Understanding Key Loudspeaker Parameters(9): Force Factor (Bl) in Loudspeakers
Published by IWISTAO
Among all Thiele–Small parameters, the Force Factor (Bl) plays one of the most crucial roles in determining a loudspeaker’s motor strength and cone control. Often called the motor constant, Bl describes how effectively the voice coil and magnet system convert electrical current into mechanical force. A driver with a strong Bl typically delivers tighter, more controlled bass, while a weak Bl can result in looser, less accurate cone motion.
In simple terms, Bl tells you how powerful the speaker’s “engine” is.
1. What Is Bl?
Bl is the product of:
- B – magnetic flux density in the gap (Tesla)
- l – length of the voice-coil wire in the magnetic field (meters)
Bl = B × lIt is measured in Tesla-meters (T·m) or Newtons per Ampere (N/A). Bl indicates how much mechanical force the motor generates per ampere of current flowing through the coil.
2. Why Bl Matters
The basic force equation is:
F = Bl × IWhere F is cone-driving force and I is input current.
- High Bl → strong force → strong cone control
- Low Bl → weak force → loose or boomy response
Bl influences:
- Cone acceleration
- Bass tightness and accuracy
- Transient response
- Sensitivity and efficiency
- Distortion levels
- Enclosure tuning and system damping
3. Typical Bl Values by Driver Type
| Driver Type | Typical Bl (T·m) | Notes |
|---|---|---|
| 1–2″ Tweeter | 2–4 | Small gap and coil |
| 3–4″ Midrange | 4–6 | Light diaphragm |
| 5–6.5″ Woofer | 6–10 | Standard Hi-Fi woofer |
| 8″ Woofer | 9–14 | Good motor control |
| 10–12″ Subwoofer | 12–20 | Heavy cone control |
| 15–18″ Pro Subwoofer | 18–30+ | High SPL, strong motor |
| SPL Competition Sub | 25–45+ | Extreme motor strength |
4. How Bl Affects Speaker Behavior
a. Cone Control
A strong Bl motor holds the diaphragm tightly, reducing:
- Overshoot
- Ringing
- Boominess
High Bl = tight, accurate bass.
b. Sensitivity and Efficiency
Bl influences sensitivity based on:
Sensitivity ∝ (Bl)² / (Re × Mms)Drivers with high Bl and low Re can achieve much higher efficiency.
c. Maximum SPL
A stronger motor accelerates the cone more effectively, allowing higher maximum output before distortion.
d. Electrical and Mechanical Damping
Bl heavily affects Qes and Qts:
- High Bl → low Qes → tight, controlled response
- Low Bl → high Qes → warm or loose bass
This also determines ideal enclosure types.
5. Bl and Enclosure Interaction
1. Sealed Enclosures
- High Bl: tight, precise bass
- Low Bl: softer, more relaxed bass
2. Bass-Reflex Enclosures
Moderate to high Bl provides improved control around port tuning.
3. Horn Systems
Horn-loaded systems require very high Bl to maintain proper loading and efficiency.
4. Open-Baffle
Lower Bl is sometimes preferred to avoid over-damping the bass response.
6. Bl Linearity (Bl(x))
A good driver maintains stable Bl across the cone’s excursion range. Sharp drops in Bl(x) cause:
- Increased distortion
- Reduced SPL capability
- Loss of control at high excursion
Premium designs use optimized magnetic structures, underhung coils, and Faraday rings to stabilize Bl(x).
7. How Bl Is Measured
Method 1 — From T/S Parameters
Bl = √((Re × Mms) / Qes) × 2πfoMethod 2 — Klippel or Laser Analysis
Precision systems measure Bl(x) across excursion.
Method 3 — Manufacturer Specifications
Most datasheets list the Bl value explicitly.
8. Real-World Examples
| Driver | Size | Mms | Re | Bl | Description |
|---|---|---|---|---|---|
| Full-range A | 3″ | 2 g | 6 Ω | 4 T·m | Fast, light diaphragm |
| Woofer B | 6.5″ | 15 g | 5.6 Ω | 7.5 T·m | Balanced Hi-Fi design |
| Subwoofer C | 12″ | 75 g | 3.2 Ω | 20 T·m | Powerful low-frequency authority |
| SPL Sub D | 15″ | 250 g | 2 Ω | 32 T·m | Extreme motor force for competitions |
9. How to Interpret Bl
High Bl Means:
- Strong motor force
- Tight cone control
- Lower distortion
- Higher SPL capability
- Good match for vented and horn systems
Low Bl Means:
- Weaker motor force
- Warmer, softer bass
- Higher Qts
- Useful for open-baffle designs
Conclusion
The Force Factor (Bl) is the core indicator of a loudspeaker’s motor strength and control. It influences bass tightness, distortion, efficiency, and how the driver interacts with its enclosure. By understanding Bl and balancing it with Mms, Re, Sd, and Xmax, designers can create speakers that deliver clean, powerful, and precise sound performance across all listening conditions.
Understanding Bl helps designers and audiophiles select the right driver for the right enclosure — whether it's a fast, articulate bookshelf speaker or a deep, high-SPL subwoofer.
