Like capacitors, all coils represent an energy storage.

How is stored energy specified?
The coil's stored energy E is dependant on the coil's inductance L and the current I applied squared. E = 0.5*L*I2. The higher the inductance L , the more energy the coil stores - at given current. The unit of measure is Henry or H, where usually only a thousandth is needed: Millhenry or mH.

What are the implications for loudspeakers?
When current is applied to a coil, a magnetic field proportional to the current is generated around the conductor. In order to build-up a magnetic field a voltage in the coil is needed. If not enough voltage is available, the magnetic field is built-up slowly. The current flow through the coil also rises only slowly. In other words: The current in the coil is linked to a magnetic field; voltage applied ensures a slow rise of both. When the current changes, the coil charges or discharges. Again, a voltage is applied to the coil, delaying the current change. If a loudspeaker is connected to the coil in series, the coil delays at quick voltage changes. For high frequencies the coil needs all available voltage to recharge - nothing is passed on anymore to the loudspeaker.

When a coil is connected in parallel to a loudspeaker something else happens. At quick current / voltage changes the coil changes its magnetic field and lets the AC voltage pass through. The coil works like a filter, letting high frequency oscillation pass through to the loudspeaker connected in parallel.

What is the ideal coil?
An ideal coil shows above mentioned properties. Ideally a coil stores energy while not dissipating any - therefore, the coil doesn't generate any heat. However, in reality no electrical component is perfect. The wires of a coil are not infinitely good conductors , that's the reason why energy is turned into heat. Additionally, losses occur when a coil recharges, especially when electrical conductors (eddy-current losses) or magnetically active material (magnetic snapback) are close by.

What types of coils are available and what properties do they have?
There are air core coils and coils with magnetic core. The latter are divided into coils with open or closed core around the wire-wrap. The list of coils in ascending order of quality:

• Coils with ferromagnetic sheet metal (iron) in order to enlarge the magnetic field may be much smaller than air core coils (at a given inductance and DC resistance).
• Coils with ferrite-magnetic materials (ferrite) in order to enlarge the magnetic field and consequently the inductance, show lower distortion at low or medium levels compared to ferromagnetic coils.
• Air core coils are the best choice, if price and size are no object.
• Foil (air) coils show the best behaviour regarding impedance response. Mundorf foil coils posses by far the best vibration performance (according to lab results in Hobby HiFi 4/2000 they far superior to baked varnish coils.

When to use which coil?
The following table lists the typical properties of coils:
 

Additional important notes: If a crossover is optimised for a high-impedance coil, the use of a low-impedance coil often doesn't show any advantage. The crossover needs to be tuned again incorporating the high quality component. A re-tuning is also advisable when exchanging a coil with core for a coil without core; since distortion is reduced the result would be rather positive.

What resistance does the ideal coil have?
A coil's resistance is frequency dependant. This resistance cannot be compared with the common resistance. For an estimate the calculation of reactance is quite helpful.
Z = 2*Pi*frequency*L)
where L=inductance, f=frequency, Pi=3.141...

Example:

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