Driver TS Parameters: BL (Motor Strength)

BL is the product of the magnetic field strength (B) in the voice coil gap and the length of wire (L) within that field, measured in tesla-meters (T·m). It represents the force generated to move the cone.

For basic speaker design, motor strength (BL) isn’t a primary concern since manufacturers optimize it for the driver’s intended purpose. However, understanding BL helps in designing high-performance enclosures and matching drivers to specific applications.

Simply put, BL quantifies the strength of the motor system. It is calculated as the product of the magnetic field (B) and the active length of the voice coil (L) within that field. A stronger BL means greater force to move the cone, resulting in better control and efficiency. For a more detailed explanation, with diagrams, please see the article on Xmax: https://speakerwizard.co.uk/driver-ts-parameters-xmax/ which includes diagrams of the magnet structure, and magnetic fields.

With ferrite magnets, increasing motor strength typically requires a larger magnet, which adds weight and cost. However, magnet size alone isn’t an indicator of quality—some large-magnet drivers are poorly designed and inefficient, and the huge magnet is just for show – although a large magnet can have a side benefit of providing a large thermal mass and surface area for cooling.

Neodymium magnets offer high motor strength with minimal weight, making them ideal for portable and high-performance systems. However, they are significantly more expensive than ferrite alternatives.

BL depends not just on magnet strength but also on the voice coil’s wire length within the field. This means coil height, diameter, wire gauge, and even multi-layer winding techniques influence BL. Many modern drivers use inside-outside wound voice coils to maximize wire length while maintaining compact designs.

BL can be thought of as the “muscle” of the speaker motor. A high BL means the motor exerts greater force on the cone, improving control and efficiency. Drivers with high BL values tend to deliver tight, accurate sound, while low BL drivers may sound softer or “looser.”

To help put things into context, for a typical 18″ woofer a high BL figure would be considered 30 or higher. A driver with a BL in this range will exhibit very price cone control. A low BL figure would be 20 or less, a driver with a low BL will be significantly less able to control the cone accurately.

Depending on your application, you may still be wondering why you should care about BL? If you are planning on building a horn loaded bass bin, or scoop bin, a high BL is pretty much essential, you wont get away with just chucking any old driver into the cabinet and get the right results. If you consider that in most horn loaded applications you are having to compress air, you want a driver that exhibits a high force to achieve this compression, a strong motor makes this possible.

In other applications, if you were to make a listening comparison between a high BL driver with a low BL driver, you would find the high BL driver will sound much tighter and more accurate. Generally for live music applications this type of sound is preferred as it will more accurately reproduce the instrument sounds. Low BL driver can sound ‘woolly’ or ‘muddy’ because the cone does not respond to transients as quickly, and has the potential to introduce some distortion. Some people prefer this sound as it can give a warmer bass sound.

High BL drivers are generally needed for high power bass applications, where a large heavy cone is used. High BL drivers are usually more efficient, as the higher motor strength equates to more pushing power. For mid-range drivers, it is normal to use a much lighter cone, and a high BL is not necessarily needed, but can contribute to a more accurate response.

High BL drivers make it possible to use smaller sealed boxes in some designs, because the motor strength is so high, the restoring force of the air in the box, and the suspension of the driver, are small in comparison, making them relatively insignificant.

High BL increases electromagnetic damping, which can be problematic in bass reflex enclosures. Excessive damping may reduce the effectiveness of the port, requiring design adjustments for optimal performance. With bass reflex designs a slightly lower BL is often more appropriate to get a more balanced result.

We’ll discuss BL and voice coil geometry more in another article.