Sound absorption by using Helmholtz resonators
The first stages in the launch of spacecrafts (ignition of the engine and first 3 seconds of the lift-off) constitute the biggest human-made event from the acoustic point of view. During these events, space vehicles are subjected to extremely high sound pressure levels that can have significant effects on lightweight transported structures, such as solar panels and antennas.
As a possible solution to be implemented, a prototype of a composed by Helmholtz resonators that can be used for maximizing the absorption of the sound waves has recently been developed. The absorption is maximized by tuning the geometry of the resonators, by means of an optimization algorithm, for a targeted range of frequencies. The geometry of the system modifies the effective properties of the medium and allows that the condition known as critical coupling is produced. This condition has a similar effect to an impedance matching between the system and the surrounding medium and produces the absorption of energy in the system.
The combined effect of the resonance frequencies of the elements and the critical couple condition, can lead to the perfect absorption of the sound waves in the working range of frequencies. As an additional property, due to the distribution of the resonators, important diffusive properties are achieved in this range.
Full article is available in the journal Applied Sciences: https://www.mdpi.com/2076-3417/10/5/1690.