In the world of audio, there are many acronyms and abbreviations that can be confusing for those who are new to the industry. One such term is speaker BL.
Speaker BL is often referred to as the force factor of a speaker and gives us an indication of the magnetic motor strength of a speaker. In very simple terms, it tells us how strong the magnet motor system of the speaker is.
Like everything with speakers, the BL value is only part of the puzzle. Knowing the BL value on its own will not tell you how great a speaker’s performance is, but used in conjunction with other speaker parameters, it can help paint a picture of speaker performance.
In this article, I will cover the fundamentals of speaker BL, covering:
- What is speaker BL?
- What is the unit of BL?
- How do you calculate speaker BL?
- How do you measure speaker BL?
- How do you design speaker BL?
- How does speaker BL impact sound?
What Is Speaker BL?
Speaker BL gives us an indication of the magnetic motor strength in a moving coil speaker driver.
Within the speaker design industry, you may hear BL referred to as the “force factor” or “shove” of a speaker.
All standard moving coil speakers have a voice coil attached to a cone or diaphragm. The voice coil sits within a magnet.
When current is applied to the voice coil in the form of our input signal, the voice coil in the magnetic field will vibrate up and down, which in turn moves the attached cone or diaphragm up and down, which in turn moves air, to produce sound.
The current that is flowing through the voice coil, which is sitting in the magnetic field, creates a force. It is this force that moves our cone/diaphragm and is calculated from the following formula:
The force factor BL is calculated from the following formula:
- BL = B (The flux density of the magnet) x L (The length of voice coil wire in the magnetic gap)
We want to know how strong our magnet system is to understand how much force it produces, so we are sure our magnet system is strong enough to move the attached cone/diaphragm and produce sound.
What Is The Unit Of BL?
The unit of BL is Tesla meters (Tm) or N/A (Newtons Per Amp)
N/A (Newtons per Amp) explains nicely what is happening in the speaker motor system as Newtons Per Amp is “Newtons of force per AMP of voice coil current”.
How Do You Calculate Speaker BL?
The force factor BL is calculated from the following formula:
- BL = B(The flux density of the magnet) x L (The length of voice coil wire in the magnetic gap)
We are going to want to know how strong our magnet system is to understand how much force it produces, so we are sure our magnet system is strong enough to move the attached cone/diaphragm and produce sound.
For example, let’s say you have a speaker with a BL = 10N/A.
In this simple example, a speaker with 1A (amps) of current in the voice coil can hold a mass of 1kg on the diaphragm/cone, which still keeps the cone in a steady position.
From this, we can understand how strong our magnetic system (speaker motor) actually is.
Unfortunately, BL is not that easy to calculate in reality, as the flux density in the speaker motor assembly gap is not constant and not all the coils on the voice coil are in the magnetic field.
For example, in a woofer, some coil length may be outside the magnetic field created by the motor assembly, and this coil, which is outside the magnetic field, contributes no force.
To be very precise, a true and actual BL product is found by calculating the integral of the field strength over the length of the coil in the gap, or by actually measuring the BL.
How Do You Measure Speaker BL?
There are many ways to measure speaker BL, but one of my favourite methods is called the “opposing force method”.
To complete a BL speaker measurement using this method, you will need the following equipment:
- DC Power Supply
- Multimeter
- Speaker
Below is a simple step-by-step guide on how to measure speaker BL using the opposing force method
- Take a known mass, let’s call it Ma (accurately measured), and place it on the cone. The weight of the mass must be heavy enough to move the cone by at least 0.6cm.
- Connect your speaker to the DC power supply.
- Gradually increase the DC voltage until the cone rises back to its original resting position (where it was before any mass was applied).
- When you reach the original resting position of the cone, note the current (I) from the multimeter.
- Using your measured values, you can calculate the BL from the following formula:
- BL = (9.8 x Ma) / I
where:
- Ma = measured mass you placed on the cone in kilograms
- I = measured current in amperes when the cone restores to its original resting position.
How Do You Design Speaker BL?
There are many variables that go into designing a speaker with the correct level of BL.
Some of these factors which help decide our BL when designing are:
- the type and size of the magnet
- the gauge and length of the voice coil wire
- the type of enclosure.
By understanding how speaker BL is calculated, you can better select components for your speaker design to ensure you achieve your desired level of force factor.
From a speaker designer’s point of view, when designing a magnetic circuit, there will be a requirement for a given BL value.
From this starting point, a speaker designer will work out some realistic voice coil specifications which include the coil diameter, coil wire thickness and length of the coil to be immersed in the magnetic field.
Using advanced simulation software, speaker designers will then create an electromagnetic simulation of the magnet motor assembly to see how well saturated the metalwork is regarding magnetic flux to ensure a sensible flux value is achieved.
How Does Speaker BL Impact Sound?
BL increases output (sensitivity) for a given cone area.
When you think about this in very simple terms, a stronger speaker motor (magnetic assembly) will be able to have much more control over the moving cone or diaphragm. This results in the ability to reproduce the input signal more precisely.
It is important to point out that a high BL value does not mean better efficiency or sensitivity by default, as there are many other factors that influence the output of a speaker.
Generally, BL can be correlated to the size of a speaker. Bigger speakers need larger BL as they are moving more mass.
However, when comparing speakers of a similar size, a higher BL will yield a better transient response since our motor is stronger and can move the cone/diaphragm with greater control, speed and, as a result, accuracy.
Final Thoughts
Speaker BL is a measure of the force factor that is exerted on the speaker cone by the magnetic field of the speaker motor.
BL affects sound quality by increasing output for a given cone area and providing a better transient response.
By understanding how BL is calculated and used, you can create better magnet and voice coil systems for your speaker design to ensure optimal sound quality.
