D, frequency-weighting-Z, and frequency-weighting-A is mandatory for all SLMs specified in IEC61672 [2]. At present, the calibration work is mostly carried out manually in our country and it is quite inefficient and time-consuming. Researchers make some efforts on calibrating SLMs automatically [5, 6]. They get good experiment results in electrical calibration method for SLMs. Their systems can realise automation only when the SLMs have electrical outputs which should have equivalent relationship with the measured sound level of SLMs. However, quite a lot of SLMs in the market don’t have electrical output interface and their measured values can be obtained only by their digital indication on the display screen. So it limits their systems’ application.