Placement of coils in crossover networks
2 x 0.56 mH in series = 1.12 mH
Measured value = 1.144 mH

Go to response to this file:

Fig. 1. Coils 20 cm apart:
1.144 mH
Fig. 2. Coils 10 cm apart:
1.137 mH
Fig. 3. Coils as close as possible:
1.109 mH
Fig. 4. Coils placed close as seen on picture, one coil turned upside down: 1.181 mH
Compare with fig. 3.
Fig. 5. Coils placed close as seen on picture:
1.147 mH
Fig. 6. Coils placed close as seen on picture:
1.175 mH
Fig. 7. Coils placed on top on one another, same orientation:
1.312 mH
Fig. 8. Coils placed on top on one another, reverse orientation:
1.015 mH

Coils placed close to aluminium
Measured value = 3.305 mH
Fig. 9.
3.3 mH cored coil, vertical orientation.
Coil on plastic surface:
3.305 mH
Fig. 10.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu:
2.832 mH
Fig. 11.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu + 10 mm plywood:
3.055 mH
Fig. 12.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu + 15 mm plywood:
3.147 mH
Fig. 13.
3.3 mH cored coil, vertical orientation.
Coil on 15 mm alu + 25 mm plywood:
3.232 mH
Fig. 14.
3.3 mH cored coil, horizontal orientation.
Coil on 15 mm alu:
3.187 mH
Fig. 15.
3.3 mH cored coil, horizontal orientation.
Coil on 15 mm alu + 15 mm plywood:
3.279 mH
Fig. 16.
3.3 mH cored coil, horizontal orientation.
Coil on 15 mm alu + 25 mm plywood:
3.301 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?

 Thanks to Karl/Canada for asking!

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.

Dear Mr. Gravesen,
As I think you don't have much time as you are designing excellent speakers at this very moment I will try to make it short:
In my opinion, you measurements about the placement of coils won't show the whole truth or the real problem of placing coils next to each other. The real problem is the crosstalk, not the change in inductivity (although this is a little problem, too, even though the changes in inductivity stay below 10 %). It would be great if you cold do the measurements again, but this time measure the crosstalk, too. You could do this by giving one coil a 50 Hz signal with a defined voltage and measuring the output voltage on the other coil. Or you could at least give a hint on your page that the change in inductivity is not the only effect...
I'm writing to you as in many internet boards people refer to your measurements and you are kind of a expert among the hobby loudspeaker designers...
Best Regards, B.M.

Dear B.M.
Thanks for you comments to my "coil placement".
The aim of this small study was to demonstrate that coils interact and people should take care when constructing crossovers. I've seen horrific examples.
What would be interesting is to know what the impact of this would be under dynamic conditions. I do not have equipment to dig deeper into this problem, but it was my intention some day to try and make a listening test with an e.g. 3-coil crossover and see if people could hear any difference from coils wide apart and bundled together. Best regards Troels