Air Equal to Compliance (Vas) Parameter 

The amount of air that equals the driver’s compliance to the cone moving.  

General Rule: The smaller this value, the stiffer the suspension is. 

Note: This parameter is not usually useful in determining the driver’s overall performance because humidity and temperature can greatly affect this measurement.  

Compliance (Cms) Parameter 

The stiffness, or compliance, of cone movement when acted upon by the force of one newton. Largely determined by the driver’s suspension components (spider and surround). 

General Rule: Stiffer movement of the cone will offer lower compliance while looser movement of the cone will offer higher compliance.  

Note: The compliance of a driver will inversely affect the driver’s resonant frequency.  

Moving Mass (Mms and Mmd) Parameter 

Mass of Moving Parts (Mmd) 

Mass of the cone, voice coil, the former, and half of the surround and spider. 

Mass of Moving Parts + Mass of Air Moving (Mms) 

Mass of the cone, voice coil, the former, and half of the surround and spider, and the air mass in front of and behind the cone.  

General Rule: The higher the mass, the lower the resonant frequency of the driver. Conversely, the lower the mass, the higher the resonant frequency of the driver. 

Cone Surface Area (Sd) Parameter 

The surface area of the driver’s cone.  

General Rule: Useful in determining how much air a subwoofer can move. 

Note: Cone designs vary from model to model, so even if the drivers are the same size, they may offer different Sd parameters. 

Distance to Distortion (Xmax) Parameter 

The distance that the voice coil can move until there are a lesser number of voice coil windings within the gap.  

General Rule: It affects how far the cone will move and thus how much air will be moved.  

Note: When there are a lesser number of voice coil windings within the gap distortion occurs. 

Electrical Parameters 

Q (Qes) Parameters 

The quality of the driver’s damping (resistance to or control of motion) capabilities. This is primarily affected by the driver’s suspension. 

Electrical Damping (Qes) 

Damping is controlled by the driver’s coil and magnet.  

Mechanical Damping + Electrical Damping (Qts) 

A value, important in enclosure designing, that is based on the driver’s qms and qes.  

Qts less than 0.4 is usually optimal for a ported enclosure. 

Qts between 0.4 and 0.7 is usually optimal for a sealed enclosure. 

Qts greater than 0.7 is usually optimal for an infinite baffle.  

General Rule: As the Q factors increase, the mechanical damping decreases.  

Note: Once the driver is installed into an enclosure, the Q parameters of that setup changes. This can be attributed to the air within the enclosure assisting in damping the movement of the driver’s soft parts. 

DC Resistance of the Voice Coil (Re) Parameter 

The measurement of resistance of the voice coil. 

General Rule: Usually measures lower than the driver’s nominal impedance, which is the impedance the amplifier will generally see with the driver hooked up.  

Note: When nearing the resonant frequency of the driver, it will require more power to move due to a spike in impedance caused by the inductance of the voice coil.  

Voice Coil Inductance (Le) Parameter 

The voice coil’s inductance, the electromotive force (EMF) generated from the current flowing in the opposite direction.

General Rule: The higher the value the steeper the rate at which the electrical impedance climbs as we move up the frequency scale. This is the reason why there is a spike in impedance near the driver’s resonant frequency and as frequency increases.  

Note: This value will be lower for drivers that are designed to play higher frequencies. Additionally, the higher the inductance, the slower the driver reacts to changes in the signal. 

Motor Strength (Bl) Parameter 

Product of the magnet’s flux density (B) and the length of wire within that flux (l); the strength of the motor that when applied to the voice coil, assists in moving the driver back and forth. 

General Rule: The larger the cone is the more motor strength that is required. This is what makes it difficult to compare motor strength from size to size (for example, an 8” driver compared to a 12” driver). You can, however, compare motor strength from model to model so long as the size of the driver stays the same.  

Note: This is a fair indication of how quickly the driver will react to input or overcome the stiffness of the driver’s suspension.  

Electro-Mechanical Parameters 

Q (Qts) Parameters 

The quality of the driver’s damping (resistance to or control of motion) capabilities. This is primarily affected by the driver’s suspension. 

Mechanical Damping + Electrical Damping (Qts) 

A value, important in enclosure designing, that is based on the driver’s qms and qes.  

Qts less than 0.4 is usually optimal for a ported enclosure due to a tightly controlled suspension. 

Qts between 0.4 and 0.7 is usually optimal for a sealed enclosure due to a moderately controlled suspension. 

Qts greater than 0.7 is usually optimal for an infinite baffle due to a minimally controlled suspension. 

General Rule: As the Q factors increase, the mechanical damping decreases.  

Note: Once the driver is installed into an enclosure, the Q parameters of that setup changes. This can be attributed to the air within the enclosure assisting in damping the movement of the driver’s soft parts. 

Sensitivity (SPL) Parameter 

The output of the driver with the input of one watt of power. 

General Rule: Tells you how efficient the driver is on one watt of power.  

Note: This can be measured differently from company to company. This is what makes it difficult to compare drivers between companies based on this parameter. You can, however, compare this parameter between drivers that are manufactured by the same company. 

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Image Credits: HexiBase