Gamma structure

Optimum choice

The surface area of a loudspeaker cabinet is infinitely greater than that of the driver diaphragms, so it can radiate sound at a level close to that of the diaphragms, which is colored and distorted by the resonances of the cabinet structure. This alters reproduction and blurs the sound image. To prevent such "interference", it's essential that the cabinet is as inert as possible, and not emissive. At Focal, we use MDF (Medium Density Fiber) to achieve this inertia. This material may seem "low tech" compared to other materials used by some competitors, but MDF has intrinsic advantages that make it an optimal choice for cabinet construction.

Firstly, it is dense and sufficiently rigid (when used in great thickness for massive front panels) to counteract the acceleration of the moving parts under the effect of magnetic force. When a cone is propelled by the voice coil, an opposing force acts in reaction on the loudspeaker frame. This reaction force is one of the main causes of vibration in the cabinet structure, and must be perfectly controlled. A front panel of very high mass is required, but stiffeners must also be judiciously positioned to control resonances from other cabinet walls. A cabinet made of a very rigid material can be just as bad as one that is insufficiently rigid, as it pushes resonances into a higher frequency zone where the ear is more sensitive.

Secondly, MDF is similar to a sandwich structure, mainly in thickness, with the faces on either side of the panel denser than the core. This contributes to rigidity, but also provides natural internal damping to help absorb any stray vibrations. For Utopia and Sopra, the fronts are made of thick MDF panels laminated together to multiply the sandwich effect.

Finally, MDF can be easily machined with numerical controls to obtain curved shapes. The first acoustic advantage is that such shapes avoid direct reflections by progressively diffracting the sound wave around the cabinet, thus eliminating secondary emissions caused by direct reflections from sharp angles. Secondly, from a structural point of view, a curved shape offers greater intrinsic rigidity than a flat panel.

Focal products using this technology

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