Leaving your phone at home only reduces by one the indeterminate count of listening devices.
A countermeasure like this takes out all opportunistic recording, however an adversary could plan specifically for recording you: if the adversary places multiple microphones [two could suffice, three would be ideal] at known locations, then they can process the joint recordings and separate the concealment noise from the desired signal.
Leaving your phone at home only reduces by one the indeterminate count of listening devices.
A countermeasure like this takes out all opportunistic recording, however an adversary could plan specifically for recording you: if the adversary places multiple microphones [two could suffice, three would be ideal] at known locations, then they can process the joint recordings and separate the concealment noise from the desired signal.