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TTC - Tetracoil Double Voice Coil

TTC - Tetracoil

Tetracoil Double Voice Coil (TTC) technology is based on an innovative magnetic structure where two different inside-outside voice coils are wound on the same former and suspended evenly in the two magnetic gaps.

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.



The Tetracoil 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.
This conventional design is shown in Fig 1.

For many years (see fig. 2), single and multi-layer voice-coils have been wound on the outside layer of the voice-coil former. 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.

To take advantage of voice-coil wire to steel intimacy, the ISV Interleaved Sandwich Voice coil technology has been used by Eighteen Sound since 19981. As shown in Figure 3 and Figure 4 the ISV voice-coil divides a single multi-layer voice-coil into two separate voice-coil segments by winding coil layers on both the inside and outside of the voice-coil former.

It is clear that the ISV coil doubles the direct surface area relation between voice-coil and steel plates, increasing thermal conductivity for improved transfer of heat from voice-coil to pole plate and front plate.

Tetracoil Technology

The Tetracoil technology extends the ISV principle into a dual gap motor structure. With two inner and two outer coils there are in effect four voice-coils, hence the name “Tetracoil”. The two Interleaved Sandwich Voice coils are wound on a single cylindrical voice coil former, suspended evenly in the two magnetic gaps as shown in Fig 5.

In the Tetracoil 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.


Tetracoil Benefits

Increased heat transfer

The first benefit provided by Tetracoil technology is the increase in thermal conductivity. As shown in Figure 5, each gap of the Tetracoil motor structure is equal to the gap of a traditional structure shown in Figure 4, however the area of direct thermal path is doubled, 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 Tetracoil provides four times the voice-coil to steel surface area contact. This is corresponding to using a 5.6” diameter voice-coil in place of a 4” diameter voice-coil. In this scenario it is easy to see that the larger diameter voice-coil has more surface area.
Fig 6 illustrates power compression comparisons between two 18 inch speakers, one with Tetracoil technology and one with ISV structure, powered with 15 minutes of pink noise filtered from 30 to 300 Hz. Measurement shows that the Tetracoil structure allows more than 1 dB less power compression.

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, as seen in Figure 7, 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 as plotted against voice-coil excursion in Fig 9. 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, Fig.10.

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.



To summarize, for a given size and class of woofer, the Tetracoil motor will produce more acoustic output than traditional voice-coil drivers. The Tetracoil innovation is a unique blend of Eighteen Sound technologies: the dual gap motor structure with four voice-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. 
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