Tech Talk

The 8-Inch Woofer for Summit AMA

George Short

Senior Principal Engineer - HARMAN Luxury Audio

When most of us dream of woofers, it looks something like this:

But to truly appreciate the Summit JW200SC 8-Inch woofer, we have to see it like this:

3D Model by Larry Brown

These design features – optimizing performance via leveraging 80 years of transducer design know-how, the latest FEA, and the most modern manufacturing techniques in the world – yielding a modest looking 8-inch transducer that sets a new standard for the state of the art.

Customarily in product descriptions one creates a “features” list…. so here goes:

Design Feature:

Extended Vented Pole Piece

Copper Faraday Sleeve

Aluminium Flux Stabilizing Cylinder and Ceramic Magnet

Kapton® Voice Coil Former

Edgewound Copper-Clad-Aluminium Ribbon Voice Coil

Dual Opposing Spiders

C4™-PMI Sandwich Cone with Rubber Surround

C4 Dust Cap

Good stuff. But what does all this do? Well, in developing this transducer, emphasis was placed on optimizing every single element for performance, durability and enduring quality.

Extended Vented Pole Piece: As discussed in my recent Tech-Talk “Improving Woofer Motor Linearity with Finite Element Analysis”, by extending the pole piece beyond the top plate we symmetrize the stray flux above and below, greatly reducing distortion. Also, the pole acts as a heat wick that pulls thermal energy from the inside of the voice coil into the pole steel. Lastly, cone motion forces air flow through the pole vent and cools the steel.

Copper Faraday Sleeve: This sleeve of formed copper hugs the pole piece. Electromagnetically, the sleeve provides a high conductivity induced sympathetic current loop just inside the voice coil cylinder, where the magnetic field generated by current flow through the voice coil is most concentrated. The sympathetic current minimizes both voice coil self-inductance and its variation with excursion, and additionally contributes to mitigating modulation of the motor’s static magnetic field by voice coil current and motion. Thermally, it acts as a proximity conduit between the voice coil and pole piece, as copper is 20-times more thermally efficient than plain steel. Whew. So, particularly in the midrange, this penny’s weight of copper gives us the highly desirable quintuple bonus.

Aluminium Flux Stabilizing Cylinder and Ceramic Magnet: in almost all HARMAN Luxury Audio woofers, these features are used together. Like the Copper Faraday Sleeve, the aluminium cylinder provides an induced sympathetic current loop, but in this case it surrounds the voice coil and is internal to the magnet, stabilizing the magnetic field within the magnet itself. Also like the Faraday Sleeve, The aluminium cylinder wicks heat from the voice coil windings and conducts it to the magnet. It is most effective at low frequencies.

The combination of the Copper Faraday Sleeve and Aluminum Flux Stabilizing Cylinder reduce the woofer’s distortion by an order of magnitude.

Kapton® Voice Coil Former: Kapton® is an amazing film, even more amazing when one considers it was invented in 1965 (by Dupont). The grade of Kapton® we use has been optimized for voice coil former applications, and features excellent dimensional stability, mechanical stiffness and unprecedented durability. The surface is specially treated to permanently bond to voice coil, spider and cone adhesives.

Edgewound Copper-Clad-Aluminium Ribbon Voice Coil: Voice coil winding density contributes directly (proportionally) to motor strength. A typical voice coil is wound with two layers of round wire, which geometrically can fit four windings in a square about twice the width of the wire. For example, a 0.5mm voice coil wire will have 4 windings in a 1mm x 1mm square. Additionally, the conductor cross sectional area is 0.78mm².

Ribbon wire is different. It is rectangular with a 5:1 ratio, so a 1mm wide wire is 0.20mm thick, and when wound on edge in a stack there are five windings in a 1mm square. For the same profile, we can fit in 25% more windings and have 22% more conductor area. This allowed us to replace the usual round copper wire with less conductive, but much lighter, copper-clad-aluminum wire, saving about 30% of the voice coil mass with no loss in motor strength. For a high performance mid-woofer, lighter is better.

Dual Opposing Spiders: Spiders are essentially round woven accordions. As they move back-and-forth, they stretch-compress-stretch about their rest shape, and invariably they have a preferred direction. By using dual spiders mounted in opposition, any nonlinear motion in one is cancelled by the opposite non-linear motion in the other, resulting in a perfectly linear suspension. Additionally, the stress on each spider is halved, doubling durability.

Transducer excursion analysis shows the Summit JW200 can move 26mm peak-to-peak – more than one inch – and still perform within its optimum ±10% linear window. To put this in perspective, if we scaled this to a 15” woofer, it would be moving 52mm (2 inches) peak-to-peak! Yikes! But….yea.

PMI-C4™ Composite Sandwich Cone with Rubber Surround: The best of the best. C4™ is Carbon Cellulose Composite Cone, a proprietary compound developed by JBL. It is a hard pressed, paper thin, rigid skin.

PMI (Polymethacrylimide) closed-cell foam was developed for the aeronautics industry, it is almost lighter than air and can be found in gliders, Formula One racecars, and now Summit woofer cones. By bonding both sides of a thin layer of PMI foam with the C4™ composite, we get a cone that is incredibly light and much stiffer than a single layer cone. For loudspeaker applications this sandwich can actually be too stiff, therefore special adhesives were developed. Applied between the foam and the skin, to add just a whisper of give, cone vibration energy is effectively transmitted to and dissipated within the rubber surround.

A theoretically perfect 8-Inch woofer should be ruler-flat from its bass tuning up to about 500Hz, above which its frequency response on axis should gently rise approaching 6dB/octave before rolling off at the top end. The Summit JW200SC approaches this ideal, behaving perfectly to about 1.2kHz and greatly simplifying crossover design. One can clearly hear this approach shining through.

The Summit 8-Inch woofer- defining the state of the art:

3D Model by Larry Brown. Rendering by Erik Lundin

The challenge is to design a transducer that can go as deep as many subwoofers, yet exhibits clarity through the center octaves that surpasses dedicated midrange transducers. But for Summit AMA, we would not settle for anything less!