Typical loudspeaker cabinets have pronounced structural resonances which are very audible and reduce the speaker’s ‘signal-to-noise ratio’. At FINKTEAM we take this aspect of loudspeaker performance very seriously because we know that a quiet cabinet allows the reproduction of low-level detail in a recording which is otherwise swamped by spurious cabinet output. Coloration and time smear are reduced, stereo image focus is improved and listener fatigue postponed.
The lower and upper cabinets of WM-4 are designed differently according to the frequency range of the vibrations to which they are subjected by their respective drive units. Compliant spacers position the upper cabinet precisely on the lower one while ensuring effective vibration isolation between the two.
With the bass cabinet the design emphasis is on making the cabinet as stiff as possible, to force panel resonances above the crossover frequency to the midrange drivers. This is achieved using internal bracing, the positioning of which was optimised using finite element analysis and confirmed with laser interferometry measurements. It is important that braces add stiffness only where needed, otherwise they can transfer energy to other parts of the cabinet, making the control of cabinet vibration harder.
With the midrange/tweeter cabinet the design emphasis is on panel damping. It is impossible to force all the panel bending resonances above the passband so instead they are damped to reduce their amplitude to below audibility. This is achieved using a multilayer construction that combines multi-thickness MDF panels with a damping layer whose internal friction converts vibration into heat. FINKTEAM-developed algorithms help specify ideal material thicknesses to achieve the best results, but the ultimate determination is made by subjective assessment.
Structural resonances are not the only ones we need to control: there are also standing waves within the enclosed volume of air. These impose forces on the enclosure walls and can also escape the cabinet either by passing through the diaphragm of the drive unit or, in the case of a vented speaker like the WM-4, through the reflex port.
The traditional solution is to fill the space with a fibrous tangle such as long-hair wool or BAF, or a flexible or rigid open-cell foam material. These subject internal air movement to frictional losses which damp the resonances. But they apply these losses at all frequencies, not just the resonance frequencies, and this can have a negative effect on sound quality. Dynamics and precision both suffer.
We’ve used a very different solution in the WM-4. Tuned quarter-wave resonators, mounted within the cabinet, act to equalise the pressure differential between the extremities and centre of the air space which occurs at the fundamental internal resonance. This virtually removes the resonance without the need for large amounts of cabinet stuffing.