June 2, 2010

What do the numbers tell you about a driver?

So you want to use a particular driver and you are wondering if it's suitable for your application. If you look at the data sheet, you will see "Thiele Small" parameters. What do the numbers mean? What can you judge about a driver based on these numbers? In this article the focus is on bass drivers.

What you can't tell ...

The parameters don't tell the whole story. They won't show how well it is built or anything about the quality or distortion performance. They also won't show if the driver has a nice flat response or if it has nasty break-up or resonances. What the parameters will do is show you if it's suitable for the box you have in mind and what kind of output and extension you can expect. This is a good starting point.

The main parameters to watch

free air resonance

This indicates the low frequency limit of the driver. In general this becomes the low frequency limit in a vented box, although you can run a driver lower than fs if the box is large. Driver excursion becomes critical as this creates an extra demand. In a sealed box, the bass limit will be one octave above fs. 12db of EQ is required to get extension down to fs however, this will quadruple driver excursion and require more than 10x as much power!

total Quality factor

This has nothing to do with quality. It is a measure of the damping of a driver. A driver with a low Qts value (<0.3)>0.5) has less damping, often resulting in a peak around fs. Mid Q drivers are a popular choice for subwoofers as they are generally flexible and will suit vented and sealed designs.

High Q drivers are often used in a sealed box use as they tend to need a very big vented box. Some of the new Exodus audio drivers have a relatively high Q. While a fairly large vented box is generally required, it will be more efficient than a lower Q driver in a smaller box.

Low Q drivers allow small boxes, but their efficiency is generally lower so that high power is required. The result is often a sub that is expensive for the performance that is offered. Many commercial subs use low Q drivers to allow small size, but due to the inefficiency, very high powered amplifiers are used. Early versions of the Peerless XLS driver were very low Q and worked in a small box but were not practical for a vented design. A workable vent would not fit in the box so passive radiators were required. The cost jumps up as a result.

equivalent volume compliance - the volume of air which has the same compliance as the driver's suspension

VAS in conjunction with Qts gives an idea of the size of the box required. Generally, driver with a large VAS need a large box. If the driver has a high Qts, it needs to be even bigger.

effective piston area

This is the total cone surface area that produces output. Generally part of the surround is included as part of the piston area. Drivers with very large surrounds have a lower SD.

maximum linear excursion (one way)

xmax is typically defined as the point where the force of the motor (BL) drops to 70% and is considered to be the maximum excursion which is useful. This is critical for determining the output capability of the driver, when combined with power handling and SD.

mechanical excursion limit

This is the limit of cone travel before damage, although some drivers avoid damage when pushed beyond xmech. Ideally you should design a subwoofer so that there is some kind of limiting so that there will never be enough power to push a driver to this point. This can be done by a combination of a few strategies:
  • limiting amplifier power
  • sealed box where the selected volume will limit excursion to safe levels
  • rumble filter in the crossover
  • DSP dynamic filtering
Electrical power handling

This number should always be taken with a pinch of salt. Often an inflated number is given and even when accurately quoted, may not be significant. Car audio subs often have a high power handling and low xmax. This works well in their application, where they will often be placed in a small sealed box in the small space of a car which provides massive acoustic bass boost. In a home theatre subwoofer application, xmax is more critical. Where a home theatre subwoofer is used for music, thermal power handling becomes the limiting factor.

Voice coil inductance

Ideally inductance should be as low as possible. In a crossover, inductors act as a low pass filter. High inductance in a driver has the same effect. If it is very high, it can limit the upper useable bandwidth. It is also said to have a deterimental impact on transient response, although this aspect is the subject of debate.

Driver moving mass

Avoid the temptation to choose a driver with low mms thinking it will sound faster. Mms is chosen to give a desired fs and when considered in isolation does not show how "fast" a driver is.

Force factor

This is an indicator of motor strength. Avoid the temptation to choose a driver because of high BL. Considered in isolation, this means nothing.


This number is misleading. The design of the box has such a dramatic impact on the actual efficiency, that this number tells us very little. A high sensitivity driver may end up less efficient than a much lower sensitivity driver.

Parameters to ignore

Most of the other parameters don't warrant consideration, at least not in a quick analysis of a driver.

Example 1 - Peerless XLS (830500)

fs: 18 Hz
Qts: 0.2
VAS: 139 L
xmax: 12 mm
xmech: 22 mm

This driver works in a very small box. It suits a sealed box or passive radiator box. A 40L box can get decent extension, but there is no way a practical vent will work. Low tuning in a small box doesn't work. You might make it as big as 85L, which is bigger than required but at this size, you can make a workable vented box with a 4" vent. Xmax is moderate at 12mm so this driver will only need 240 - 350w. xmech is quite high at 22mm due to the large rubber surround so there is a fair safety margin. SD is quite low for a 12" driver.

Example 2 - Exodus Tempest-X2

fs: 20 Hz
Qts: 0.42
VAS: 255 L
xmax: 26 mm
PE: 1kw

The parameters show this is a very different driver. It needs a much bigger box as seen in the higher Qts and VAS. It also has more than double the excursion and very high thermal power handling. For home theatre use, a large vented box with a high power amp will yield more extension and output than the modest XLS driver.

The next step

After taking a quick look at the parameters, it's time to simulate the driver. A great deal can be learnt this way. After trying different settings, you will find that you can force fit a driver into different applications.

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