Eighteen Sound Expands Tetracoil Dual Voice Coil Subwoofer Series

May 2 2019, 00:50
Prolight+Sound 2019 was a reassuring show for all companies working with Eighteen Sound drivers. The company is now, together with Ciare, part of the B&C Speakers SpA Group of companies. At the Frankfurt show, Eighteen Sound and Ciare had their own separate booth (side by side with B&C), showing lots of new products in all areas of the portfolio. Of course, Eighteen Sound is renowned for its low frequency drivers, and during Prolight+Sound the company impressed with the new 18NTLW500 next generation high performance subwoofer using Tetracoil technology.

Following the acquisition of Ciare, and the acquisition of the two brands by B&C, Eighteen Sound, located in Reggio Emilia, Italy, continues to expand its research & development activities and develop its state-of-the-art transducer technologies, now consolidated as an expanded portfolio for multiple application areas, with Eighteen Sound focusing on the pro audio segment. The Eighteen Sound engineering team continues to operate independently, even if coordination with B&C is now a reality.

The new 18NTLW500 subwoofer complements the already existing 21NTLW5000 model and the similarly impressive 12NTLW3500, all sharing the same design and technologies. Low frequency drivers are an area where Eighteen Sound excels and the new subwoofers were naturally a big focus for the company at the Frankfurt show, with the booth showing front an center not only the new 18NTLW500 and 12NTLW3500 models, but also showing the units' impressive voice coils.

The 18NTLW5000 subwoofer is specified as an 18 inch diameter, high-power, high excursion subwoofer, with 95 dB average sensitivity (1W/1m) and 4000 W program power handling (1800 W AES) with 8Ω nominal impedance (6.8Ω minimum), using an impressive dual 115mm (4.5") Tetracoil aluminum voice coil. Specifically designed for high SPL at very low frequency applications (30 - 1800 Hz) in either a reflex, bandpass or horn loaded configuration, the 18NTLW5000 was optimized to be used with the highest power amplifiers currently available, able to deliver 4000 watts of program power without clipping.

To sustain the required performance, Eighteen Sound explored its  proprietary Tetracoil technology, where two different, axially separated magnetic gaps and two inside-outside 4.5" diameter aluminum voice coils are wound on the same former and suspended evenly in the two magnetic gaps. The advantages of this Tetracoil design results in symmetric flux density versus displacement behavior, that minimizes even distortion products, with a very symmetric and flat inductance curve, and achieving better heat dissipation, since the equivalent voice coil diameter of a 4.6" Tetracoil speaker is a larger surface area, greater than 6". This allows driving AES power handling up to 1800 watts.

Of course, to achieve extreme excursions and SPL levels, the 18NTLW5000 design also features a large displacement suspension system which, in conjunction with a fiberglass reinforced, straight ribbed cone, allows an ultra-linear piston action and provides full mechanical control across the entire working range. In order to further increase power handling and reduce power compression, a low-density material air diffraction system is placed into the backplate venting hole, acting as a cooling system, increasing power handling capability and lowering the power compression figure.

Since the typical large power subwoofer configurations where the 18NTLW5000 will be used demand a design that is able to perform properly under inclement weather conditions, the cone is treated with a water repellent component that improves pulp strength and provides protection on both sides of the membrane. In addition, the epoxy coating on the magnetic structure of the metal plates is far more resistant than standard zinc coating to the corrosive effects of salts and oxidization.

More about Tetracoil
The 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. For many years, single and multi-layer voice-coils have been wound on the outside layer of the voice-coil formers, made from polyamides or glass fiber, to 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.
Interleaved Sandwich Voice coil (ISV) technology has been used by Eighteen Sound since 1998, dividing 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. This design 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.
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”. In this 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.

This topology, providing four times the conductive surface area, translates in improved thermal conductivity, leading to increased power handling and lower power compression. The magnetic circuit generates a perfectly symmetric flux in the horizontal plane. 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 always the same, creating a constant inductance curve.

For a given size and class of woofer, the Tetracoil motor will produce more acoustic output than traditional voice-coil drivers, with the 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.

Companion Models
Together with the 18-inch 18NTLW5000 model, Eighteen Sound introduced the 12-inch 12NTLW3500, allowing alternative high-quality, and low-distortion applications, using the same basic Tetracoil neodymium motor structure design, and achieving 96dB average sensitivity (1W/1m) and 900W AES power handling. The Dual Gap technology uses the same Tetracoil concept to maximize its benefits in therms of thermal dissipation and Bl symmetry to a wider frequency band, making the 12NTLW3500 a perfect component, both as a woofer and a midbass, with the lowest intermodulation distortion in the professional segment. These features, together with the woofer's extreme low weight (only 4.8 Kg) make the 12NTLW3500 a natural choice for highest quality line arrays and two-way systems. 

Alternatively, the 21NTLW5000 is a 21-inch diameter high performance subwoofer, specifically designed for high SPL very low frequency applications in either a reflex, bandpass or horn loaded configuration. Rated for 4000W program power without clipping and 96.5 dB SPL sensitivity (1W/1m), the 21NTLW5000 has similar features as the newer, smaller 12- and 18-inch models, including the proprietary Tetracoil technology, here applied in a massive dual 115mm (4.5") aluminum voice coil.

Comparing the specifications of this larger 21NTLW5000 subwoofer - introduced originally in 2017 - with the new 18NTLW5000 design just accentuates the impressive work done by Eighteen Sound in optimizing the whole concept. The  new 18" will enable very impressive low frequency reinforcement applications in compact cabinets.
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