The SV307 outdoor noise monitor is fitted with a MEMS microphone, for additional ruggedness and reliability. As such, none of the other two methods can be used. In the case of the SV307, a small microspeaker is installed in the weather protection, which generates an acoustical signal which can be used for a calibration check.
The SV307 also includes the possibility of sending alerts if the measured level is outside tolerances, or there is another issue with the patented microphone system.
In this way, you can be sure that measured results are valid.