I use three values here, namely 0.3, 0.7 and 1 mH (These are the values used in the Ariel loudspeakers). Here's how to get from inductance value to actual-coil-in-your-hot-grubby-little-paws.

- The
calculator at Mogami-wire
gives the following for 8, 10, 12 and 14 gauge copper wire:
Wire size Area Diameter (mil) Diameter (mm) Resistance 8 AWG 8.336mm^2 128.5 mil 3.264mm 0.00206 Ohm/m 10 AWG 5.261mm^2 101.9 mil 2.588mm 0.00328 Ohm/m 12 AWG 3.309mm^2 80.8 mil 2.053mm 0.00521 Ohm/m 14 AWG 2.081mm^2 64.1 mil 1.628mm 0.00829 Ohm/m - Lynn Olson recommends
North Creek 10 or 12 gauge air-cored inductors for
the Ariel crossover.
__And so do I__, if you can easily get hold of them. We on this side of the pond have to deal with ~20% import duties, expensive shipping, and a terrible exchange rate, so I'm going to try and make my own inductors. - The North Creek inductors have the following DC
resistances, from which I calculated how much
wire they used. (You must agree that this is pretty quick and
dirty... :-)
Coil Inductance 10 Gauge R Equivalent length 12 Gauge R Equivalent length 0.3 mH 0.04 Ohm 12.195m 0.07 Ohm 13.436m 0.7 mH 0.06 Ohm 18.293m 0.11 Ohm 21.113m 1.0 mH 0.09 Ohm 27.439m 0.14 Ohm 26.871m - I've sourced the enamelled copper wire locally, from
WST (021 511 6287). They gave me the following information:
Wire diameter Length/kg Price/kg Price/100m 2.12mm 31.49m/kg ~R36 R114 (2001) 2.5mm 22.67m/kg ~R36 R159

You can use the calculator at Shavano Music Online, which gives

Inductance | Re | height | radius | turns |
---|---|---|---|---|

1 mH | 0.11 ohm | 30.83 mm | 61.66 mm | 75 turns |

0.7 mH | 0.09 ohm | 28.52 mm | 57.04 mm | 65 turns |

for 10 gauge and

Inductance | Re | height | radius | turns |
---|---|---|---|---|

1 mH | 0.16 ohm | 25.56 mm | 51.13 mm | 82 turns |

0.7 mH | 0.13 ohm | 23.73 mm | 47.46 mm | 71 turns |

0.3 mH | 0.08 ohm | 19.8 mm | 39.6 mm | 51 turns |

for 12 gauge wire.

Alternatively, the calculator at Lalena (using Shavano's dimensions) gives

Inductance | Re | height | radius | turns |
---|---|---|---|---|

1 mH | 0.1 ohm | 30.83 mm | 61.66 mm | 73 turns |

0.7 mH | 0.08 ohm | 28.52 mm | 57.04 mm | 63 turns |

for 10 gauge and

Inductance | Re | height | radius | turns |
---|---|---|---|---|

1 mH | 0.15 ohm | 25.56 mm | 51.13 mm | 79 turns |

0.7 mH | 0.12 ohm | 23.73 mm | 47.46 mm | 69 turns |

0.3 mH | 0.07 ohm | 19.8 mm | 39.6 mm | 50 turns |

for 12 gauge wire.

The
formula they use is

L=((D^2 * T^2)/(1000*(18*D+40*C)))

Where D is the average turn diameter, C is the coil length, and
T is the number of turns.

There's also a calculator on
Peter Ortner's site which is based on A.N. Thiele's paper *Air
Cored Inductors for Audio*. To summarise the summary :-) Thiele derives
a *time constant * K = (L/R) (with L in uH, R in ohm) which he uses
to get the optimum dimensions for an inductor. The inner radius, thickness
and width of the coil are all equal, namely C = sqrt( K / 8.66).

Apparently, the length of the wire required is W = 0.1873 * sqrt(L*C)

The number of turns is N = 19.88 * sqrt(L/C)

The wire diameter is D = 0.841*C/sqrt(N)

The mass of the wire is (C^3)/21.4.

For the 1mH 10 gauge coil above, L = 1000, R = 0.11

K = 9090.9, C = 32.4.

W = 33.7 m

N = 110.4 turns (Lalena gives 103 turns for a coil with these dimensions)

D = 2.59 mm

and you will need 1589 grams of wire (which is 36 metres long
according to WST, so things seem to check out).

Looking at North Creek's resistance values, and the ones obtained by the calculators and Thiele's formula, I must assume that they have a kick-ass formula that tells them how to wind coils with such a low resistance... (1.0 mH and 0.09 ohm can be achieved using 2.93 mm diameter (about 9 gauge) wire with Thiele's formula)

The classical coil inductance formula is L=(r^2 n^2)/(9r+10l) with the dimensions in inches, or L=(r^2 n^2)/(25.4*(9r+10l)) with r and l in mm, but I doubt if that would be at all relevant here :-)

The impedance of a woofer will show a peak at resonance, and then a slow rise from the nominal impedance with increasing frequency.

A Zobel network consists of a series network of a capacitor and a resistor (a high wattage unit) connected in parallel with the woofer.

The purpose of the Zobel network is to cancel the rise in impedance, which is caused by the driver's voice coil impedance. In effect, the voice coil impedance is resonanted out using a capacitor.

With R the nominal impedance of the loudspeaker (4 or 8 ohms) and Le the voice coil impedance, C=Le/(R^2). Other sources suggest that the resistor should be 1.25 times larger than the voice coil resistance.