If you're willing to use a typewriter you're willing to use a computer with no network connection, so all you need to do is block radio transmissions. A faraday cage can't cost that much compared to the other costs involved in espionage.
What if the room had a length of surgical or other type of long tubing embedded into the wall so that the detector was a fair distance away? Essentially, a very crude stethoscope originating in a wall outlet or other place in the room that already has an orifice(s) and terminating at another location with a passive resonator.
By decoupling the transmitter from the audio source by using a non-electronic medium, it's possible to still eavesdrop on keystrokes even in an em-shielded room.
To what attack are you referring here?