Transmission loss of plates with embedded multi-scale and tuned acoustic black holes

An acoustic black hole (ABH) plate is a lightweight and high loss panel structure for effective reduction of vibration and radiated sound. It is understood that the high loss local ABH modes can be designed at desired frequencies by changing the size of the ABH cell(s). The ABH cell diameter (size) and minimum thickness play dominant roles in the performance of the ABH effect. In addition, attaching tuning masses at the center of the ABH cells has been shown to alter the local ABH modes with the result of improved low-frequency performance. In this work, the transmission loss (TL) of an embedded multi-scale ABH plate was investigated. The embedded large and small ABH cells were particularly designed to cut-on below and above the critical frequency of the plate, respectively. The results were compared with a uniform plate and an embedded single-scale ABH plate. Discrete tuning masses were attached at the ABH cells' center to manipulate the ABH cut-on modes to increase the TL further. The results show that the damped multi-scale ABH plate achieved a 10 dB TL increase, flattened the TL curve, and nearly eliminated the plate coincidence dip. Manipulating the high loss ABH modes by adding tuning masses (20 g each) demonstrated a 2 dB increase at low frequencies within the mass-law range. Although damping material was applied, adding some mass, an overall weight advantage was still attained compared to the uniform plate. The damped multi-scale ABH plate is 7% lighter than the uniform plate.