i remember when some mocked me few years ago after i called my grid of 100 tunable resonators : "mechanical equalizer" for system/room...😊
In my acoustic frame of mind i claimed that it was most powerful than many electronical equalizer ( will not explain why again here) and cost nothing except time and "trained ears" for the tuning .,..
Read this extract quote from "Helmholtz resonance" Wiki entry :
«A set of varied size resonators was sold to be used as discrete acoustic filters for the spectral analysis of complex sounds. There is also an adjustable type, called a universal resonator, which consists of two cylinders, one inside the other, which can slide in or out to change the volume of the cavity over a continuous range. An array of 14 of this type of resonator has been employed in a mechanical Fourier sound analyzer. »