How is microphone's frequency response determined
Reciprocity calibration is the technique used to calibrate a microphone without the need for any calibrated sources.  It relies on the basic physics of transducers that show that the ratio of the receiving response to transmitting response of most transducers (designated as reciprocal transducers) is independent of their construction and depends only on easily measured parameters such as frequency, air density and sound speed.  General Radio used to make a reciprocity calibrator that included all of the necessary equipment including a cavity to hold the microphone under calibration.  The process requires a reciprocal microphone.  A series of electrical measurements are made while using the reciprocal microphone as both a transmitter and receiver. These measurements are combined to yield the sensitivity of the microphone under test.  By varying the test frequency, a complete frequency response can be obtained.  Once that is done, the test microphone is calibrated and can be used to calibrate other hydrophones by using comparison measurements.  Details of reciprocity calibration can be found in any acoustic textbook as well as Wikipedia and other on-line sources. Accuracy of reciprocity calibrations is very high because all of the required measurements are electrical and, with the proper procedures, only ratios of measurements are required so a calibrated meter is not even needed (but it must be linear).