Researchers have reported several hazardous impacts of low-frequency noise human health such as annoyance, headache, and fatigue. To attenuate low frequency sound, passive solutions such as foam layers, sound diffusing and sandwich panels, are listed as the most widespread means for acoustic treatment in a vast range of applications, from room acoustics to loudspeaker enclosures. However, these means present certain limitations in the low frequency range, namely below 300 Hz approximately, which is very important regarding noise control in transportation media or inside buildings, among other applications. Generally, since the ‘mass law’ that governs the mid-frequency range above the fundamental frequency states that the level of STL (i.e Sound transmission loss – a quantification metric of how much sound energy is prevented from traveling through an acoustic treatment) can be improved only by increasing the mass density of the panel, this leaves only the mitigation of the first panel resonance region as in Figure 1 especially when the addition of extra mass needs to be avoided.
Figure 1. Typical STL profile of a panel
Owing to this need of low frequency sound absorption, it is desired to develop a new generation of partitions capable of attenuating noise in low-to-high frequency range. The practical constraints relating to the thickness of the panel for adequate noise absorption, are compensated through the unique damping properties of the proposed KDamper mounts together with their ability to attenuate low frequency excitation in a considerable range.
Figure 2. Indicative implementation of a KDamper mounted flexible panel
Therefore, a preliminary implementation of an acoustic panel with appropriately designed elastic mounts incorporating negative stiffness elements, is proposed as depicted in Figure 2. The problem is approached as in the case of classic vibration isolation of a rigid mass, due to the nature of the low frequency excitation. However, the elasticity of the deformable thin plate is taken into account by modelling its dynamic behavior using generalized modal values according to the model described in Figure 3. Early results regarding the noise control capabilities of such a panel configuration, indicate the potential of this proposition in multiple applications.
Figure 3. Lamped parameter model of a KDamper mounted deformable plate
More information is openly available in a recent publication (Paradeisiotis et al., 2020): https://doi.org/10.47964/1120.9335.19276. The video contains the associated conference presentation in the XI International Conference on Structural Dynamics (EURODYN2020): https://www.youtube.com/watch?v=V0aaz5Ql2Oo&feature=youtu.be