It seems this is the chicken and egg problem. To be able to determine the frequency response of a microphone one needs a speaker that has a known response. To be able to know the response of a speaker one needs a mic with a flat response.
Is there some trick used or some device(essentially a microphone) that is "provably" to have a known frequency response?
I know there are reference microphones that supposedly have a flat frequency response BUT the question is, how did they get to that point in the first place.