A CPU generates the reference value (root mean squared value) of sound level and outputs it to a DSP, the DSP generates a measured value of sound level (root mean squared value), and compares thus-generated measured value of sound level (root mean squared value) with the reference value (root mean squared value) of sound level received from the CPU, and outputs a signal expressing the result of comparison to a display device, to thereby enable comparison of the measured value of sound level (root mean squared value) with the reference value (root mean squared value) of sound level, every time the sampling takes place in synchronization with the sampling frequency in the A/D converter.
