The term is used for both RF and Audio wave absorbing rooms, but in this case they are talking explicitly about a room that doesn't reflect Sound.
That's why an RF anechoic chamber looks a lot like an acoustical anechoic chamber, and often sounds like one too.
An acoustic wavelength at STP is around 1ft/30cm, not 3m (which is closer to 100 Hz).
Point stands, though.
EM waves vs pressure waves, they're not the same thing (although the math is basically the same, just with different constants). Something that affects an EM wave won't necessarily affect a pressure wave - a Faraday cage for example will block EM waves but won't do anything to block sound waves.
There are some mathematical differences between transverse and longitudinal waves, so it's not just the same math with a different medium.
While they are both considered waves, it is not frequency regimes that differentiate acoustic pressure waves and RF waves -- RF waves can be anywhere within the "audio" spectrum and beyond, and audio waves can be well into the ultrasonic range and into the UHF regime.
The idea of the anechoic chamber is the same (blocking all reflections), but it is not just as simple as tuning to different frequencies. They do however need to be designed for a specific frequency range.
I am not an expert on anechoic chamber design (or acoustic wave propagation for that matter), but an RF anechoic chamber is not the same as an acoustic anechoic chamber, although it will show some functionality as one. Does that make sense, or am I just confusing the conversation? It did when I started writing all this. :/
Neither of the RF chambers I used had the foam cones on the floor; just carbon tiles.
Theoretically, at sea level on earth, the mean free path of air molecules allows for 3.4 GHz. But attenuation is proportional to f^2 which results in an upper limit of 100MHz. You need a medium denser than air at sea level for higher frequencies.