Panel absorber with the entire surface of the panel glued to a foam layer.
Panel absorber with laterally clamped panel. The two lowest natural oscillations are shown in colour.
Panel absorbers are yet another form of resonant oscillating mass-spring systems. A panel absorber consists of a flat panel made of wood, metal, gypsum board, or plastic material that is arranged in front of an enclosed air volume. The air volume is partly or completely filled with mineral wool or foam. Such a system has several resonance frequencies that can be excited by airborne sound.
If the panel is glued over its entire surface area to a foam layer located in front of a rigid wall, the panel as a whole is allowed to oscillate on the spring formed by the foam layer. The resonance frequency of the panel absorber can be calculated from the mass of the panel and the stiffness of the spring formed by the foam layer. The heavier the panel and the softer the foam layer, the lower the resonance frequency.
If, however, the panel is clamped in any way, it will no longer be able to move as a whole but will only be able to perform bending oscillations. Generally, calculation of the associated eigenfrequencies requires a lot of effort since they depend on the way of clamping, the dimensions, thickness, and material of the panel as well as on the air volume and its filling. Ford and McGormick offer solutions for some special cases. The bending oscillations of the panel are slowed down by the friction between the molecules of the panel material. So the sound energy is first converted into oscillation energy and only then into heat. Often the air space behind the panel requires additional damping by means of mineral wool or foam. By skilful adjustment many of the possible resonances can be arranged next to each other and damped in the best possible way to create a bass absorber effective over a comparatively broad frequency band.