1. Woofer Design using non-linear BL(x) curves
ALMA 2009 Paper Presentation:
Woofer Design using non-linear BL(x) curves
Peter Larsen

2. Drivers can be designed for:
Target Thiele-Small parameters
Target System response
The sound quality depends on:
Excursion
Shape of the BL(x) curve
Voice Coil offset
Amount of steel
Compliance linearity
High power stability
Etc
Demand for cost savings

3. Design for target TS parameters
Target Specifications:
Woofer inch
Nominal impedance 8 ohms
Total Qts
Resonance Fs <40 Hz
Xmax ~5mm (Vas <40 liters)
Magnetically Shielded

4. Available driver components:
Voice coil
Ø25.60mm ID
DCR (Re)=6.2 ohms
0.05 mm Kapton former/bobbin
6.5” paper Cone + Sur.
13.11cm diameter
Cone Fo=22.5 Hz
(Cone + dust cap + ½surround + ½spider =8g)
Top plate
ID 27.20mm
5mm thickness

5. Design for target TS parameters

6. BL(x) for 3 different Pole Geometries

7. Magnetic FEA of extended Pole geometry

8. Alternative Solution
Woofer to be designed for the target specs, using a 4-layer Voice Coil.
The 4-layer wire dimension, VC winding height and number of windings etc. is easily calculated, but the air-gap diameter must be increased to accommodate the larger winding outer diameter. The question is how much.

9. 4-layer VC Design for target TS parameters
Smaller magnets and less steel
Cost savings

10. Bass Reflex responses of 2-layer VC___/ 4-layer VC___

11. Stiffness (Spider___/ Surround___/Total___)
Suspension
Stiffness (Spider___/ Surround___/Total___)

12. Suspension Non-linearity
Fs of 2½ inch driver measured at 5V___ / 0.5V___ / 0.05V___ Input

13. Influence of Suspension Stiffness in System
2-layer VC woofer in 20L Reflex box ___Fs=38Hz/ ___Fs=55Hz

14. Suspension
Simulation of 6in woofer with changed Surround___ / standard___

15. Voice Coil Induction
Basic formula:

16. Two examples of VC Inductance Variation

17. Two alternative ways to minimize Voice Coil Inductance
Inside Neodymium motor with minimum Steel
1-layer Flat Wire Voice Coil

18. “Modern Design” Example
Vifa Woofer P17WJ from 1987

19. Compliance with Gravity
The total driver compliance will actually determine how far a woofer cone/VC will sink when placed horizontally
Therefore high compliant woofers with heavy cone/VCs may sink more than 1 mm. So storage is sometimes a problem

20. Appendix A

21. Appendix B