Bass Trap or Snake Oil

Yep, I see that but these are completely different products.  The RHR-21 is not active.

I know the RHR-21 isn't active. If you are looking for a small form factor solution the ELF is a consideration. I doubt that the RHR-21 does much of anything.

Absorbing bass with a passive device takes up more space.

I thought (maybe mistakenly) that you were looking for a solution to a problem.

Got it.  Thank you, @jperry.  How many you used and where do you place them?

I have one in the front left corner of my room.

it is a Helmholtz resonator kind of . It is designed to tune the room. Location matter. I used 100 different one of different size but tubes (bottle) with variable neck lenght and volume...They not only absorb some sound frequencies but diffuse also some others.

Those who think this did not work do not know acoustics basics...Most consumers buy they dont experiment then called "snake oil" products derived directly from Helmholtz research ...

I dont think this one will do anything else than refine the sound, it takes many  of different bigger size and well tuned to really  eliminate  bass standing waves...

But i never bought any resonators sold in commerce because it is too costly and it was more fun to make them and tune them using my ears...