The speaker cabinet is like the foundation of a house. A bad foundation and the house collapses in the next storm. It is absolutely fundamental that a loudspeaker cabinet is extremely rigid and low in vibration. Unlike a musical instrument, where the wood is responsible for the specific sound, a loudspeaker should not produce any inherent sound Therefore, the ideal speaker cabinet is acoustically dead, which means that it does not resonate at all.
With an enclosure material, it is important to distinguish between two properties, which are often mixed. There is the insulation and the damping. Insulation means how much sound passes through a material, or how much the sound is damped. This depends primarily on the specific density (kg/m3) and the rigidity of the material. In simple terms, damping means how fast a material resonates when it is excited. For most materials, this can be determined simply by tapping on them. Metals generally resonate longer than soft materials.
We have tested and measured different materials for our speaker systems. This was done with the help of a solid concrete block, where panels with different materials could be tested on one side. Materials such as MDF, aluminum and steel were used. The result was that steel showed over 20 dB better damping than the other materials, especially in the low frequency range. The disadvantage of metals is, however, as already mentioned above, the long reverberation. The decisive question, however, is at which frequency the reverberation is present. Each surface forms a so-called spring-mass system with at least one resonant frequency.
With the prototypes, we were able to make initial measurements that show the resonance frequencies. In the diagram above, these are relatively pronounced at around 600 Hz. These are clearly above the operating range of the two bass drivers (below 100 Hz), which work on the large volume. Thus, these enclosure surfaces are not excited at their resonance and therefore the damping property is not relevant in this case. This is different for a two-way system with bass-midrange driver, which is used up to 2 kHz or higher. Here, the resonances would have to be damped in some way or the resonance frequency would have to be pushed into a significantly higher range by dividing it into smaller sub-surfaces.
The whole problem with unwanted resonance frequencies is also known for every loudspeaker chassis. Every loudspeaker developer tries to use the transducers in such a way that the diaphragm resonances are clearly above the operating range. In this case they are completely harmless for the sound.