Eighteen Sound - Professional loudspeakers

DG - Dual Gap

Dual gap double voice coil (DG) technology, similarly to the Tetracoil one, is based on an innovative magnetic structure where two different voice coils are wound on the same former and suspended evenly in the two magnetic gaps. The difference between Dual Gap and Tetracoil is that in this case, the coils are wound only on the outer surface of the former, reducing the overall moving mass.

The key advantages are:
- Ideal motor symmetry over large displacement providing flat inductance and minimal even-order distortion.

- Excellent thermal dissipation and reduction of thermaldistortion resulting from: (a) twice the voice coil surface area of a standard single voice coil of the same diameter and, (b) reduced power compression for up to 50% more output at high power.

Background

The Dual Gap technology can best be understood by reviewing the evolution of conventional loudspeaker motors. Traditional electro-dynamic loudspeakers incorporate a motor with single copper or aluminum voice coil immersed in a static magnetic field created by a dedicated permanent magnetic circuit. Due to current flowing in the voice-coil a Lorentz force will move the coil in a direction perpendicular to the magnetic field.

For many years, single and multi-layer voice-coils have been wound on the outside layer of the voice-coil former and suspended in a single gap. High power voice-coil formers made from polyamides or glass fiber effectively acts as a thermal insulation between the inner voice-coil layer and the motor pole piece. This thermal insulation reduces thermal radiation from the voice-coil to the steel parts.

18Sound developed first the ISV wound method, to create a major coil surface exposed to the air flux and reduce thermal eating and then applyed it to dual gap motors creating the TTC (Tetracoil) technology which is nowadays the flagship technology for low frequency reproduction.

Dual Gap motors are similar to Tetracoils, but the coils are just wound outside the former to reduce weight but still taking advantage of the extremely high motor force simmetry.

In the Dual Gap design the flux flowing in the upper gap has an equal flux density but with opposite direction from the flux flowing across the lower gap. The current flowing in the upper voice-coil is equal to but with opposite direction from the current flowing in the lower voice-coil. It is important to notice that the motor structure is perfectly symmetric about the horizontal axis. Consequently, this dual gap, dual voice-coil topology provides an equal Lorentz force in each voice-coil with motive force in the same direction.

Dual Gap Benefits

Increased heat transfer

The first benefit provided by Dual Gap technology is the increase in thermal conductivity the area of direct thermal path is doubled then in a traditional single gap motor resulting in a reduction of thermal resistance. This leads to an increased power handling and lower power compression. Compared to a traditional voice-coil with single gap topology the Dual Gap provides two times the voice-coil to steel surface area contact.

Motor Symmetry: Bl(x) symmetry

As stated before, the magnetic circuit is perfectly symmetric in the horizontal plane. This leads to a perfectly symmetric flux density shape and since the coil is symmetric,the Bl factor will be symmetric as well, minimizing the even distortion usually caused by Bl (x) asymmetry and the DC component effect.

L(x) regularity

The motor symmetry creates a very symmetric and quite flat inductance curve. When the cone moves up, the lower coil goes into the gap while the upper one leaves the gap. Consequently, the magnetic path "seen" by the whole coil is quite always the same, with constant inductance curve, in contrast with the inductance of a traditional magnetic circuit.

The benefit of a regular inductance with displacement is low reluctance force. This attraction force is described

by so a high slope inductance will cause a DC component and second harmonic distortion.

AC flux canceling

The coils are wounded in the opposite direction, so the AC magnetic flux generated by one coil is equal and opposite to the flux generated by the other one and this brings about a flux cancellation, reducing inductance.

Conclusion

To summarize, for a given size and class of woofer, the Dual Gapmotor will produce more acoustic output than traditional voice-coil drivers. The Dual Gap innovation is a unique blend of Eighteen Sound technologies: the dual gap motor structure with twovoice-coils wound in an innovative manner increases the power handling and reduces power compression while balancing motor symmetry. Consequently, it has the great advantage of reducing the intrinsic distortions and DC component offsets caused by Bl(x) and L(x) asymmetry for extreme linear excursion with maximum low frequency SPL.

More info and technical charts are provided in the Tetracoil Technology page.

DG - Dual Gap

Dual Gap Double Voice Coil (DG) technology is based on an innovative magnetic structure where two different voice coils are wound on the same former and suspended evenly in the two magnetic gaps. The motor structure is the same of the TTC Tetracoil tech, but the coils are wound only outside the former, reducing the moving mass weight.

The key advantages are:
- Ideal motor symmetry over large displacement providing flat inductance and minimal even-order distortion.