Coils are often categorised in air core coils and core coils. Air core coils usually don't have a core (they might have a synthetic core without electromagnetic properties). Core coils have a core made of ferromagnetic material (ferrite, iron sheet etc.) to increase its inductance.
Air core coils are best since they don't show any loss due to reverse flux. They often have the disadvantage of being too large at a given inductance and DC resistance. In this case, a ferromagnetic core has to be added to reduce the size.
Cores made of ferromagnetic materials to increase coil inductance show two types of loss:
Both loss types result in sound degradation and not applicable to air core coils.
The wire used for the coil possesses a DC resistance. Reducing the DC resistance by employing a coil in series to a bass driver, results in tighter and cleaner bass. The DC resistance doesn't cause any harmonic distortion and, therefore, sound degradation.
Besides the reverse flux loss, the microphonics effect also plays a role in all types of coils.
The microphonics effect (interaction of current flowing through the coil -> magnetic field -> -> mechanical movement of coil wires -> and the resulting induced voltage in the coil) adds information to the original signal. Details of the original signal are overlaid and alienated; the music lacks three-dimensionality and transparency. To avoid this, the wires should be more secured in place.
This can be done by
The table below shows which coil works best for a specific driver type.
Please note: the coil's polarity is of no importance!
Audaphon air coils L7 L10 L13 core coils M12 M14 Mundorf Luft L100 L140 H coreH71 H100 H140 Monacor Trafo
All listed coils are made of copper lacquer wire with a purity of>99.99%. The coil body is made of platic. The wire ends are tinned and labelled. Inductance is measured at 25 degrees Celsius; tolerance is +/- 3%.
L denotes inductance, R the DC resistance, D the diameter, Le the length, Wire-W the width of the wire, DxH the diameter and the height.
Pmax 8 Ohm is the maximum power that this coil is able to deliver to the bass driver. The values are valid for free air convection and 20°C. For 4 Ohm speakers this value has to be halved.
All listed coils are made of copper lacquer wire with a purity of>99.99%. The coil body is made of glass fibre reinforced polyamide. The wire ends are tinned and labelled. Inductance is measured at 25 degrees Celsius; tolerance is +/- 3%.
Monacor transformer core coil with high power handling. The coil has OFC wire turns and is vacuum impregnated. These characteristics are available at an amazingly low price. This coil is also suited for subwoofers and bass drivers in hi-fi loudspeakers.