Infinity Kappa 600 Design and Construction

The Kappa 600's were engineered and manufactured by Infinity in Denmark, originally for the European market. They were available in the United States for a time, and are still available in Europe. Product information for the Kappa series is available on Infinity's international web site .

The Kappa 600's are floor-standing speakers with a narrow, tapered enclosure just over three feet tall. The cabinets are finished with furniture grade Danish veneers in maple, cherry, or the review pair's black ash and weigh 71.5 pounds each. The grilles and baffles for all the variants are a contrasting gray/silver. The front baffle is sloped back at 5 degrees to provide phase coherency between the midrange and tweeter and has rounded corners to minimize diffraction effects. The speakers have gold plated dual binding posts with large gauge wire jumpers allowing for bi-wiring/bi-amplification. The crossovers use air core inductors in a low loss design for increased power handling and reduced degradation of sound quality. Threaded carpet spikes are provided to stabilize the speaker on carpeted floors.

The cabinets are formed of ¾" to 1" MDF with wood veneer. The inside of the cabinets are completely covered with foam to provide internal damping. Horizontal bracing couples the sidewalls together at several points for additional rigidity. All the internal wiring is done with 16 gauge, braided copper wire with soldered connections.

The speaker is a three-way design with a driver compliment that consists of a 1" dome tweeter, a 6 ½" cone midrange, and a 10" side firing cone woofer in a rear ported design. This woofer placement allows for the narrow front baffle to improve imaging in a dedicated left/right speaker configuration. Crossover frequencies are at 100 and 3000 Hz, effectively making the system behave similar to a two-way system with a passive subwoofer. All the drivers use Infinity's Ceramic Metal Matrix Diaphragms (CMMD) for the cones.

CMMD is a composite material for which Harman International Industries Incorporated holds two patents: 6,327,372 and 6,404,897 . One is related to the composite material and the other related to the manufacturing process for the material. The composite material consists of an aluminum core with alumina anodized to the outer surfaces in a 20/60/20% arrangement.

Alumina is a ceramic material also known chemically as aluminum oxide (Al2O3). It is the primary component of bauxite, aluminum ore, used to produce alumninum and is found naturally occurring in clay soils and other minerals. Metallic aluminum is highly reactive with oxygen such that any exposed aluminum surface will naturally form a thin layer of alumina that prevents further corrosion. Anodizing can be used to thicken and modify the mechanical properties of this naturally occurring layer as Infinity has done.

Alumina is a widely used engineering ceramic that has strong interatomic ionic bonds that give rise to a number of advantageous mechanical properties. In addition to having a very high stiffness, alumnia is hard and wear resistant, has good electrical and thermal insulating properties, has a melting point of 2000 deg C, and resists chemical reactions including acid and alkaline attack. It is used in a wide range of applications including: abrasives, cutting tools, high voltage insulators, furnace liners, ballistic armor, and high temperature laboratory equipment among other uses.

Infinity literature on CMMD provides a comparison between alumina and a number of other materials commonly used to construct speaker cones. Materials such as polypropylene, Kevlar, paper, and titanium are compared with the aluminum and alumina components of CMMD. Material properties such as Young's Modulus (E) that represents the linear elastic stiffness of a material, mass density (Dm), and the speed of sound in the material are provided. Young's Modulus can be calculated by taking a piece of material, of a known cross sectional area, usually rectangular, and measuring how its length changes (strain) as an applied load increases on the cross section (stress). The data can be plotted and the slope of the initial portion of the data, before any significant kinks or bends in the curve, is Young's Modulus: stress/strain. The speed of sound in a material is an indirect measure of the material stiffness and natural vibrational frequency of the material as wave propagation velocity increases with stiffness.

Of particular interest is to calculate the ratio of stiffness to a unit mass (density) (E/Dm):

As illustrated by the calculations above, alumina offers a substantial advantage in stiffness for a given mass of material. Based on a linear ratio of Infinity's material thickness percentages for the components of CMMD, the combined material still retains a ratio of 56.7x10 ^9 , more than double metallic titanium and ten times greater than paper. Now consider that a material's stiffness (k) to resist deformation to an applied axial load is ka = AE/L ((area*Young's Modulus)/length) and to an applied bending load is kb = 4EI/L (where the moment of inertia I = (width*(thickness^3))/(length^4) for a rectangular cross section), is it becomes clear that for a given geometry that it will take a substantially greater mass of these other materials to achieve the same stiffness as a CMMD cone. While the mechanics of a cone loaded at its apex are a little more complicated than for a rectangular, end loaded section, these fundamentals serve to illustrate the importance of the properties Infinity discusses.

A composite material such as CMMD attempts to combine advantageous mechanical behavior for each material while minimizing the disadvantages. Aluminum has a high stiffness to mass ratio, is ductile, but has little internal damping; like all diaphragms made of a metal it will tend to ring at resonance as an underdamped mechanical system. Alumina has an even greater stiffness to mass ratio, has higher internal damping, but is brittle, with poor tensile capacity to resist the kind of cyclic loading that speaker diaphragms undergo. Combined, Infinity contends that the material is light, stiff, has improved internal damping, and is a durable material for cone construction.

The mechanical advantages have several important effects on performance. Low inertial mass allows the drivers to respond quickly to the ever-changing audio signal for improved transient response. High stiffness has a twofold effect: to minimize distortion under load, as illustrated above, and to push the natural frequencies for internal vibration modes higher. Natural circular frequency is calculated by w = (k/m)^½ (the square root of stiffness/mass) and the frequency in hertz (Hz) is f = w/2pi (the natural circular frequency/2*pi). Optimum behavior is rigid body motion of the diaphragm without any natural vibrational frequencies occurring over the audible frequency range of the driver. Any natural frequencies within the cones operational range would, under cyclic load, cause resonance and produce audible distortion of the signal. Improved internal damping will attenuate any vibrational modes within the diaphragm more quickly. Couple that with the butyl surrounds on the cones, which will damp the rigid body vibrations of the diaphragm and you should have a driver able to more faithfully reproduce a variable frequency signal. The test of the success in implementing these design goals is in the listening.