sound is absorbed well through mass and multiple layers (sound absorbtion through layering). in fact an air cavity can also help in the absortion of sound. also if you have 2 parallel walls with an air cavity it is best if they are not quite perfectly parallel, this alows the sound to be better disapated as differnt wavelengths can 'bounce' around inside the cavity allowing the absorbtion co-effeciancy of the walls to better absorb a wider frequency bandwidth.

you can even purchase a ready sealed room designed for drum kits to be recorded in small project studios, however this is a small structure but is very portable unlike a breeze block and sand strucutre as mentioned earlier.

it would almost be better for large events to build their own bespoke 'bunker' for each event (especially if they have budjets of up to 100000)