Placement of coils in crossover networks
Copyright 2009-22 Troels Gravesen

The study below may suggest you need a square meter board to construct a crossover. Not so. We can and have to compromise.
The general rule of thumb is that we should not be able to see one coil through the center hole of another.
Also coils carry different loads meaning the electromagnetic field around a coil will me different and not equal to the two 0.56 mH coils used in this study.
Coils in midrange and tweeter circuits may carry very little current and can be placed relatively close, where a huge coil for bass may need more distance.
And another thing is what we can measure - and what we can actually hear. How much interaction is allowed before we can hear an impact on sound. Not easy!

About C-coils


2 x 0.56 mH in series = 1.12 mH
Measured value = 1.14 mH
Fig. 1. Coils 20 cm apart:
1.14 mH
Fig. 2. Coils 10 cm apart:
1.14 mH
Fig. 3. Coils as close as possible:
1.11 mH
Fig. 4. Coils placed close as seen on picture, one coil turned upside down: 1.18 mH
Compare with fig. 3.
Fig. 5. Coils placed close as seen on picture:
1.15 mH
This is OK.
Fig. 6. Coils placed close as seen on picture:
1.18 mH
Don't do this.
Fig. 7. Coils placed on top on one another, same orientation:
1.31 mH
Never do this.
Fig. 8. Coils placed on top on one another, reverse orientation:
1.02 mH
Never do this.

 

 

Coils placed close to aluminium
Measured value = 3.31 mH

Fig. 9.
3.3 mH cored coil, vertical orientation.
Coil on plastic surface:
3.31 mH
Fig. 10.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu:
2.83 mH
Fig. 11.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu + 10 mm plywood:
3.06 mH
Fig. 12.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu + 15 mm plywood:
3.15 mH
Fig. 13.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu + 25 mm plywood:
3.23 mH
Fig. 14.
3.3 mH cored coil, horizontal orientation.
Coil on 15 mm alu:
3.19 mH
Fig. 15.
3.3 mH cored coil, horizontal orientation.
Coil on 15 mm alu + 15 mm plywood:
3.28 mH
Fig. 16.
3.3 mH cored coil, horizontal orientation.
Coil on 15 mm alu + 25 mm plywood:
3.30 mH
  Conclusion to this small additional study:
Coils placed vertically near to aluminium display a significant reduction in inductivity. Coils should at least be 5 cm/2 inches from nearest aluminium surface. More is recommended.
Coils placed horizontally near to aluminium display only minor impact on inductivity, however 3-4 cm distance is recommended.

How about caps?

 

3.9 mH air cored coil
Measures 3.97 mH

Obbligato cap in copper tube:
Very little impact when placed besides the coil.

Obbligato cap in copper tube:
Reduction in inductance when placed directly in the electromagnetic field of the coil.

Capacitor in alu tube:
Little impact when placed aside coil.

Capacitor in alu tube:
Reduction in inductance when placed directly in the electromagnetic field of the coil.

Silver mica cap in metal (Fe) housing:
Fortunately we never use these for crossovers.
  All other caps in non-metal wrap had no impact on inductance.

 

C-coils:


You may find difficulty measuring the inductance of C-coils due to an extremely low DCR.
Here's a link to a page that displays 3 options for measuring inductance. I use Method 2 for C-coils and some 2% Superior-Z caps.
http://daycounter.com/Articles/How-To-Measure-Inductance.phtml