A separate calibration of your microphone will be able to show a flat response curve or one where several frequencies may be off. You might miss this valuable piece of information if the sound level meter is checked as a whole unit (SLM + microphone combined).
MICROHONE CALIBRATION AT CIH EQUIPMENT COMPANY - Measurements made in accordance with IEC61094 - Sensitivity of the microphone as mV/Pa and dB rel.1V/Pa - Frequency response using an electrostatic actuator in the frequency range 20-20,000Hz
The expanded uncertainty of the sensitivity measurements is calculated as a combination of the uncertainty of the test system and the repeatability of the measurements. This includes the calculation of the standard deviations, degrees of freedom and coverage factor which are needed to estimate the 95% expanded uncertainty.