TS Parameters in Subwoofers: Explained

The terms Thiele and Small in TS are named after two experts (Neville Thiele and Richard Small) for their discovery and calculations in setting a list of parameters that define the quality and requirement of subwoofers and speakers.

This is how T/S stands for Thiele/Small and is also referred to as TSP (Thiele/Small Parameters).

Simply put, TS parameters define the specified low-frequency performance of a Subwoofer or speaker.

It consists of a range of measurements obtained from these units in free-air testing situations.

However, the advancement in technology in this modern era has made it easier to calculate specified parameters using various software or applications.

These software programs work on the calculations quite accurately making it easy for the manufacturers to define the parameters.

Surprisingly the subwoofers and speakers that we use in the sound systems have various parameters and calculations that we will discuss in this article below.

The parameters are typically categorized into Mechanical, Electrical, and Electro-mechanical.

  • The mechanical parameter defines the physical characteristics of the subwoofer’s suspension and cone.
  • The relation between the subwoofer magnets and the voice coils is specified by the electrical parameters.
  • Electro-mechanical parameters define the relation between the mechanical and electrical parameters.

T/S Parameters that define a subwoofer

To understand your subwoofer’s parameters defined by manufacturers you will need to first understand the terms and abbreviations.

Below is a list of the different types of parameters that manufacturers can use to define their subwoofers.

Parameter ‘Fs’ (or ‘F0’)

This particular code defines the free-air resonance frequency of the subwoofer which is measured in Hz.

At this frequency, the weight of the moving parts is perfectly balanced with the subwoofer suspension system.

Parameter ‘Q’

The parameter ‘Q’ refers to the relative damping factor or quality factor of a subwoofer.

It is basically relative to the suspension, magnet, and enclosure of the subwoofer’s driver.

The damping effect is responsible for reducing the amplitude of oscillations within the subwoofer and speaker.

It simply calculates the suspension’s reactions and as a general rule, the better the suspension the better the driver’s movement with control and accuracy in recreating sound effects.

Parameter ‘Qms’

This particular parameter defines the subwoofer and speaker’s relative damping (Q factor) at the resonance frequency (Fs). It also includes the mechanical losses.

Additionally, it is also referred to as the mechanical damping of the subwoofer.

It is among the unitless measurements that usually vary from 0.5 to 10.

Parameter ‘Qes’

‘Qes’ relates to the speakers and subwoofers damping (Q) at the resonance frequency (Fs or F0) considering the electrical losses.

It is also referred to as the mechanical damping of the subwoofer or speaker.

The unitless measurement usually varies between 0.5 and 10 which is similar to the parameter Qms.

Parameter ‘Qts’

It includes all the dimensional losses with the subwoofer’s relative damping (Q) at resonance frequency (Fs or F0).

The quality of the driver’s damping capabilities is primarily affected by the driver’s suspension. 

This is basically a combination of Qms and Qes and the equation used to calculate ‘Qts’ is 1/Qms + 1/Qes this will give you the ‘Q’ factor of the driver.

The value defines the best enclosure type suited to the driver and hence it’s essential for designing perfect enclosures based on the driver’s ‘Qms’ and ‘Qes’.

A Qts valued at 0.6 or more is suited to larger ported enclosures or free air due to a minimally controlled suspension.

On the other hand, values lower than 0.6 are well suited with ported enclosures due to a tightly controlled suspension.

In general, the mechanical damping decreases as the Q factors increase.

Additionally, the Q parameter of the setup usually changes once the driver is installed in an enclosure.

Simply put, after the driver is installed into an enclosure, the Q parameters of that setup can vary a bit.

This can be associated with the enclosure’s air helping to dampen the motion of the driver’s gentle parts.

Parameter ‘Mms’ and ‘Mmd’

‘Mms’ and ‘Mmd’ are two ways of calculating the total mass of the cone, coil, and other moving parts of the Subwoofer typically measured in grams.

Mmd is the direct physical weight of a percentage of the soft parts of the driver, the cone, VC, half the surround, and half the spider makeup in grams.

The Mms parameter is basically the weight of the air on the cone such as a small pocket of air fit on the cone.

Generally, the larger mass makes it more difficult to move the driver, because the heavier the cone the lower the Fs impacting its overall efficiency.

Parameter ‘Rms’

‘Rms’ is measured in kg/s and is the mechanical resistance of the subwoofer accounting for the driver’s suspension losses or damping.

Parameter ‘Vas’

‘Vas’ is measured in Liters and is the amount of air that has a similar stiffness as the subwoofer’s suspension does.

Greater Vas ratings mean the stiffness is lower and as per the parameter rating standards, larger enclosures are recommended for such units.

Parameter ‘Xmax’

This specific parameter defines the maximum amount of linear excursion the subwoofer is capable of and is typically measured in millimeters.

It is also referred to as the maximum peak-to-peak or linear peak excursion of the subwoofer.

Note: The amount of linear movement a voice coil can travel is referred to as excursion.

Parameter ‘Xmech’

The Xmech is the Excursion Distance before Damage such as the distance of travel for a VC before physical damage occurs to a woofer.

It usually exceeds the ‘Xmax’ parameter and considers the point where the surround and spider can become overstretched risking the VC of hitting the magnet bottom plate.

Parameter ‘D’

It defines the effective diameter of the subwoofer and is usually measured in metric units.

The measurement may not be the same with every subwoofer brand as some manufacturers will list them using different terminology.

Parameter ‘Sd’

Sd stands for the surface area of the driver’s cone which is measured in square centimeters.

It is quite useful in determining the amount of air a subwoofer can move. 

Cone designs vary from model to model, so 

Even if the drivers are of the same size, it is possible that they may offer different ‘Sd’ parameters because cone designs vary from model to model.

Parameter ‘Vd’

“Vd” parameter simply refers to the maximum amount of linear volume displacement the subwoofer is able to produce.

In other words, it means the total air volume the subwoofer’s cone moves, this metric is measured in cubic meters.

The formula used to measure the ‘Vd’ value is (Xmax) multiplied by the surface area (Sd).

Parameter ‘Re’

This specific parameter is the measurement of the voice coil’s resistance. 

It measures the impedance the amplifier will typically detect with the driver hooked up which simply means it measures the lower than the driver’s negligible impedance.

When it nears the resonant frequency of the driver, it generally needs more power to move.

This is due to a spike in impedance driven by the inductance of the voice coil.

Parameter ‘BL’

It measures the total magnetic strength of the motor structure or the motor force calculated in Tesla meters.

As a general rule, the higher the BL the higher the motor force, simply meaning the higher the motor force the better the efficiency.

However, this can also be affected by many other factors and is relative to the size of the woofer.

For example; A ‘BL’ of 10 is fairly normal, but on a 4-inch Mid is very high, and on an 18-inch sub would be rather low.

Parameter ‘n0’

It is the reference efficiency of the system and is usually more precise than the manufacturer’s sensitivity rating. This particular rating is described in percentage (%).

Parameter ‘SPLo’ or ‘SPL’

‘SPLo’ or ‘SPL’ denotes the subwoofer’s Sound Pressure Level (SPL). 

The SPL or sound pressure level of the speaker is measured in decibels (dB) and the rating is typically measured in two ways such as 1 watt at 1 meter or 2.83v at 1m.

Decibels or dB are determined by various factors that create efficiency, however, a larger SPL is better for the efficiency of the driver.

The SPL for a subwoofer that ranges around 88 to 90 dB is considered fair, however, midrange speakers and tweeters can be much more efficient and even exceed 100 dB due to their lighter construction.

Parameter ‘Pe’

This is the thermal capacity of the subwoofer and is measured in watts.

It defines the ratings at which the drivers run and using the driver at higher ratings can lead to its failure or simply damage.

Parameter ‘Pt’

It is another parameter that falls under the mechanical parameters category of the subwoofer.

It defines the thermal power handling of the subwoofer measured in watts, such as the recommended RMS rating of the unit.

Final Thoughts

The parameters are too many, however, you can get an insight into the workings of your driver and these parameters vary for different subwoofer enclosures.

To get you a nice working enclosure will only need to consider Fs, Qts, and Vas from the list of TS parameters.

To conclude, not all parameters are mentioned in this article. However, the manufacturers may recommend the type of enclosures or boxes in different codes such as Vc (volume of the closed box), Vb (volume of the bass reflex box), Fc (Resonance frequency of the closed box), Fb (resonance frequency of the bass reflex box), etc,.

Armed with a Diploma in electrical engineering and a remarkable 12 years of expertise, he excels in rejuvenating music systems, washing machines, dryers, and laundry-related appliances. Manish's profound insights and practical know-how establish him as a credible authority in appliance and music system repairs. Contact: manish.singh (at) subwooferblog.com. He's always ready to lend a helping hand!